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Next: Introduction, Previous: (dir), Up: (dir) [Contents][Index] Bash FeaturesThis text is a brief description of the features that are present in the Bash shell (version 4.4, 7 September 2016). The Bash home page is http://www.gnu.org/software/bash/. This is Edition 4.4, last updated 7 September 2016,
of The GNU Bash Reference Manual,
for Bash contains features that appear in other popular shells, and some features that only appear in Bash. Some of the shells that Bash has borrowed concepts from are the Bourne Shell (sh), the Korn Shell (ksh), and the C-shell (csh and its successor, tcsh). The following menu breaks the features up into categories, noting which features were inspired by other shells and which are specific to Bash. This manual is meant as a brief introduction to features found in Bash. The Bash manual page should be used as the definitive reference on shell behavior.
Next: Definitions, Up: Top [Contents][Index] 1 Introduction
Next: What is a shell?, Up: Introduction [Contents][Index] 1.1 What is Bash?Bash is the shell, or command language interpreter,
for the GNU operating system.
The name is an acronym for the ‘Bourne-Again SHell’,
a pun on Stephen Bourne, the author of the direct ancestor of
the current Unix shell Bash is largely compatible with While the GNU operating system provides other shells, including
a version of Previous: What is Bash?, Up: Introduction [Contents][Index] 1.2 What is a shell?At its base, a shell is simply a macro processor that executes commands. The term macro processor means functionality where text and symbols are expanded to create larger expressions. A Unix shell is both a command interpreter and a programming language. As a command interpreter, the shell provides the user interface to the rich set of GNU utilities. The programming language features allow these utilities to be combined. Files containing commands can be created, and become commands themselves. These new commands have the same status as system commands in directories such as /bin, allowing users or groups to establish custom environments to automate their common tasks. Shells may be used interactively or non-interactively. In interactive mode, they accept input typed from the keyboard. When executing non-interactively, shells execute commands read from a file. A shell allows execution of GNU commands, both synchronously and asynchronously. The shell waits for synchronous commands to complete before accepting more input; asynchronous commands continue to execute in parallel with the shell while it reads and executes additional commands. The redirection constructs permit fine-grained control of the input and output of those commands. Moreover, the shell allows control over the contents of commands’ environments. Shells also provide a small set of built-in
commands (builtins) implementing functionality impossible
or inconvenient to obtain via separate utilities.
For example, While executing commands is essential, most of the power (and complexity) of shells is due to their embedded programming languages. Like any high-level language, the shell provides variables, flow control constructs, quoting, and functions. Shells offer features geared specifically for interactive use rather than to augment the programming language. These interactive features include job control, command line editing, command history and aliases. Each of these features is described in this manual. Next: Basic Shell Features, Previous: Introduction, Up: Top [Contents][Index] 2 DefinitionsThese definitions are used throughout the remainder of this manual.
Next: Shell Builtin Commands, Previous: Definitions, Up: Top [Contents][Index] 3 Basic Shell FeaturesBash is an acronym for ‘Bourne-Again SHell’. The Bourne shell is the traditional Unix shell originally written by Stephen Bourne. All of the Bourne shell builtin commands are available in Bash, The rules for evaluation and quoting are taken from the POSIX specification for the ‘standard’ Unix shell. This chapter briefly summarizes the shell’s ‘building blocks’: commands, control structures, shell functions, shell parameters, shell expansions, redirections, which are a way to direct input and output from and to named files, and how the shell executes commands.
Next: Shell Commands, Up: Basic Shell Features [Contents][Index] 3.1 Shell Syntax
When the shell reads input, it proceeds through a sequence of operations. If the input indicates the beginning of a comment, the shell ignores the comment symbol (‘#’), and the rest of that line. Otherwise, roughly speaking, the shell reads its input and divides the input into words and operators, employing the quoting rules to select which meanings to assign various words and characters. The shell then parses these tokens into commands and other constructs, removes the special meaning of certain words or characters, expands others, redirects input and output as needed, executes the specified command, waits for the command’s exit status, and makes that exit status available for further inspection or processing. Next: Quoting, Up: Shell Syntax [Contents][Index] 3.1.1 Shell OperationThe following is a brief description of the shell’s operation when it reads and executes a command. Basically, the shell does the following:
Next: Comments, Previous: Shell Operation, Up: Shell Syntax [Contents][Index] 3.1.2 Quoting
Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. Each of the shell metacharacters (see Definitions) has special meaning to the shell and must be quoted if it is to represent itself. When the command history expansion facilities are being used (see History Interaction), the history expansion character, usually ‘!’, must be quoted to prevent history expansion. See Bash History Facilities, for more details concerning history expansion. There are three quoting mechanisms: the escape character, single quotes, and double quotes. Next: Single Quotes, Up: Quoting [Contents][Index] 3.1.2.1 Escape CharacterA non-quoted backslash ‘\’ is the Bash escape character.
It preserves the literal value of the next character that follows,
with the exception of Next: Double Quotes, Previous: Escape Character, Up: Quoting [Contents][Index] 3.1.2.2 Single QuotesEnclosing characters in single quotes (‘'’) preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. Next: ANSI-C Quoting, Previous: Single Quotes, Up: Quoting [Contents][Index] 3.1.2.3 Double QuotesEnclosing characters in double quotes (‘"’) preserves the literal value
of all characters within the quotes, with the exception of
‘$’, ‘`’, ‘\’,
and, when history expansion is enabled, ‘!’.
When the shell is in
POSIX mode (see Bash POSIX Mode),
the ‘!’ has no special meaning
within double quotes, even when history expansion is enabled.
The characters ‘$’ and ‘`’
retain their special meaning within double quotes (see Shell Expansions).
The backslash retains its special meaning only when followed by one of
the following characters:
‘$’, ‘`’, ‘"’, ‘\’, or The special parameters ‘*’ and ‘@’ have special meaning when in double quotes (see Shell Parameter Expansion). Next: Locale Translation, Previous: Double Quotes, Up: Quoting [Contents][Index] 3.1.2.4 ANSI-C QuotingWords of the form
The expanded result is single-quoted, as if the dollar sign had not been present. Previous: ANSI-C Quoting, Up: Quoting [Contents][Index] 3.1.2.5 Locale-Specific TranslationA double-quoted string preceded by a dollar sign (‘$’) will cause
the string to be translated according to the current locale.
If the current locale is Some systems use the message catalog selected by the Previous: Quoting, Up: Shell Syntax [Contents][Index] 3.1.3 CommentsIn a non-interactive shell, or an interactive shell in which the
Next: Shell Functions, Previous: Shell Syntax, Up: Basic Shell Features [Contents][Index] 3.2 Shell CommandsA simple shell command such as More complex shell commands are composed of simple commands arranged together in a variety of ways: in a pipeline in which the output of one command becomes the input of a second, in a loop or conditional construct, or in some other grouping.
Next: Pipelines, Up: Shell Commands [Contents][Index] 3.2.1 Simple CommandsA simple command is the kind of command encountered most often.
It’s just a sequence of words separated by The return status (see Exit Status) of a simple command is
its exit status as provided
by the POSIX 1003.1 Next: Lists, Previous: Simple Commands, Up: Shell Commands [Contents][Index] 3.2.2 PipelinesA The format for a pipeline is [time [-p]] [!] command1 [ | or |& command2 ] … The output of each command in the pipeline is connected via a pipe to the input of the next command. That is, each command reads the previous command’s output. This connection is performed before any redirections specified by the command. If ‘|&’ is used, command1’s standard error, in addition to
its standard output, is connected to
command2’s standard input through the pipe;
it is shorthand for The reserved word When the shell is in POSIX mode (see Bash POSIX Mode), If the pipeline is not executed asynchronously (see Lists), the shell waits for all commands in the pipeline to complete. Each command in a pipeline is executed in its own subshell
(see Command Execution Environment). The exit
status of a pipeline is the exit status of the last command in the
pipeline, unless the Next: Compound Commands, Previous: Pipelines, Up: Shell Commands [Contents][Index] 3.2.3 Lists of CommandsA Of these list operators, ‘&&’ and ‘||’ have equal precedence, followed by ‘;’ and ‘&’, which have equal precedence. A sequence of one or more newlines may appear in a If a command is terminated by the control operator ‘&’,
the shell executes the command asynchronously in a subshell.
This is known as executing the command in the background.
The shell does not wait for the command to finish, and the return
status is 0 (true).
When job control is not active (see Job Control),
the standard input for asynchronous commands, in the absence of any
explicit redirections, is redirected from Commands separated by a ‘;’ are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed. AND and OR lists are sequences of one or more pipelines separated by the control operators ‘&&’ and ‘||’, respectively. AND and OR lists are executed with left associativity. An AND list has the form command1 && command2 command2 is executed if, and only if, command1 returns an exit status of zero. An OR list has the form command1 || command2 command2 is executed if, and only if, command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list. Next: Coprocesses, Previous: Lists, Up: Shell Commands [Contents][Index] 3.2.4 Compound Commands
Compound commands are the shell programming constructs. Each construct begins with a reserved word or control operator and is terminated by a corresponding reserved word or operator. Any redirections (see Redirections) associated with a compound command apply to all commands within that compound command unless explicitly overridden. In most cases a list of commands in a compound command’s description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon. Bash provides looping constructs, conditional commands, and mechanisms to group commands and execute them as a unit. Next: Conditional Constructs, Up: Compound Commands [Contents][Index] 3.2.4.1 Looping ConstructsBash supports the following looping constructs. Note that wherever a ‘;’ appears in the description of a command’s syntax, it may be replaced with one or more newlines.
The Next: Command Grouping, Previous: Looping Constructs, Up: Compound Commands [Contents][Index] 3.2.4.2 Conditional Constructs
Previous: Conditional Constructs, Up: Compound Commands [Contents][Index] 3.2.4.3 Grouping CommandsBash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
In addition to the creation of a subshell, there is a subtle difference
between these two constructs due to historical reasons. The braces
are The exit status of both of these constructs is the exit status of list. Next: GNU Parallel, Previous: Compound Commands, Up: Shell Commands [Contents][Index] 3.2.5 CoprocessesA The format for a coprocess is: coproc [NAME] command [redirections] This creates a coprocess named NAME. If NAME is not supplied, the default name is COPROC. NAME must not be supplied if command is a simple command (see Simple Commands); otherwise, it is interpreted as the first word of the simple command. When the coprocess is executed, the shell creates an array variable
(see Arrays)
named The process ID of the shell spawned to execute the coprocess is
available as the value of the variable Since the coprocess is created as an asynchronous command,
the Previous: Coprocesses, Up: Shell Commands [Contents][Index] 3.2.6 GNU ParallelThere are ways to run commands in parallel that are not built into Bash. GNU Parallel is a tool to do just that. GNU Parallel, as its name suggests, can be used to build and run commands
in parallel. You may run the same command with different arguments, whether
they are filenames, usernames, hostnames, or lines read from files. GNU
Parallel provides shorthand references to many of the most common operations
(input lines, various portions of the input line, different ways to specify
the input source, and so on). Parallel can replace For a complete description, refer to the GNU Parallel documentation. A few examples should provide a brief introduction to its use. For example, it is easy to replace find . -type f -name '*.html' -print | parallel gzip If you need to protect special characters such as newlines in file names, use find’s -print0 option and parallel’s -0 option. You can use Parallel to move files from the current directory when the
number of files is too large to process with one ls | parallel mv {} destdir
As you can see, the {} is replaced with each line read from standard input.
While using find . -depth 1 \! -name '.*' -print0 | parallel -0 mv {} destdir
as alluded to above. This will run as many find . -depth 1 \! -name '.*' -print0 | parallel -0 -X mv {} destdir
GNU Parallel can replace certain common idioms that operate on lines read from a file (in this case, filenames listed one per line): while IFS= read -r x; do do-something1 "$x" "config-$x" do-something2 < "$x" done < file | process-output with a more compact syntax reminiscent of lambdas: cat list | parallel "do-something1 {} config-{} ; do-something2 < {}" | process-output
Parallel provides a built-in mechanism to remove filename extensions, which lends itself to batch file transformations or renaming: ls *.gz | parallel -j+0 "zcat {} | bzip2 >{.}.bz2 && rm {}"
This will recompress all files in the current directory with names ending
in .gz using bzip2, running one job per CPU (-j+0) in parallel.
(We use parallel "zcat {} | bzip2 >{.}.bz2 && rm {}" ::: *.gz
If a command generates output, you may want to preserve the input order in the output. For instance, the following command { echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel traceroute
will display as output the traceroute invocation that finishes first. Adding the -k option { echo foss.org.my ; echo debian.org; echo freenetproject.org; } | parallel -k traceroute
will ensure that the output of Finally, Parallel can be used to run a sequence of shell commands in parallel, similar to ‘cat file | bash’. It is not uncommon to take a list of filenames, create a series of shell commands to operate on them, and feed that list of commnds to a shell. Parallel can speed this up. Assuming that file contains a list of shell commands, one per line, parallel -j 10 < file will evaluate the commands using the shell (since no explicit command is supplied as an argument), in blocks of ten shell jobs at a time. Next: Shell Parameters, Previous: Shell Commands, Up: Basic Shell Features [Contents][Index] 3.3 Shell FunctionsShell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them. Functions are declared using this syntax: name () compound-command [ redirections ] or function name [()] compound-command [ redirections ] This defines a shell function named name. The reserved
word A function definition may be deleted using the -f option to the
The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. Note that for historical reasons, in the most common usage the curly braces
that surround the body of the function must be separated from the body by
When a function is executed, the arguments to the
function become the positional parameters
during its execution (see Positional Parameters).
The special parameter ‘#’ that expands to the number of
positional parameters is updated to reflect the change.
Special parameter All other aspects of the shell execution
environment are identical between a function and its caller
with these exceptions:
the The If the builtin command Variables local to the function may be declared with the
Function names and definitions may be listed with the
-f option to the Functions may be recursive.
The Next: Shell Expansions, Previous: Shell Functions, Up: Basic Shell Features [Contents][Index] 3.4 Shell Parameters
A parameter is an entity that stores values.
It can be a A parameter is set if it has been assigned a value. The null string is
a valid value. Once a variable is set, it may be unset only by using
the A variable may be assigned to by a statement of the form name=[value] If value
is not given, the variable is assigned the null string. All
values undergo tilde expansion, parameter and variable expansion,
command substitution, arithmetic expansion, and quote
removal (detailed below). If the variable has its In the context where an assignment statement is assigning a value
to a shell variable or array index (see Arrays), the ‘+=’
operator can be used to
append to or add to the variable’s previous value.
This includes arguments to builtin commands such as A variable can be assigned the nameref attribute using the
-n option to the declare -n ref=$1 inside the function creates a nameref variable ref whose value is
the variable name passed as the first argument.
References and assignments to ref, and changes to its attributes,
are treated as references, assignments, and attribute modifications
to the variable whose name was passed as If the control variable in a Next: Special Parameters, Up: Shell Parameters [Contents][Index] 3.4.1 Positional ParametersA positional parameter is a parameter denoted by one or more
digits, other than the single digit When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces. Previous: Positional Parameters, Up: Shell Parameters [Contents][Index] 3.4.2 Special ParametersThe shell treats several parameters specially. These parameters may only be referenced; assignment to them is not allowed.
Next: Redirections, Previous: Shell Parameters, Up: Basic Shell Features [Contents][Index] 3.5 Shell ExpansionsExpansion is performed on the command line after it has been split into
The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion. On systems that can support it, there is an additional expansion available: process substitution. This is performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution. After these expansions are performed, quote characters present in the original word are removed unless they have been quoted themselves (quote removal). Only brace expansion, word splitting, and filename expansion
can change the number of words of the expansion; other expansions
expand a single word to a single word.
The only exceptions to this are the expansions of
After all expansions, Next: Tilde Expansion, Up: Shell Expansions [Contents][Index] 3.5.1 Brace ExpansionBrace expansion is a mechanism by which arbitrary strings may be generated. This mechanism is similar to filename expansion (see Filename Expansion), but the filenames generated need not exist. Patterns to be brace expanded take the form of an optional preamble, followed by either a series of comma-separated strings or a sequence expression between a pair of braces, followed by an optional postscript. The preamble is prefixed to each string contained within the braces, and the postscript is then appended to each resulting string, expanding left to right. Brace expansions may be nested. The results of each expanded string are not sorted; left to right order is preserved. For example, bash$ echo a{d,c,b}e
ade ace abe
A sequence expression takes the form Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or the text between the braces. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion. A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. A { or ‘,’ may be quoted with a backslash to prevent its being considered part of a brace expression. To avoid conflicts with parameter expansion, the string ‘${’ is not considered eligible for brace expansion. This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the above example: mkdir /usr/local/src/bash/{old,new,dist,bugs}
or chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}
Next: Shell Parameter Expansion, Previous: Brace Expansion, Up: Shell Expansions [Contents][Index] 3.5.2 Tilde ExpansionIf a word begins with an unquoted tilde character (‘~’), all of the
characters up to the first unquoted slash (or all characters,
if there is no unquoted slash) are considered a tilde-prefix.
If none of the characters in the tilde-prefix are quoted, the
characters in the tilde-prefix following the tilde are treated as a
possible login name.
If this login name is the null string, the tilde is replaced with the
value of the If the tilde-prefix is ‘~+’, the value of
the shell variable If the characters following the tilde in the tilde-prefix consist of a
number N, optionally prefixed by a ‘+’ or a ‘-’,
the tilde-prefix is replaced with the
corresponding element from the directory stack, as it would be displayed
by the If the login name is invalid, or the tilde expansion fails, the word is left unchanged. Each variable assignment is checked for unquoted tilde-prefixes immediately
following a ‘:’ or the first ‘=’.
In these cases, tilde expansion is also performed.
Consequently, one may use filenames with tildes in assignments to
The following table shows how Bash treats unquoted tilde-prefixes:
Next: Command Substitution, Previous: Tilde Expansion, Up: Shell Expansions [Contents][Index] 3.5.3 Shell Parameter ExpansionThe ‘$’ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name. When braces are used, the matching ending brace is the first ‘}’ not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion. The basic form of parameter expansion is ${parameter}. The value of parameter is substituted. The parameter is a shell parameter as described above (see Shell Parameters) or an array reference (see Arrays). The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that is not to be interpreted as part of its name. If the first character of parameter is an exclamation point (!),
and parameter is not a nameref,
it introduces a level of variable indirection.
Bash uses the value of the variable formed from the rest of
parameter as the name of the variable; this variable is then
expanded and that value is used in the rest of the substitution, rather
than the value of parameter itself.
This is known as In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. When not performing substring expansion, using the form described below (e.g., ‘:-’), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter’s existence and that its value is not null; if the colon is omitted, the operator tests only for existence.
Next: Arithmetic Expansion, Previous: Shell Parameter Expansion, Up: Shell Expansions [Contents][Index] 3.5.4 Command SubstitutionCommand substitution allows the output of a command to replace the command itself. Command substitution occurs when a command is enclosed as follows: $(command) or `command` Bash performs the expansion by executing command in a subshell environment
and replacing the command substitution with the standard output of the
command, with any trailing newlines deleted.
Embedded newlines are not deleted, but they may be removed during
word splitting.
The command substitution When the old-style backquote form of substitution is used,
backslash retains its literal meaning except when followed by
‘$’, ‘`’, or ‘\’.
The first backquote not preceded by a backslash terminates the
command substitution.
When using the Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes. If the substitution appears within double quotes, word splitting and filename expansion are not performed on the results. Next: Process Substitution, Previous: Command Substitution, Up: Shell Expansions [Contents][Index] 3.5.5 Arithmetic ExpansionArithmetic expansion allows the evaluation of an arithmetic expression and the substitution of the result. The format for arithmetic expansion is: $(( expression )) The expression is treated as if it were within double quotes, but a double quote inside the parentheses is not treated specially. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Arithmetic expansions may be nested. The evaluation is performed according to the rules listed below (see Shell Arithmetic). If the expression is invalid, Bash prints a message indicating failure to the standard error and no substitution occurs. Next: Word Splitting, Previous: Arithmetic Expansion, Up: Shell Expansions [Contents][Index] 3.5.6 Process SubstitutionProcess substitution allows a process’s input or output to be referred to using a filename. It takes the form of <(list) or >(list) The process list is run asynchronously, and its input or output
appears as a filename.
This filename is
passed as an argument to the current command as the result of the
expansion.
If the When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion. Next: Filename Expansion, Previous: Process Substitution, Up: Shell Expansions [Contents][Index] 3.5.7 Word SplittingThe shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. The shell treats each character of Explicit null arguments ( Note that if no expansion occurs, no splitting is performed. Next: Quote Removal, Previous: Word Splitting, Up: Shell Expansions [Contents][Index] 3.5.8 Filename Expansion
After word splitting, unless the -f option has been set
(see The Set Builtin), Bash scans each word for the characters
‘*’, ‘?’, and ‘[’.
If one of these characters appears, then the word is
regarded as a pattern,
and replaced with an alphabetically sorted list of
filenames matching the pattern (see Pattern Matching).
If no matching filenames are found,
and the shell option When a pattern is used for filename expansion, the character ‘.’
at the start of a filename or immediately following a slash
must be matched explicitly, unless the shell option See the description of The Up: Filename Expansion [Contents][Index] 3.5.8.1 Pattern MatchingAny character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally. The special pattern characters have the following meanings:
If the
Previous: Filename Expansion, Up: Shell Expansions [Contents][Index] 3.5.9 Quote RemovalAfter the preceding expansions, all unquoted occurrences of the characters ‘\’, ‘'’, and ‘"’ that did not result from one of the above expansions are removed. Next: Executing Commands, Previous: Shell Expansions, Up: Basic Shell Features [Contents][Index] 3.6 RedirectionsBefore a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection allows commands’ file handles to be duplicated, opened, closed, made to refer to different files, and can change the files the command reads from and writes to. Redirection may also be used to modify file handles in the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right. Each redirection that may be preceded by a file descriptor number may instead be preceded by a word of the form {varname}. In this case, for each redirection operator except >&- and <&-, the shell will allocate a file descriptor greater than 10 and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of varname defines the file descriptor to close. In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is ‘<’, the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is ‘>’, the redirection refers to the standard output (file descriptor 1). The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error. Note that the order of redirections is significant. For example, the command ls > dirlist 2>&1 directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command ls 2>&1 > dirlist directs only the standard output to file dirlist, because the standard error was made a copy of the standard output before the standard output was redirected to dirlist. Bash handles several filenames specially when they are used in redirections, as described in the following table. If the operating system on which Bash is running provides these special files, bash will use them; otherwise it will emulate them internally with the behavior described below.
A failure to open or create a file causes the redirection to fail. Redirections using file descriptors greater than 9 should be used with care, as they may conflict with file descriptors the shell uses internally. 3.6.1 Redirecting InputRedirection of input causes the file whose name results from
the expansion of word
to be opened for reading on file descriptor The general format for redirecting input is: [n]<word 3.6.2 Redirecting OutputRedirection of output causes the file whose name results from the expansion of word to be opened for writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created; if it does exist it is truncated to zero size. The general format for redirecting output is: [n]>[|]word If the redirection operator is ‘>’, and the 3.6.3 Appending Redirected OutputRedirection of output in this fashion causes the file whose name results from the expansion of word to be opened for appending on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the file does not exist it is created. The general format for appending output is: [n]>>word 3.6.4 Redirecting Standard Output and Standard ErrorThis construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word. There are two formats for redirecting standard output and standard error: &>word and >&word Of the two forms, the first is preferred. This is semantically equivalent to >word 2>&1 When using the second form, word may not expand to a number or ‘-’. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons. 3.6.5 Appending Standard Output and Standard ErrorThis construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be appended to the file whose name is the expansion of word. The format for appending standard output and standard error is: &>>word This is semantically equivalent to >>word 2>&1 (see Duplicating File Descriptors below). 3.6.6 Here DocumentsThis type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input (or file descriptor n if n is specified) for a command. The format of here-documents is: [n]<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution,
arithmetic expansion, or filename expansion is performed on
word. If any part of word is quoted, the
delimiter is the result of quote removal on word,
and the lines in the here-document are not expanded.
If word is unquoted,
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence If the redirection operator is ‘<<-’, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion. 3.6.7 Here StringsA variant of here documents, the format is: [n]<<< word The word undergoes brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. Pathname expansion and word splitting are not performed. The result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified). 3.6.8 Duplicating File DescriptorsThe redirection operator [n]<&word is used to duplicate input file descriptors. If word expands to one or more digits, the file descriptor denoted by n is made to be a copy of that file descriptor. If the digits in word do not specify a file descriptor open for input, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed. If n is not specified, the standard input (file descriptor 0) is used. The operator [n]>&word is used similarly to duplicate output file descriptors. If n is not specified, the standard output (file descriptor 1) is used. If the digits in word do not specify a file descriptor open for output, a redirection error occurs. If word evaluates to ‘-’, file descriptor n is closed. As a special case, if n is omitted, and word does not expand to one or more digits or ‘-’, the standard output and standard error are redirected as described previously. 3.6.9 Moving File DescriptorsThe redirection operator [n]<&digit- moves the file descriptor digit to file descriptor n, or the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n. Similarly, the redirection operator [n]>&digit- moves the file descriptor digit to file descriptor n, or the standard output (file descriptor 1) if n is not specified. 3.6.10 Opening File Descriptors for Reading and WritingThe redirection operator [n]<>word causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created. Next: Shell Scripts, Previous: Redirections, Up: Basic Shell Features [Contents][Index] 3.7 Executing Commands
Next: Command Search and Execution, Up: Executing Commands [Contents][Index] 3.7.1 Simple Command ExpansionWhen a simple command is executed, the shell performs the following expansions, assignments, and redirections, from left to right.
If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status. If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status. If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero. Next: Command Execution Environment, Previous: Simple Command Expansion, Up: Executing Commands [Contents][Index] 3.7.2 Command Search and ExecutionAfter a command has been split into words, if it results in a simple command and an optional list of arguments, the following actions are taken.
Next: Environment, Previous: Command Search and Execution, Up: Executing Commands [Contents][Index] 3.7.3 Command Execution EnvironmentThe shell has an execution environment, which consists of the following:
When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.
A command invoked in this separate environment cannot affect the shell’s execution environment. Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell’s execution environment. Subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When not in POSIX mode, Bash clears the -e option in such subshells. If a command is followed by a ‘&’ and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections. Next: Exit Status, Previous: Command Execution Environment, Up: Executing Commands [Contents][Index] 3.7.4 EnvironmentWhen a program is invoked it is given an array of strings
called the environment.
This is a list of name-value pairs, of the form Bash provides several ways to manipulate the environment.
On invocation, the shell scans its own environment and
creates a parameter for each name found, automatically marking
it for export
to child processes. Executed commands inherit the environment.
The The environment for any simple command or function may be augmented temporarily by prefixing it with parameter assignments, as described in Shell Parameters. These assignment statements affect only the environment seen by that command. If the -k option is set (see The Set Builtin), then all parameter assignments are placed in the environment for a command, not just those that precede the command name. When Bash invokes an external command, the variable ‘$_’ is set to the full pathname of the command and passed to that command in its environment. Next: Signals, Previous: Environment, Up: Executing Commands [Contents][Index] 3.7.5 Exit StatusThe exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes. For the shell’s purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status. If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126. If a command fails because of an error during expansion or redirection, the exit status is greater than zero. The exit status is used by the Bash conditional commands (see Conditional Constructs) and some of the list constructs (see Lists). All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage, generally invalid options or missing arguments. Previous: Exit Status, Up: Executing Commands [Contents][Index] 3.7.6 SignalsWhen Bash is interactive, in the absence of any traps, it ignores
Non-builtin commands started by Bash have signal handlers set to the
values inherited by the shell from its parent.
When job control is not in effect, asynchronous commands
ignore The shell exits by default upon receipt of a If the If Bash is waiting for a command to complete and receives a signal
for which a trap has been set, the trap will not be executed until
the command completes.
When Bash is waiting for an asynchronous
command via the Previous: Executing Commands, Up: Basic Shell Features [Contents][Index] 3.8 Shell ScriptsA shell script is a text file containing shell commands. When such
a file is used as the first non-option argument when invoking Bash,
and neither the -c nor -s option is supplied
(see Invoking Bash),
Bash reads and executes commands from the file, then exits. This
mode of operation creates a non-interactive shell. The shell first
searches for the file in the current directory, and looks in the
directories in When Bash runs
a shell script, it sets the special parameter A shell script may be made executable by using the filename arguments is equivalent to executing bash filename arguments if Most versions of Unix make this a part of the operating system’s command
execution mechanism. If the first line of a script begins with
the two characters ‘#!’, the remainder of the line specifies
an interpreter for the program.
Thus, you can specify Bash, The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the script file, followed by the name of the script file, followed by the rest of the arguments. Bash will perform this action on operating systems that do not handle it themselves. Note that some older versions of Unix limit the interpreter name and argument to a maximum of 32 characters. Bash scripts often begin with Next: Shell Variables, Previous: Basic Shell Features, Up: Top [Contents][Index] 4 Shell Builtin Commands
Builtin commands are contained within the shell itself. When the name of a builtin command is used as the first word of a simple command (see Simple Commands), the shell executes the command directly, without invoking another program. Builtin commands are necessary to implement functionality impossible or inconvenient to obtain with separate utilities. This section briefly describes the builtins which Bash inherits from the Bourne Shell, as well as the builtin commands which are unique to or have been extended in Bash. Several builtin commands are described in other chapters: builtin commands which provide the Bash interface to the job control facilities (see Job Control Builtins), the directory stack (see Directory Stack Builtins), the command history (see Bash History Builtins), and the programmable completion facilities (see Programmable Completion Builtins). Many of the builtins have been extended by POSIX or Bash. Unless otherwise noted, each builtin command documented as accepting
options preceded by ‘-’ accepts ‘--’
to signify the end of the options.
The Next: Bash Builtins, Up: Shell Builtin Commands [Contents][Index] 4.1 Bourne Shell BuiltinsThe following shell builtin commands are inherited from the Bourne Shell. These commands are implemented as specified by the POSIX standard.
Next: Modifying Shell Behavior, Previous: Bourne Shell Builtins, Up: Shell Builtin Commands [Contents][Index] 4.2 Bash Builtin CommandsThis section describes builtin commands which are unique to or have been extended in Bash. Some of these commands are specified in the POSIX standard.
Next: Special Builtins, Previous: Bash Builtins, Up: Shell Builtin Commands [Contents][Index] 4.3 Modifying Shell Behavior
Next: The Shopt Builtin, Up: Modifying Shell Behavior [Contents][Index] 4.3.1 The Set BuiltinThis builtin is so complicated that it deserves its own section.
Previous: The Set Builtin, Up: Modifying Shell Behavior [Contents][Index] 4.3.2 The Shopt BuiltinThis builtin allows you to change additional shell optional behavior.
Previous: Modifying Shell Behavior, Up: Shell Builtin Commands [Contents][Index] 4.4 Special BuiltinsFor historical reasons, the POSIX standard has classified several builtin commands as special. When Bash is executing in POSIX mode, the special builtins differ from other builtin commands in three respects:
When Bash is not executing in POSIX mode, these builtins behave no differently than the rest of the Bash builtin commands. The Bash POSIX mode is described in Bash POSIX Mode. These are the POSIX special builtins: break : . continue eval exec exit export readonly return set shift trap unset Next: Bash Features, Previous: Shell Builtin Commands, Up: Top [Contents][Index] 5 Shell Variables
This chapter describes the shell variables that Bash uses. Bash automatically assigns default values to a number of variables. Next: Bash Variables, Up: Shell Variables [Contents][Index] 5.1 Bourne Shell VariablesBash uses certain shell variables in the same way as the Bourne shell. In some cases, Bash assigns a default value to the variable.
Previous: Bourne Shell Variables, Up: Shell Variables [Contents][Index] 5.2 Bash VariablesThese variables are set or used by Bash, but other shells do not normally treat them specially. A few variables used by Bash are described in different chapters: variables for controlling the job control facilities (see Job Control Variables).
Next: Job Control, Previous: Shell Variables, Up: Top [Contents][Index] 6 Bash FeaturesThis chapter describes features unique to Bash.
Next: Bash Startup Files, Up: Bash Features [Contents][Index] 6.1 Invoking Bashbash [long-opt] [-ir] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument …] bash [long-opt] [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] -c string [argument …] bash [long-opt] -s [-abefhkmnptuvxdBCDHP] [-o option] [-O shopt_option] [argument …] All of the single-character options used with the
There are several single-character options that may be supplied at
invocation which are not available with the
A login shell is one whose first character of argument zero is ‘-’, or one invoked with the --login option. An interactive shell is one started without non-option arguments,
unless -s is specified,
without specifying the -c option, and whose input and output are both
connected to terminals (as determined by If arguments remain after option processing, and neither the
-c nor the -s
option has been supplied, the first argument is assumed to
be the name of a file containing shell commands (see Shell Scripts).
When Bash is invoked in this fashion, Next: Interactive Shells, Previous: Invoking Bash, Up: Bash Features [Contents][Index] 6.2 Bash Startup FilesThis section describes how Bash executes its startup files. If any of the files exist but cannot be read, Bash reports an error. Tildes are expanded in filenames as described above under Tilde Expansion (see Tilde Expansion). Interactive shells are described in Interactive Shells. Invoked as an interactive login shell, or with --loginWhen Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior. When an interactive login shell exits,
or a non-interactive login shell executes the Invoked as an interactive non-login shellWhen an interactive shell that is not a login shell is started, Bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force Bash to read and execute commands from file instead of ~/.bashrc. So, typically, your ~/.bash_profile contains the line after (or before) any login-specific initializations. Invoked non-interactivelyWhen Bash is started non-interactively, to run a shell script,
for example, it looks for the variable but the value of the As noted above, if a non-interactive shell is invoked with the --login option, Bash attempts to read and execute commands from the login shell startup files. Invoked with name
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| • What is an Interactive Shell?: | What determines whether a shell is Interactive. | |
| • Is this Shell Interactive?: | How to tell if a shell is interactive. | |
| • Interactive Shell Behavior: | What changes in a interactive shell? |
Next: Is this Shell Interactive?, Up: Interactive Shells [Contents][Index]
6.3.1 What is an Interactive Shell?
An interactive shell
is one started without non-option arguments, unless -s is
specified, without specifying the -c option, and
whose input and error output are both
connected to terminals (as determined by isatty(3)),
or one started with the -i option.
An interactive shell generally reads from and writes to a user’s terminal.
The -s invocation option may be used to set the positional parameters when an interactive shell is started.
Next: Interactive Shell Behavior, Previous: What is an Interactive Shell?, Up: Interactive Shells [Contents][Index]
6.3.2 Is this Shell Interactive?
To determine within a startup script whether or not Bash is
running interactively,
test the value of the ‘-’ special parameter.
It contains i when the shell is interactive. For example:
case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac
Alternatively, startup scripts may examine the variable
PS1; it is unset in non-interactive shells, and set in
interactive shells. Thus:
if [ -z "$PS1" ]; then
echo This shell is not interactive
else
echo This shell is interactive
fi
Previous: Is this Shell Interactive?, Up: Interactive Shells [Contents][Index]
6.3.3 Interactive Shell Behavior
When the shell is running interactively, it changes its behavior in several ways.
- Startup files are read and executed as described in Bash Startup Files.
- Job Control (see Job Control) is enabled by default. When job
control is in effect, Bash ignores the keyboard-generated job control
signals
SIGTTIN,SIGTTOU, andSIGTSTP. - Bash expands and displays
PS1before reading the first line of a command, and expands and displaysPS2before reading the second and subsequent lines of a multi-line command. Bash displaysPS0after it reads a command but before executing it. - Bash executes the value of the
PROMPT_COMMANDvariable as a command before printing the primary prompt,$PS1(see Bash Variables). - Readline (see Command Line Editing) is used to read commands from the user’s terminal.
- Bash inspects the value of the
ignoreeofoption toset -oinstead of exiting immediately when it receives anEOFon its standard input when reading a command (see The Set Builtin). - Command history (see Bash History Facilities)
and history expansion (see History Interaction)
are enabled by default.
Bash will save the command history to the file named by
$HISTFILEwhen a shell with history enabled exits. - Alias expansion (see Aliases) is performed by default.
- In the absence of any traps, Bash ignores
SIGTERM(see Signals). - In the absence of any traps,
SIGINTis caught and handled ((see Signals).SIGINTwill interrupt some shell builtins. - An interactive login shell sends a
SIGHUPto all jobs on exit if thehuponexitshell option has been enabled (see Signals). - The -n invocation option is ignored, and ‘set -n’ has no effect (see The Set Builtin).
- Bash will check for mail periodically, depending on the values of the
MAIL,MAILPATH, andMAILCHECKshell variables (see Bash Variables). - Expansion errors due to references to unbound shell variables after ‘set -u’ has been enabled will not cause the shell to exit (see The Set Builtin).
- The shell will not exit on expansion errors caused by var being unset
or null in
${var:?word}expansions (see Shell Parameter Expansion). - Redirection errors encountered by shell builtins will not cause the shell to exit.
- When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see Bash POSIX Mode).
- A failed
execwill not cause the shell to exit (see Bourne Shell Builtins). - Parser syntax errors will not cause the shell to exit.
- Simple spelling correction for directory arguments to the
cdbuiltin is enabled by default (see the description of thecdspelloption to theshoptbuiltin in The Shopt Builtin). - The shell will check the value of the
TMOUTvariable and exit if a command is not read within the specified number of seconds after printing$PS1(see Bash Variables).
Next: Shell Arithmetic, Previous: Interactive Shells, Up: Bash Features [Contents][Index]
6.4 Bash Conditional Expressions
Conditional expressions are used by the [[ compound command
and the test and [ builtin commands.
Expressions may be unary or binary. Unary expressions are often used to examine the status of a file. There are string operators and numeric comparison operators as well. Bash handles several filenames specially when they are used in expressions. If the operating system on which Bash is running provides these special files, Bash will use them; otherwise it will emulate them internally with this behavior: If the file argument to one of the primaries is of the form /dev/fd/N, then file descriptor N is checked. If the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2, respectively, is checked.
When used with [[, the ‘<’ and ‘>’ operators sort
lexicographically using the current locale.
The test command uses ASCII ordering.
Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself.
-a fileTrue if file exists.
-b fileTrue if file exists and is a block special file.
-c fileTrue if file exists and is a character special file.
-d fileTrue if file exists and is a directory.
-e fileTrue if file exists.
-f fileTrue if file exists and is a regular file.
-g fileTrue if file exists and its set-group-id bit is set.
-h fileTrue if file exists and is a symbolic link.
-k fileTrue if file exists and its "sticky" bit is set.
-p fileTrue if file exists and is a named pipe (FIFO).
-r fileTrue if file exists and is readable.
-s fileTrue if file exists and has a size greater than zero.
-t fdTrue if file descriptor fd is open and refers to a terminal.
-u fileTrue if file exists and its set-user-id bit is set.
-w fileTrue if file exists and is writable.
-x fileTrue if file exists and is executable.
-G fileTrue if file exists and is owned by the effective group id.
-L fileTrue if file exists and is a symbolic link.
-N fileTrue if file exists and has been modified since it was last read.
-O fileTrue if file exists and is owned by the effective user id.
-S fileTrue if file exists and is a socket.
file1 -ef file2True if file1 and file2 refer to the same device and inode numbers.
file1 -nt file2True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2True if file1 is older than file2, or if file2 exists and file1 does not.
-o optnameTrue if the shell option optname is enabled. The list of options appears in the description of the -o option to the
setbuiltin (see The Set Builtin).-v varnameTrue if the shell variable varname is set (has been assigned a value).
-R varnameTrue if the shell variable varname is set and is a name reference.
-z stringTrue if the length of string is zero.
-n stringstringTrue if the length of string is non-zero.
string1 == string2string1 = string2True if the strings are equal. When used with the
[[command, this performs pattern matching as described above (see Conditional Constructs).‘=’ should be used with the
testcommand for POSIX conformance.string1 != string2True if the strings are not equal.
string1 < string2True if string1 sorts before string2 lexicographically.
string1 > string2True if string1 sorts after string2 lexicographically.
arg1 OP arg2OPis one of ‘-eq’, ‘-ne’, ‘-lt’, ‘-le’, ‘-gt’, or ‘-ge’. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. Arg1 and arg2 may be positive or negative integers.
Next: Aliases, Previous: Bash Conditional Expressions, Up: Bash Features [Contents][Index]
6.5 Shell Arithmetic
The shell allows arithmetic expressions to be evaluated, as one of
the shell expansions or by using the (( compound command, the
let builtin, or the -i option to the declare builtin.
Evaluation is done in fixed-width integers with no check for overflow, though division by 0 is trapped and flagged as an error. The operators and their precedence, associativity, and values are the same as in the C language. The following list of operators is grouped into levels of equal-precedence operators. The levels are listed in order of decreasing precedence.
id++ id--variable post-increment and post-decrement
++id --idvariable pre-increment and pre-decrement
- +unary minus and plus
! ~logical and bitwise negation
**exponentiation
* / %multiplication, division, remainder
+ -addition, subtraction
<< >>left and right bitwise shifts
<= >= < >comparison
== !=equality and inequality
&bitwise AND
^bitwise exclusive OR
|bitwise OR
&&logical AND
||logical OR
expr ? expr : exprconditional operator
= *= /= %= += -= <<= >>= &= ^= |=assignment
expr1 , expr2comma
Shell variables are allowed as operands; parameter expansion is performed before the expression is evaluated. Within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax. The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using ‘declare -i’ is assigned a value. A null value evaluates to 0. A shell variable need not have its integer attribute turned on to be used in an expression.
Constants with a leading 0 are interpreted as octal numbers.
A leading ‘0x’ or ‘0X’ denotes hexadecimal. Otherwise,
numbers take the form [base#]n, where the optional base
is a decimal number between 2 and 64 representing the arithmetic
base, and n is a number in that base.
If base# is omitted, then base 10 is used.
When specifying n,
the digits greater than 9 are represented by the lowercase letters,
the uppercase letters, ‘@’, and ‘_’, in that order.
If base is less than or equal to 36, lowercase and uppercase
letters may be used interchangeably to represent numbers between 10
and 35.
Operators are evaluated in order of precedence. Sub-expressions in parentheses are evaluated first and may override the precedence rules above.
Next: Arrays, Previous: Shell Arithmetic, Up: Bash Features [Contents][Index]
6.6 Aliases
Aliases allow a string to be substituted for a word when it is used
as the first word of a simple command.
The shell maintains a list of aliases that may be set and unset with
the alias and unalias builtin commands.
The first word of each simple command, if unquoted, is checked to see
if it has an alias.
If so, that word is replaced by the text of the alias.
The characters ‘/’, ‘$’, ‘`’, ‘=’ and any of the
shell metacharacters or quoting characters listed above may not appear
in an alias name.
The replacement text may contain any valid
shell input, including shell metacharacters.
The first word of the replacement text is tested for
aliases, but a word that is identical to an alias being expanded
is not expanded a second time.
This means that one may alias ls to "ls -F",
for instance, and Bash does not try to recursively expand the
replacement text.
If the last character of the alias value is a
blank, then the next command word following the
alias is also checked for alias expansion.
Aliases are created and listed with the alias
command, and removed with the unalias command.
There is no mechanism for using arguments in the replacement text,
as in csh.
If arguments are needed, a shell function should be used
(see Shell Functions).
Aliases are not expanded when the shell is not interactive,
unless the expand_aliases shell option is set using
shopt (see The Shopt Builtin).
The rules concerning the definition and use of aliases are
somewhat confusing. Bash
always reads at least one complete line
of input before executing any
of the commands on that line. Aliases are expanded when a
command is read, not when it is executed. Therefore, an
alias definition appearing on the same line as another
command does not take effect until the next line of input is read.
The commands following the alias definition
on that line are not affected by the new alias.
This behavior is also an issue when functions are executed.
Aliases are expanded when a function definition is read,
not when the function is executed, because a function definition
is itself a command. As a consequence, aliases
defined in a function are not available until after that
function is executed. To be safe, always put
alias definitions on a separate line, and do not use alias
in compound commands.
For almost every purpose, shell functions are preferred over aliases.
Next: The Directory Stack, Previous: Aliases, Up: Bash Features [Contents][Index]
6.7 Arrays
Bash provides one-dimensional indexed and associative array variables.
Any variable may be used as an indexed array;
the declare builtin will explicitly declare an array.
There is no maximum
limit on the size of an array, nor any requirement that members
be indexed or assigned contiguously.
Indexed arrays are referenced using integers (including arithmetic
expressions (see Shell Arithmetic)) and are zero-based;
associative arrays use arbitrary strings.
Unless otherwise noted, indexed array indices must be non-negative integers.
An indexed array is created automatically if any variable is assigned to using the syntax
name[subscript]=value
The subscript is treated as an arithmetic expression that must evaluate to a number. To explicitly declare an array, use
declare -a name
The syntax
declare -a name[subscript]
is also accepted; the subscript is ignored.
Associative arrays are created using
declare -A name.
Attributes may be
specified for an array variable using the declare and
readonly builtins. Each attribute applies to all members of
an array.
Arrays are assigned to using compound assignments of the form
name=(value1 value2 … )
where each
value is of the form [subscript]=string.
Indexed array assignments do not require anything but string.
When assigning to indexed arrays, if
the optional subscript is supplied, that index is assigned to;
otherwise the index of the element assigned is the last index assigned
to by the statement plus one. Indexing starts at zero.
When assigning to an associative array, the subscript is required.
This syntax is also accepted by the declare
builtin. Individual array elements may be assigned to using the
name[subscript]=value syntax introduced above.
When assigning to an indexed array, if name is subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of name, so negative indices count back from the end of the array, and an index of -1 references the last element.
Any element of an array may be referenced using
${name[subscript]}.
The braces are required to avoid
conflicts with the shell’s filename expansion operators. If the
subscript is ‘@’ or ‘*’, the word expands to all members
of the array name. These subscripts differ only when the word
appears within double quotes.
If the word is double-quoted,
${name[*]} expands to a single word with
the value of each array member separated by the first character of the
IFS variable, and ${name[@]} expands each element of
name to a separate word. When there are no array members,
${name[@]} expands to nothing.
If the double-quoted expansion occurs within a word, the expansion of
the first parameter is joined with the beginning part of the original
word, and the expansion of the last parameter is joined with the last
part of the original word.
This is analogous to the
expansion of the special parameters ‘@’ and ‘*’.
${#name[subscript]} expands to the length of
${name[subscript]}.
If subscript is ‘@’ or
‘*’, the expansion is the number of elements in the array.
If the subscript
used to reference an element of an indexed array
evaluates to a number less than zero, it is
interpreted as relative to one greater than the maximum index of the array,
so negative indices count back from the end of the array,
and an index of -1 refers to the last element.
Referencing an array variable without a subscript is equivalent to
referencing with a subscript of 0.
Any reference to a variable using a valid subscript is legal, and
bash will create an array if necessary.
An array variable is considered set if a subscript has been assigned a value. The null string is a valid value.
It is possible to obtain the keys (indices) of an array as well as the values. ${!name[@]} and ${!name[*]} expand to the indices assigned in array variable name. The treatment when in double quotes is similar to the expansion of the special parameters ‘@’ and ‘*’ within double quotes.
The unset builtin is used to destroy arrays.
unset name[subscript]
destroys the array element at index subscript.
Negative subscripts to indexed arrays are interpreted as described above.
Care must be taken to avoid unwanted side effects caused by filename
expansion.
unset name, where name is an array, removes the
entire array. A subscript of ‘*’ or ‘@’ also removes the
entire array.
The declare, local, and readonly
builtins each accept a -a option to specify an indexed
array and a -A option to specify an associative array.
If both options are supplied, -A takes precedence.
The read builtin accepts a -a
option to assign a list of words read from the standard input
to an array, and can read values from the standard input into
individual array elements. The set and declare
builtins display array values in a way that allows them to be
reused as input.
Next: Controlling the Prompt, Previous: Arrays, Up: Bash Features [Contents][Index]
6.8 The Directory Stack
| • Directory Stack Builtins: | Bash builtin commands to manipulate the directory stack. |
The directory stack is a list of recently-visited directories. The
pushd builtin adds directories to the stack as it changes
the current directory, and the popd builtin removes specified
directories from the stack and changes the current directory to
the directory removed. The dirs builtin displays the contents
of the directory stack. The current directory is always the "top"
of the directory stack.
The contents of the directory stack are also visible
as the value of the DIRSTACK shell variable.
Up: The Directory Stack [Contents][Index]
6.8.1 Directory Stack Builtins
dirs-
dirs [-clpv] [+N | -N]
Display the list of currently remembered directories. Directories are added to the list with the
pushdcommand; thepopdcommand removes directories from the list. The current directory is always the first directory in the stack.-cClears the directory stack by deleting all of the elements.
-lProduces a listing using full pathnames; the default listing format uses a tilde to denote the home directory.
-pCauses
dirsto print the directory stack with one entry per line.-vCauses
dirsto print the directory stack with one entry per line, prefixing each entry with its index in the stack.+NDisplays the Nth directory (counting from the left of the list printed by
dirswhen invoked without options), starting with zero.-NDisplays the Nth directory (counting from the right of the list printed by
dirswhen invoked without options), starting with zero.
popd-
popd [-n] [+N | -N]
When no arguments are given,
popdremoves the top directory from the stack and performs acdto the new top directory. The elements are numbered from 0 starting at the first directory listed withdirs; that is,popdis equivalent topopd +0.-nSuppresses the normal change of directory when removing directories from the stack, so that only the stack is manipulated.
+NRemoves the Nth directory (counting from the left of the list printed by
dirs), starting with zero.-NRemoves the Nth directory (counting from the right of the list printed by
dirs), starting with zero.
pushdpushd [-n] [+N | -N | dir]
Save the current directory on the top of the directory stack and then
cdto dir. With no arguments,pushdexchanges the top two directories and makes the new top the current directory.-nSuppresses the normal change of directory when rotating or adding directories to the stack, so that only the stack is manipulated.
+NBrings the Nth directory (counting from the left of the list printed by
dirs, starting with zero) to the top of the list by rotating the stack.-NBrings the Nth directory (counting from the right of the list printed by
dirs, starting with zero) to the top of the list by rotating the stack.dirMakes dir be the top of the stack, making it the new current directory as if it had been supplied as an argument to the
cdbuiltin.
Next: The Restricted Shell, Previous: The Directory Stack, Up: Bash Features [Contents][Index]
6.9 Controlling the Prompt
The value of the variable PROMPT_COMMAND is examined just before
Bash prints each primary prompt. If PROMPT_COMMAND is set and
has a non-null value, then the
value is executed just as if it had been typed on the command line.
In addition, the following table describes the special characters which
can appear in the prompt variables PS1 to PS4:
\aA bell character.
\dThe date, in "Weekday Month Date" format (e.g., "Tue May 26").
\D{format}The format is passed to
strftime(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required.\eAn escape character.
\hThe hostname, up to the first ‘.’.
\HThe hostname.
\jThe number of jobs currently managed by the shell.
\lThe basename of the shell’s terminal device name.
\nA newline.
\rA carriage return.
\sThe name of the shell, the basename of
$0(the portion following the final slash).\tThe time, in 24-hour HH:MM:SS format.
\TThe time, in 12-hour HH:MM:SS format.
\@The time, in 12-hour am/pm format.
\AThe time, in 24-hour HH:MM format.
\uThe username of the current user.
\vThe version of Bash (e.g., 2.00)
\VThe release of Bash, version + patchlevel (e.g., 2.00.0)
\wThe current working directory, with
$HOMEabbreviated with a tilde (uses the$PROMPT_DIRTRIMvariable).\WThe basename of
$PWD, with$HOMEabbreviated with a tilde.\!The history number of this command.
\#The command number of this command.
\$If the effective uid is 0,
#, otherwise$.\nnnThe character whose ASCII code is the octal value nnn.
\\A backslash.
\[Begin a sequence of non-printing characters. This could be used to embed a terminal control sequence into the prompt.
\]End a sequence of non-printing characters.
The command number and the history number are usually different: the history number of a command is its position in the history list, which may include commands restored from the history file (see Bash History Facilities), while the command number is the position in the sequence of commands executed during the current shell session.
After the string is decoded, it is expanded via
parameter expansion, command substitution, arithmetic
expansion, and quote removal, subject to the value of the
promptvars shell option (see Bash Builtins).
Next: Bash POSIX Mode, Previous: Controlling the Prompt, Up: Bash Features [Contents][Index]
6.10 The Restricted Shell
If Bash is started with the name rbash, or the
--restricted
or
-r
option is supplied at invocation, the shell becomes restricted.
A restricted shell is used to
set up an environment more controlled than the standard shell.
A restricted shell behaves identically to bash
with the exception that the following are disallowed or not performed:
- Changing directories with the
cdbuiltin. - Setting or unsetting the values of the
SHELL,PATH,ENV, orBASH_ENVvariables. - Specifying command names containing slashes.
- Specifying a filename containing a slash as an argument to the
.builtin command. - Specifying a filename containing a slash as an argument to the -p
option to the
hashbuiltin command. - Importing function definitions from the shell environment at startup.
- Parsing the value of
SHELLOPTSfrom the shell environment at startup. - Redirecting output using the ‘>’, ‘>|’, ‘<>’, ‘>&’, ‘&>’, and ‘>>’ redirection operators.
- Using the
execbuiltin to replace the shell with another command. - Adding or deleting builtin commands with the
-f and -d options to the
enablebuiltin. - Using the
enablebuiltin command to enable disabled shell builtins. - Specifying the -p option to the
commandbuiltin. - Turning off restricted mode with ‘set +r’ or ‘set +o restricted’.
These restrictions are enforced after any startup files are read.
When a command that is found to be a shell script is executed
(see Shell Scripts), rbash turns off any restrictions in
the shell spawned to execute the script.
Previous: The Restricted Shell, Up: Bash Features [Contents][Index]
6.11 Bash POSIX Mode
Starting Bash with the --posix command-line option or executing ‘set -o posix’ while Bash is running will cause Bash to conform more closely to the POSIX standard by changing the behavior to match that specified by POSIX in areas where the Bash default differs.
When invoked as sh, Bash enters POSIX mode after reading the
startup files.
The following list is what’s changed when ‘POSIX mode’ is in effect:
- When a command in the hash table no longer exists, Bash will re-search
$PATHto find the new location. This is also available with ‘shopt -s checkhash’. - The message printed by the job control code and builtins when a job exits with a non-zero status is ‘Done(status)’.
- The message printed by the job control code and builtins when a job
is stopped is ‘Stopped(signame)’, where signame is, for
example,
SIGTSTP. - Alias expansion is always enabled, even in non-interactive shells.
- Reserved words appearing in a context where reserved words are recognized do not undergo alias expansion.
- The POSIX
PS1andPS2expansions of ‘!’ to the history number and ‘!!’ to ‘!’ are enabled, and parameter expansion is performed on the values ofPS1andPS2regardless of the setting of thepromptvarsoption. - The POSIX startup files are executed (
$ENV) rather than the normal Bash files. - Tilde expansion is only performed on assignments preceding a command name, rather than on all assignment statements on the line.
- The default history file is ~/.sh_history (this is the
default value of
$HISTFILE). - Redirection operators do not perform filename expansion on the word in the redirection unless the shell is interactive.
- Redirection operators do not perform word splitting on the word in the redirection.
- Function names must be valid shell
names. That is, they may not contain characters other than letters, digits, and underscores, and may not start with a digit. Declaring a function with an invalid name causes a fatal syntax error in non-interactive shells. - Function names may not be the same as one of the POSIX special builtins.
- POSIX special builtins are found before shell functions during command lookup.
- When printing shell function definitions (e.g., by
type), Bash does not print thefunctionkeyword. - Literal tildes that appear as the first character in elements of
the
PATHvariable are not expanded as described above under Tilde Expansion. - The
timereserved word may be used by itself as a command. When used in this way, it displays timing statistics for the shell and its completed children. TheTIMEFORMATvariable controls the format of the timing information. - When parsing and expanding a ${…} expansion that appears within double quotes, single quotes are no longer special and cannot be used to quote a closing brace or other special character, unless the operator is one of those defined to perform pattern removal. In this case, they do not have to appear as matched pairs.
- The parser does not recognize
timeas a reserved word if the next token begins with a ‘-’. - The ‘!’ character does not introduce history expansion within a
double-quoted string, even if the
histexpandoption is enabled. - If a POSIX special builtin returns an error status, a non-interactive shell exits. The fatal errors are those listed in the POSIX standard, and include things like passing incorrect options, redirection errors, variable assignment errors for assignments preceding the command name, and so on.
- A non-interactive shell exits with an error status if a variable assignment error occurs when no command name follows the assignment statements. A variable assignment error occurs, for example, when trying to assign a value to a readonly variable.
- A non-interactive shell exits with an error status if a variable assignment error occurs in an assignment statement preceding a special builtin, but not with any other simple command.
- A non-interactive shell exits with an error status if the iteration
variable in a
forstatement or the selection variable in aselectstatement is a readonly variable. - Non-interactive shells exit if filename in
.filename is not found. - Non-interactive shells exit if a syntax error in an arithmetic expansion results in an invalid expression.
- Non-interactive shells exit if a parameter expansion error occurs.
- Non-interactive shells exit if there is a syntax error in a script read
with the
.orsourcebuiltins, or in a string processed by theevalbuiltin. - Process substitution is not available.
- While variable indirection is available, it may not be applied to the ‘#’ and ‘?’ special parameters.
- When expanding the ‘*’ special parameter in a pattern context where the
expansion is double-quoted does not treat the
$*as if it were double-quoted. - Assignment statements preceding POSIX special builtins persist in the shell environment after the builtin completes.
- Assignment statements preceding shell function calls persist in the shell environment after the function returns, as if a POSIX special builtin command had been executed.
- The
commandbuiltin does not prevent builtins that take assignment statements as arguments from expanding them as assignment statements; when not in POSIX mode, assignment builtins lose their assignment statement expansion properties when preceded bycommand. - The
bgbuiltin uses the required format to describe each job placed in the background, which does not include an indication of whether the job is the current or previous job. - The output of ‘kill -l’ prints all the signal names on a single line, separated by spaces, without the ‘SIG’ prefix.
- The
killbuiltin does not accept signal names with a ‘SIG’ prefix. - The
exportandreadonlybuiltin commands display their output in the format required by POSIX. - The
trapbuiltin displays signal names without the leadingSIG. - The
trapbuiltin doesn’t check the first argument for a possible signal specification and revert the signal handling to the original disposition if it is, unless that argument consists solely of digits and is a valid signal number. If users want to reset the handler for a given signal to the original disposition, they should use ‘-’ as the first argument. - The
.andsourcebuiltins do not search the current directory for the filename argument if it is not found by searchingPATH. - Enabling POSIX mode has the effect of setting the
inherit_errexitoption, so subshells spawned to execute command substitutions inherit the value of the -e option from the parent shell. When theinherit_errexitoption is not enabled, Bash clears the -e option in such subshells. - When the
aliasbuiltin displays alias definitions, it does not display them with a leading ‘alias ’ unless the -p option is supplied. - When the
setbuiltin is invoked without options, it does not display shell function names and definitions. - When the
setbuiltin is invoked without options, it displays variable values without quotes, unless they contain shell metacharacters, even if the result contains nonprinting characters. - When the
cdbuiltin is invoked in logical mode, and the pathname constructed from$PWDand the directory name supplied as an argument does not refer to an existing directory,cdwill fail instead of falling back to physical mode. - The
pwdbuiltin verifies that the value it prints is the same as the current directory, even if it is not asked to check the file system with the -P option. - When listing the history, the
fcbuiltin does not include an indication of whether or not a history entry has been modified. - The default editor used by
fcised. - The
typeandcommandbuiltins will not report a non-executable file as having been found, though the shell will attempt to execute such a file if it is the only so-named file found in$PATH. - The
viediting mode will invoke thevieditor directly when the ‘v’ command is run, instead of checking$VISUALand$EDITOR. - When the
xpg_echooption is enabled, Bash does not attempt to interpret any arguments toechoas options. Each argument is displayed, after escape characters are converted. - The
ulimitbuiltin uses a block size of 512 bytes for the -c and -f options. - The arrival of
SIGCHLDwhen a trap is set onSIGCHLDdoes not interrupt thewaitbuiltin and cause it to return immediately. The trap command is run once for each child that exits. - The
readbuiltin may be interrupted by a signal for which a trap has been set. If Bash receives a trapped signal while executingread, the trap handler executes andreadreturns an exit status greater than 128. - Bash removes an exited background process’s status from the list of such
statuses after the
waitbuiltin is used to obtain it.
There is other POSIX behavior that Bash does not implement by default even when in POSIX mode. Specifically:
- The
fcbuiltin checks$EDITORas a program to edit history entries ifFCEDITis unset, rather than defaulting directly toed.fcusesedifEDITORis unset. - As noted above, Bash requires the
xpg_echooption to be enabled for theechobuiltin to be fully conformant.
Bash can be configured to be POSIX-conformant by default, by specifying
the --enable-strict-posix-default to configure when building
(see Optional Features).
Next: Command Line Editing, Previous: Bash Features, Up: Top [Contents][Index]
7 Job Control
This chapter discusses what job control is, how it works, and how Bash allows you to access its facilities.
| • Job Control Basics: | How job control works. | |
| • Job Control Builtins: | Bash builtin commands used to interact with job control. | |
| • Job Control Variables: | Variables Bash uses to customize job control. |
Next: Job Control Builtins, Up: Job Control [Contents][Index]
7.1 Job Control Basics
Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the operating system kernel’s terminal driver and Bash.
The shell associates a job with each pipeline. It keeps a
table of currently executing jobs, which may be listed with the
jobs command. When Bash starts a job
asynchronously, it prints a line that looks
like:
[1] 25647
indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.
To facilitate the implementation of the user interface to job
control, the operating system maintains the notion of a current terminal
process group ID. Members of this process group (processes whose
process group ID is equal to the current terminal process group
ID) receive keyboard-generated signals such as SIGINT.
These processes are said to be in the foreground. Background
processes are those whose process group ID differs from the
terminal’s; such processes are immune to keyboard-generated
signals. Only foreground processes are allowed to read from or, if
the user so specifies with stty tostop, write to the terminal.
Background processes which attempt to
read from (write to when stty tostop is in effect) the
terminal are sent a SIGTTIN (SIGTTOU)
signal by the kernel’s terminal driver,
which, unless caught, suspends the process.
If the operating system on which Bash is running supports
job control, Bash contains facilities to use it. Typing the
suspend character (typically ‘^Z’, Control-Z) while a
process is running causes that process to be stopped and returns
control to Bash. Typing the delayed suspend character
(typically ‘^Y’, Control-Y) causes the process to be stopped
when it attempts to read input from the terminal, and control to
be returned to Bash. The user then manipulates the state of
this job, using the bg command to continue it in the
background, the fg command to continue it in the
foreground, or the kill command to kill it. A ‘^Z’
takes effect immediately, and has the additional side effect of
causing pending output and typeahead to be discarded.
There are a number of ways to refer to a job in the shell. The character ‘%’ introduces a job specification (jobspec).
Job number n may be referred to as ‘%n’.
The symbols ‘%%’ and ‘%+’ refer to the shell’s notion of the
current job, which is the last job stopped while it was in the foreground
or started in the background.
A single ‘%’ (with no accompanying job specification) also refers
to the current job.
The previous job may be referenced using ‘%-’.
If there is only a single job, ‘%+’ and ‘%-’ can both be used
to refer to that job.
In output pertaining to jobs (e.g., the output of the jobs
command), the current job is always flagged with a ‘+’, and the
previous job with a ‘-’.
A job may also be referred to
using a prefix of the name used to start it, or using a substring
that appears in its command line. For example, ‘%ce’ refers
to a stopped ce job. Using ‘%?ce’, on the
other hand, refers to any job containing the string ‘ce’ in
its command line. If the prefix or substring matches more than one job,
Bash reports an error.
Simply naming a job can be used to bring it into the foreground: ‘%1’ is a synonym for ‘fg %1’, bringing job 1 from the background into the foreground. Similarly, ‘%1 &’ resumes job 1 in the background, equivalent to ‘bg %1’
The shell learns immediately whenever a job changes state.
Normally, Bash waits until it is about to print a prompt
before reporting changes in a job’s status so as to not interrupt
any other output.
If the -b option to the set builtin is enabled,
Bash reports such changes immediately (see The Set Builtin).
Any trap on SIGCHLD is executed for each child process
that exits.
If an attempt to exit Bash is made while jobs are stopped, (or running, if
the checkjobs option is enabled – see The Shopt Builtin), the
shell prints a warning message, and if the checkjobs option is
enabled, lists the jobs and their statuses.
The jobs command may then be used to inspect their status.
If a second attempt to exit is made without an intervening command,
Bash does not print another warning, and any stopped jobs are terminated.
Next: Job Control Variables, Previous: Job Control Basics, Up: Job Control [Contents][Index]
7.2 Job Control Builtins
bg-
bg [jobspec …]
Resume each suspended job jobspec in the background, as if it had been started with ‘&’. If jobspec is not supplied, the current job is used. The return status is zero unless it is run when job control is not enabled, or, when run with job control enabled, any jobspec was not found or specifies a job that was started without job control.
fg-
fg [jobspec]
Resume the job jobspec in the foreground and make it the current job. If jobspec is not supplied, the current job is used. The return status is that of the command placed into the foreground, or non-zero if run when job control is disabled or, when run with job control enabled, jobspec does not specify a valid job or jobspec specifies a job that was started without job control.
jobs-
jobs [-lnprs] [jobspec] jobs -x command [arguments]
The first form lists the active jobs. The options have the following meanings:
-lList process IDs in addition to the normal information.
-nDisplay information only about jobs that have changed status since the user was last notified of their status.
-pList only the process ID of the job’s process group leader.
-rDisplay only running jobs.
-sDisplay only stopped jobs.
If jobspec is given, output is restricted to information about that job. If jobspec is not supplied, the status of all jobs is listed.
If the -x option is supplied,
jobsreplaces any jobspec found in command or arguments with the corresponding process group ID, and executes command, passing it arguments, returning its exit status. kill-
kill [-s sigspec] [-n signum] [-sigspec] jobspec or pid kill -l|-L [exit_status]
Send a signal specified by sigspec or signum to the process named by job specification jobspec or process ID pid. sigspec is either a case-insensitive signal name such as
SIGINT(with or without theSIGprefix) or a signal number; signum is a signal number. If sigspec and signum are not present,SIGTERMis used. The -l option lists the signal names. If any arguments are supplied when -l is given, the names of the signals corresponding to the arguments are listed, and the return status is zero. exit_status is a number specifying a signal number or the exit status of a process terminated by a signal. The -L option is equivalent to -l. The return status is zero if at least one signal was successfully sent, or non-zero if an error occurs or an invalid option is encountered. wait-
wait [-n] [jobspec or pid …]
Wait until the child process specified by each process ID pid or job specification jobspec exits and return the exit status of the last command waited for. If a job spec is given, all processes in the job are waited for. If no arguments are given, all currently active child processes are waited for, and the return status is zero. If the -n option is supplied,
waitwaits for any job to terminate and returns its exit status. If neither jobspec nor pid specifies an active child process of the shell, the return status is 127. disown-
disown [-ar] [-h] [jobspec … | pid … ]
Without options, remove each jobspec from the table of active jobs. If the -h option is given, the job is not removed from the table, but is marked so that
SIGHUPis not sent to the job if the shell receives aSIGHUP. If jobspec is not present, and neither the -a nor the -r option is supplied, the current job is used. If no jobspec is supplied, the -a option means to remove or mark all jobs; the -r option without a jobspec argument restricts operation to running jobs. suspend-
suspend [-f]
Suspend the execution of this shell until it receives a
SIGCONTsignal. A login shell cannot be suspended; the -f option can be used to override this and force the suspension.
When job control is not active, the kill and wait
builtins do not accept jobspec arguments. They must be
supplied process IDs.
Previous: Job Control Builtins, Up: Job Control [Contents][Index]
7.3 Job Control Variables
auto_resumeThis variable controls how the shell interacts with the user and job control. If this variable exists then single word simple commands without redirections are treated as candidates for resumption of an existing job. There is no ambiguity allowed; if there is more than one job beginning with the string typed, then the most recently accessed job will be selected. The name of a stopped job, in this context, is the command line used to start it. If this variable is set to the value ‘exact’, the string supplied must match the name of a stopped job exactly; if set to ‘substring’, the string supplied needs to match a substring of the name of a stopped job. The ‘substring’ value provides functionality analogous to the ‘%?’ job ID (see Job Control Basics). If set to any other value, the supplied string must be a prefix of a stopped job’s name; this provides functionality analogous to the ‘%’ job ID.
Next: Using History Interactively, Previous: Job Control, Up: Top [Contents][Index]
8 Command Line Editing
This chapter describes the basic features of the GNU
command line editing interface.
Command line editing is provided by the Readline library, which is
used by several different programs, including Bash.
Command line editing is enabled by default when using an interactive shell,
unless the --noediting option is supplied at shell invocation.
Line editing is also used when using the -e option to the
read builtin command (see Bash Builtins).
By default, the line editing commands are similar to those of Emacs.
A vi-style line editing interface is also available.
Line editing can be enabled at any time using the -o emacs or
-o vi options to the set builtin command
(see The Set Builtin), or disabled using the +o emacs or
+o vi options to set.
| • Introduction and Notation: | Notation used in this text. | |
| • Readline Interaction: | The minimum set of commands for editing a line. | |
| • Readline Init File: | Customizing Readline from a user’s view. | |
| • Bindable Readline Commands: | A description of most of the Readline commands available for binding | |
| • Readline vi Mode: | A short description of how to make Readline behave like the vi editor. | |
| • Programmable Completion: | How to specify the possible completions for a specific command. | |
| • Programmable Completion Builtins: | Builtin commands to specify how to complete arguments for a particular command. | |
| • A Programmable Completion Example: | An example shell function for generating possible completions. |
Next: Readline Interaction, Up: Command Line Editing [Contents][Index]
8.1 Introduction to Line Editing
The following paragraphs describe the notation used to represent keystrokes.
The text C-k is read as ‘Control-K’ and describes the character produced when the k key is pressed while the Control key is depressed.
The text M-k is read as ‘Meta-K’ and describes the character produced when the Meta key (if you have one) is depressed, and the k key is pressed. The Meta key is labeled ALT on many keyboards. On keyboards with two keys labeled ALT (usually to either side of the space bar), the ALT on the left side is generally set to work as a Meta key. The ALT key on the right may also be configured to work as a Meta key or may be configured as some other modifier, such as a Compose key for typing accented characters.
If you do not have a Meta or ALT key, or another key working as a Meta key, the identical keystroke can be generated by typing ESC first, and then typing k. Either process is known as metafying the k key.
The text M-C-k is read as ‘Meta-Control-k’ and describes the character produced by metafying C-k.
In addition, several keys have their own names. Specifically, DEL, ESC, LFD, SPC, RET, and TAB all stand for themselves when seen in this text, or in an init file (see Readline Init File). If your keyboard lacks a LFD key, typing C-j will produce the desired character. The RET key may be labeled Return or Enter on some keyboards.
Next: Readline Init File, Previous: Introduction and Notation, Up: Command Line Editing [Contents][Index]
8.2 Readline Interaction
Often during an interactive session you type in a long line of text, only to notice that the first word on the line is misspelled. The Readline library gives you a set of commands for manipulating the text as you type it in, allowing you to just fix your typo, and not forcing you to retype the majority of the line. Using these editing commands, you move the cursor to the place that needs correction, and delete or insert the text of the corrections. Then, when you are satisfied with the line, you simply press RET. You do not have to be at the end of the line to press RET; the entire line is accepted regardless of the location of the cursor within the line.
| • Readline Bare Essentials: | The least you need to know about Readline. | |
| • Readline Movement Commands: | Moving about the input line. | |
| • Readline Killing Commands: | How to delete text, and how to get it back! | |
| • Readline Arguments: | Giving numeric arguments to commands. | |
| • Searching: | Searching through previous lines. |
Next: Readline Movement Commands, Up: Readline Interaction [Contents][Index]
8.2.1 Readline Bare Essentials
In order to enter characters into the line, simply type them. The typed character appears where the cursor was, and then the cursor moves one space to the right. If you mistype a character, you can use your erase character to back up and delete the mistyped character.
Sometimes you may mistype a character, and not notice the error until you have typed several other characters. In that case, you can type C-b to move the cursor to the left, and then correct your mistake. Afterwards, you can move the cursor to the right with C-f.
When you add text in the middle of a line, you will notice that characters to the right of the cursor are ‘pushed over’ to make room for the text that you have inserted. Likewise, when you delete text behind the cursor, characters to the right of the cursor are ‘pulled back’ to fill in the blank space created by the removal of the text. A list of the bare essentials for editing the text of an input line follows.
- C-b
Move back one character.
- C-f
Move forward one character.
- DEL or Backspace
Delete the character to the left of the cursor.
- C-d
Delete the character underneath the cursor.
- Printing characters
Insert the character into the line at the cursor.
- C-_ or C-x C-u
Undo the last editing command. You can undo all the way back to an empty line.
(Depending on your configuration, the Backspace key be set to delete the character to the left of the cursor and the DEL key set to delete the character underneath the cursor, like C-d, rather than the character to the left of the cursor.)
Next: Readline Killing Commands, Previous: Readline Bare Essentials, Up: Readline Interaction [Contents][Index]
8.2.2 Readline Movement Commands
The above table describes the most basic keystrokes that you need in order to do editing of the input line. For your convenience, many other commands have been added in addition to C-b, C-f, C-d, and DEL. Here are some commands for moving more rapidly about the line.
- C-a
Move to the start of the line.
- C-e
Move to the end of the line.
- M-f
Move forward a word, where a word is composed of letters and digits.
- M-b
Move backward a word.
- C-l
Clear the screen, reprinting the current line at the top.
Notice how C-f moves forward a character, while M-f moves forward a word. It is a loose convention that control keystrokes operate on characters while meta keystrokes operate on words.
Next: Readline Arguments, Previous: Readline Movement Commands, Up: Readline Interaction [Contents][Index]
8.2.3 Readline Killing Commands
Killing text means to delete the text from the line, but to save it away for later use, usually by yanking (re-inserting) it back into the line. (‘Cut’ and ‘paste’ are more recent jargon for ‘kill’ and ‘yank’.)
If the description for a command says that it ‘kills’ text, then you can be sure that you can get the text back in a different (or the same) place later.
When you use a kill command, the text is saved in a kill-ring. Any number of consecutive kills save all of the killed text together, so that when you yank it back, you get it all. The kill ring is not line specific; the text that you killed on a previously typed line is available to be yanked back later, when you are typing another line.
Here is the list of commands for killing text.
- C-k
Kill the text from the current cursor position to the end of the line.
- M-d
Kill from the cursor to the end of the current word, or, if between words, to the end of the next word. Word boundaries are the same as those used by M-f.
- M-DEL
Kill from the cursor the start of the current word, or, if between words, to the start of the previous word. Word boundaries are the same as those used by M-b.
- C-w
Kill from the cursor to the previous whitespace. This is different than M-DEL because the word boundaries differ.
Here is how to yank the text back into the line. Yanking means to copy the most-recently-killed text from the kill buffer.
- C-y
Yank the most recently killed text back into the buffer at the cursor.
- M-y
Rotate the kill-ring, and yank the new top. You can only do this if the prior command is C-y or M-y.
Next: Searching, Previous: Readline Killing Commands, Up: Readline Interaction [Contents][Index]
8.2.4 Readline Arguments
You can pass numeric arguments to Readline commands. Sometimes the argument acts as a repeat count, other times it is the sign of the argument that is significant. If you pass a negative argument to a command which normally acts in a forward direction, that command will act in a backward direction. For example, to kill text back to the start of the line, you might type ‘M-- C-k’.
The general way to pass numeric arguments to a command is to type meta digits before the command. If the first ‘digit’ typed is a minus sign (‘-’), then the sign of the argument will be negative. Once you have typed one meta digit to get the argument started, you can type the remainder of the digits, and then the command. For example, to give the C-d command an argument of 10, you could type ‘M-1 0 C-d’, which will delete the next ten characters on the input line.
Previous: Readline Arguments, Up: Readline Interaction [Contents][Index]
8.2.5 Searching for Commands in the History
Readline provides commands for searching through the command history (see Bash History Facilities) for lines containing a specified string. There are two search modes: incremental and non-incremental.
Incremental searches begin before the user has finished typing the
search string.
As each character of the search string is typed, Readline displays
the next entry from the history matching the string typed so far.
An incremental search requires only as many characters as needed to
find the desired history entry.
To search backward in the history for a particular string, type
C-r. Typing C-s searches forward through the history.
The characters present in the value of the isearch-terminators variable
are used to terminate an incremental search.
If that variable has not been assigned a value, the ESC and
C-J characters will terminate an incremental search.
C-g will abort an incremental search and restore the original line.
When the search is terminated, the history entry containing the
search string becomes the current line.
To find other matching entries in the history list, type C-r or C-s as appropriate. This will search backward or forward in the history for the next entry matching the search string typed so far. Any other key sequence bound to a Readline command will terminate the search and execute that command. For instance, a RET will terminate the search and accept the line, thereby executing the command from the history list. A movement command will terminate the search, make the last line found the current line, and begin editing.
Readline remembers the last incremental search string. If two C-rs are typed without any intervening characters defining a new search string, any remembered search string is used.
Non-incremental searches read the entire search string before starting to search for matching history lines. The search string may be typed by the user or be part of the contents of the current line.
Next: Bindable Readline Commands, Previous: Readline Interaction, Up: Command Line Editing [Contents][Index]
8.3 Readline Init File
Although the Readline library comes with a set of Emacs-like
keybindings installed by default, it is possible to use a different set
of keybindings.
Any user can customize programs that use Readline by putting
commands in an inputrc file, conventionally in his home directory.
The name of this
file is taken from the value of the shell variable INPUTRC. If
that variable is unset, the default is ~/.inputrc. If that
file does not exist or cannot be read, the ultimate default is
/etc/inputrc.
When a program which uses the Readline library starts up, the init file is read, and the key bindings are set.
In addition, the C-x C-r command re-reads this init file, thus
incorporating any changes that you might have made to it.
| • Readline Init File Syntax: | Syntax for the commands in the inputrc file. | |
| • Conditional Init Constructs: | Conditional key bindings in the inputrc file. | |
| • Sample Init File: | An example inputrc file. | |
Next: Conditional Init Constructs, Up: Readline Init File [Contents][Index]
8.3.1 Readline Init File Syntax
There are only a few basic constructs allowed in the Readline init file. Blank lines are ignored. Lines beginning with a ‘#’ are comments. Lines beginning with a ‘$’ indicate conditional constructs (see Conditional Init Constructs). Other lines denote variable settings and key bindings.
- Variable Settings
You can modify the run-time behavior of Readline by altering the values of variables in Readline using the
setcommand within the init file. The syntax is simple:set variable value
Here, for example, is how to change from the default Emacs-like key binding to use
viline editing commands:set editing-mode vi
Variable names and values, where appropriate, are recognized without regard to case. Unrecognized variable names are ignored.
Boolean variables (those that can be set to on or off) are set to on if the value is null or empty, on (case-insensitive), or 1. Any other value results in the variable being set to off.
The
bind -Vcommand lists the current Readline variable names and values. See Bash Builtins.A great deal of run-time behavior is changeable with the following variables.
bell-style-
Controls what happens when Readline wants to ring the terminal bell. If set to ‘none’, Readline never rings the bell. If set to ‘visible’, Readline uses a visible bell if one is available. If set to ‘audible’ (the default), Readline attempts to ring the terminal’s bell.
bind-tty-special-chars-
If set to ‘on’ (the default), Readline attempts to bind the control characters treated specially by the kernel’s terminal driver to their Readline equivalents.
blink-matching-paren-
If set to ‘on’, Readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted. The default is ‘off’.
colored-completion-prefix-
If set to ‘on’, when listing completions, Readline displays the common prefix of the set of possible completions using a different color. The color definitions are taken from the value of the
LS_COLORSenvironment variable. The default is ‘off’. colored-stats-
If set to ‘on’, Readline displays possible completions using different colors to indicate their file type. The color definitions are taken from the value of the
LS_COLORSenvironment variable. The default is ‘off’. comment-begin-
The string to insert at the beginning of the line when the
insert-commentcommand is executed. The default value is"#". completion-display-width-
The number of screen columns used to display possible matches when performing completion. The value is ignored if it is less than 0 or greater than the terminal screen width. A value of 0 will cause matches to be displayed one per line. The default value is -1.
completion-ignore-case-
If set to ‘on’, Readline performs filename matching and completion in a case-insensitive fashion. The default value is ‘off’.
completion-map-case-
If set to ‘on’, and completion-ignore-case is enabled, Readline treats hyphens (‘-’) and underscores (‘_’) as equivalent when performing case-insensitive filename matching and completion.
completion-prefix-display-length-
The length in characters of the common prefix of a list of possible completions that is displayed without modification. When set to a value greater than zero, common prefixes longer than this value are replaced with an ellipsis when displaying possible completions.
completion-query-items-
The number of possible completions that determines when the user is asked whether the list of possibilities should be displayed. If the number of possible completions is greater than this value, Readline will ask the user whether or not he wishes to view them; otherwise, they are simply listed. This variable must be set to an integer value greater than or equal to 0. A negative value means Readline should never ask. The default limit is
100. convert-meta-
If set to ‘on’, Readline will convert characters with the eighth bit set to an ASCII key sequence by stripping the eighth bit and prefixing an ESC character, converting them to a meta-prefixed key sequence. The default value is ‘on’, but will be set to ‘off’ if the locale is one that contains eight-bit characters.
disable-completion-
If set to ‘On’, Readline will inhibit word completion. Completion characters will be inserted into the line as if they had been mapped to
self-insert. The default is ‘off’. echo-control-characters-
When set to ‘on’, on operating systems that indicate they support it, readline echoes a character corresponding to a signal generated from the keyboard. The default is ‘on’.
editing-mode-
The
editing-modevariable controls which default set of key bindings is used. By default, Readline starts up in Emacs editing mode, where the keystrokes are most similar to Emacs. This variable can be set to either ‘emacs’ or ‘vi’. emacs-mode-string-
This string is displayed immediately before the last line of the primary prompt when emacs editing mode is active. The value is expanded like a key binding, so the standard set of meta- and control prefixes and backslash escape sequences is available. Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘@’.
enable-bracketed-paste-
When set to ‘On’, Readline will configure the terminal in a way that will enable it to insert each paste into the editing buffer as a single string of characters, instead of treating each character as if it had been read from the keyboard. This can prevent pasted characters from being interpreted as editing commands. The default is ‘off’.
enable-keypad-
When set to ‘on’, Readline will try to enable the application keypad when it is called. Some systems need this to enable the arrow keys. The default is ‘off’.
enable-meta-keyWhen set to ‘on’, Readline will try to enable any meta modifier key the terminal claims to support when it is called. On many terminals, the meta key is used to send eight-bit characters. The default is ‘on’.
expand-tilde-
If set to ‘on’, tilde expansion is performed when Readline attempts word completion. The default is ‘off’.
history-preserve-point-
If set to ‘on’, the history code attempts to place the point (the current cursor position) at the same location on each history line retrieved with
previous-historyornext-history. The default is ‘off’. history-size-
Set the maximum number of history entries saved in the history list. If set to zero, any existing history entries are deleted and no new entries are saved. If set to a value less than zero, the number of history entries is not limited. By default, the number of history entries is not limited. If an attempt is made to set history-size to a non-numeric value, the maximum number of history entries will be set to 500.
horizontal-scroll-mode-
This variable can be set to either ‘on’ or ‘off’. Setting it to ‘on’ means that the text of the lines being edited will scroll horizontally on a single screen line when they are longer than the width of the screen, instead of wrapping onto a new screen line. By default, this variable is set to ‘off’.
input-meta-
If set to ‘on’, Readline will enable eight-bit input (it will not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default value is ‘off’, but Readline will set it to ‘on’ if the locale contains eight-bit characters. The name
meta-flagis a synonym for this variable. isearch-terminators-
The string of characters that should terminate an incremental search without subsequently executing the character as a command (see Searching). If this variable has not been given a value, the characters ESC and C-J will terminate an incremental search.
keymap-
Sets Readline’s idea of the current keymap for key binding commands. Acceptable
keymapnames areemacs,emacs-standard,emacs-meta,emacs-ctlx,vi,vi-move,vi-command, andvi-insert.viis equivalent tovi-command(vi-moveis also a synonym);emacsis equivalent toemacs-standard. The default value isemacs. The value of theediting-modevariable also affects the default keymap. keyseq-timeoutSpecifies the duration Readline will wait for a character when reading an ambiguous key sequence (one that can form a complete key sequence using the input read so far, or can take additional input to complete a longer key sequence). If no input is received within the timeout, Readline will use the shorter but complete key sequence. Readline uses this value to determine whether or not input is available on the current input source (
rl_instreamby default). The value is specified in milliseconds, so a value of 1000 means that Readline will wait one second for additional input. If this variable is set to a value less than or equal to zero, or to a non-numeric value, Readline will wait until another key is pressed to decide which key sequence to complete. The default value is500.mark-directoriesIf set to ‘on’, completed directory names have a slash appended. The default is ‘on’.
mark-modified-lines-
This variable, when set to ‘on’, causes Readline to display an asterisk (‘*’) at the start of history lines which have been modified. This variable is ‘off’ by default.
mark-symlinked-directories-
If set to ‘on’, completed names which are symbolic links to directories have a slash appended (subject to the value of
mark-directories). The default is ‘off’. match-hidden-files-
This variable, when set to ‘on’, causes Readline to match files whose names begin with a ‘.’ (hidden files) when performing filename completion. If set to ‘off’, the leading ‘.’ must be supplied by the user in the filename to be completed. This variable is ‘on’ by default.
menu-complete-display-prefix-
If set to ‘on’, menu completion displays the common prefix of the list of possible completions (which may be empty) before cycling through the list. The default is ‘off’.
output-meta-
If set to ‘on’, Readline will display characters with the eighth bit set directly rather than as a meta-prefixed escape sequence. The default is ‘off’, but Readline will set it to ‘on’ if the locale contains eight-bit characters.
page-completions-
If set to ‘on’, Readline uses an internal
more-like pager to display a screenful of possible completions at a time. This variable is ‘on’ by default. print-completions-horizontallyIf set to ‘on’, Readline will display completions with matches sorted horizontally in alphabetical order, rather than down the screen. The default is ‘off’.
revert-all-at-newline-
If set to ‘on’, Readline will undo all changes to history lines before returning when
accept-lineis executed. By default, history lines may be modified and retain individual undo lists across calls toreadline. The default is ‘off’. show-all-if-ambiguous-
This alters the default behavior of the completion functions. If set to ‘on’, words which have more than one possible completion cause the matches to be listed immediately instead of ringing the bell. The default value is ‘off’.
show-all-if-unmodified-
This alters the default behavior of the completion functions in a fashion similar to show-all-if-ambiguous. If set to ‘on’, words which have more than one possible completion without any possible partial completion (the possible completions don’t share a common prefix) cause the matches to be listed immediately instead of ringing the bell. The default value is ‘off’.
show-mode-in-prompt-
If set to ‘on’, add a character to the beginning of the prompt indicating the editing mode: emacs, vi command, or vi insertion. The mode strings are user-settable. The default value is ‘off’.
skip-completed-text-
If set to ‘on’, this alters the default completion behavior when inserting a single match into the line. It’s only active when performing completion in the middle of a word. If enabled, readline does not insert characters from the completion that match characters after point in the word being completed, so portions of the word following the cursor are not duplicated. For instance, if this is enabled, attempting completion when the cursor is after the ‘e’ in ‘Makefile’ will result in ‘Makefile’ rather than ‘Makefilefile’, assuming there is a single possible completion. The default value is ‘off’.
vi-cmd-mode-string-
This string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in command mode. The value is expanded like a key binding, so the standard set of meta- and control prefixes and backslash escape sequences is available. Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘(cmd)’.
vi-ins-mode-string-
This string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in insertion mode. The value is expanded like a key binding, so the standard set of meta- and control prefixes and backslash escape sequences is available. Use the ‘\1’ and ‘\2’ escapes to begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. The default is ‘(ins)’.
visible-stats-
If set to ‘on’, a character denoting a file’s type is appended to the filename when listing possible completions. The default is ‘off’.
- Key Bindings
The syntax for controlling key bindings in the init file is simple. First you need to find the name of the command that you want to change. The following sections contain tables of the command name, the default keybinding, if any, and a short description of what the command does.
Once you know the name of the command, simply place on a line in the init file the name of the key you wish to bind the command to, a colon, and then the name of the command. There can be no space between the key name and the colon – that will be interpreted as part of the key name. The name of the key can be expressed in different ways, depending on what you find most comfortable.
In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro).
The
bind -pcommand displays Readline function names and bindings in a format that can put directly into an initialization file. See Bash Builtins.- keyname: function-name or macro
keyname is the name of a key spelled out in English. For example:
Control-u: universal-argument Meta-Rubout: backward-kill-word Control-o: "> output"
In the above example, C-u is bound to the function
universal-argument, M-DEL is bound to the functionbackward-kill-word, and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text ‘> output’ into the line).A number of symbolic character names are recognized while processing this key binding syntax: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT, SPACE, SPC, and TAB.
- "keyseq": function-name or macro
keyseq differs from keyname above in that strings denoting an entire key sequence can be specified, by placing the key sequence in double quotes. Some GNU Emacs style key escapes can be used, as in the following example, but the special character names are not recognized.
"\C-u": universal-argument "\C-x\C-r": re-read-init-file "\e[11~": "Function Key 1"
In the above example, C-u is again bound to the function
universal-argument(just as it was in the first example), ‘C-x C-r’ is bound to the functionre-read-init-file, and ‘ESC [ 1 1 ~’ is bound to insert the text ‘Function Key 1’.
The following GNU Emacs style escape sequences are available when specifying key sequences:
\C-control prefix
\M-meta prefix
\ean escape character
\\backslash
\"", a double quotation mark
\'', a single quote or apostrophe
In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:
\aalert (bell)
\bbackspace
\ddelete
\fform feed
\nnewline
\rcarriage return
\thorizontal tab
\vvertical tab
\nnnthe eight-bit character whose value is the octal value nnn (one to three digits)
\xHHthe eight-bit character whose value is the hexadecimal value HH (one or two hex digits)
When entering the text of a macro, single or double quotes must be used to indicate a macro definition. Unquoted text is assumed to be a function name. In the macro body, the backslash escapes described above are expanded. Backslash will quote any other character in the macro text, including ‘"’ and ‘'’. For example, the following binding will make ‘C-x \’ insert a single ‘\’ into the line:
"\C-x\\": "\\"
Next: Sample Init File, Previous: Readline Init File Syntax, Up: Readline Init File [Contents][Index]
8.3.2 Conditional Init Constructs
Readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor which allows key bindings and variable settings to be performed as the result of tests. There are four parser directives used.
$ifThe
$ifconstruct allows bindings to be made based on the editing mode, the terminal being used, or the application using Readline. The text of the test extends to the end of the line; no characters are required to isolate it.modeThe
mode=form of the$ifdirective is used to test whether Readline is inemacsorvimode. This may be used in conjunction with the ‘set keymap’ command, for instance, to set bindings in theemacs-standardandemacs-ctlxkeymaps only if Readline is starting out inemacsmode.termThe
term=form may be used to include terminal-specific key bindings, perhaps to bind the key sequences output by the terminal’s function keys. The word on the right side of the ‘=’ is tested against both the full name of the terminal and the portion of the terminal name before the first ‘-’. This allowssunto match bothsunandsun-cmd, for instance.applicationThe application construct is used to include application-specific settings. Each program using the Readline library sets the application name, and you can test for a particular value. This could be used to bind key sequences to functions useful for a specific program. For instance, the following command adds a key sequence that quotes the current or previous word in Bash:
$if Bash # Quote the current or previous word "\C-xq": "\eb\"\ef\"" $endif
$endifThis command, as seen in the previous example, terminates an
$ifcommand.$elseCommands in this branch of the
$ifdirective are executed if the test fails.$includeThis directive takes a single filename as an argument and reads commands and bindings from that file. For example, the following directive reads from /etc/inputrc:
$include /etc/inputrc
Previous: Conditional Init Constructs, Up: Readline Init File [Contents][Index]
8.3.3 Sample Init File
Here is an example of an inputrc file. This illustrates key binding, variable assignment, and conditional syntax.
# This file controls the behaviour of line input editing for
# programs that use the GNU Readline library. Existing
# programs include FTP, Bash, and GDB.
#
# You can re-read the inputrc file with C-x C-r.
# Lines beginning with '#' are comments.
#
# First, include any system-wide bindings and variable
# assignments from /etc/Inputrc
$include /etc/Inputrc
#
# Set various bindings for emacs mode.
set editing-mode emacs
$if mode=emacs
Meta-Control-h: backward-kill-word Text after the function name is ignored
#
# Arrow keys in keypad mode
#
#"\M-OD": backward-char
#"\M-OC": forward-char
#"\M-OA": previous-history
#"\M-OB": next-history
#
# Arrow keys in ANSI mode
#
"\M-[D": backward-char
"\M-[C": forward-char
"\M-[A": previous-history
"\M-[B": next-history
#
# Arrow keys in 8 bit keypad mode
#
#"\M-\C-OD": backward-char
#"\M-\C-OC": forward-char
#"\M-\C-OA": previous-history
#"\M-\C-OB": next-history
#
# Arrow keys in 8 bit ANSI mode
#
#"\M-\C-[D": backward-char
#"\M-\C-[C": forward-char
#"\M-\C-[A": previous-history
#"\M-\C-[B": next-history
C-q: quoted-insert
$endif
# An old-style binding. This happens to be the default.
TAB: complete
# Macros that are convenient for shell interaction
$if Bash
# edit the path
"\C-xp": "PATH=${PATH}\e\C-e\C-a\ef\C-f"
# prepare to type a quoted word --
# insert open and close double quotes
# and move to just after the open quote
"\C-x\"": "\"\"\C-b"
# insert a backslash (testing backslash escapes
# in sequences and macros)
"\C-x\\": "\\"
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
# Add a binding to refresh the line, which is unbound
"\C-xr": redraw-current-line
# Edit variable on current line.
"\M-\C-v": "\C-a\C-k$\C-y\M-\C-e\C-a\C-y="
$endif
# use a visible bell if one is available
set bell-style visible
# don't strip characters to 7 bits when reading
set input-meta on
# allow iso-latin1 characters to be inserted rather
# than converted to prefix-meta sequences
set convert-meta off
# display characters with the eighth bit set directly
# rather than as meta-prefixed characters
set output-meta on
# if there are more than 150 possible completions for
# a word, ask the user if he wants to see all of them
set completion-query-items 150
# For FTP
$if Ftp
"\C-xg": "get \M-?"
"\C-xt": "put \M-?"
"\M-.": yank-last-arg
$endif
Next: Readline vi Mode, Previous: Readline Init File, Up: Command Line Editing [Contents][Index]
8.4 Bindable Readline Commands
| • Commands For Moving: | Moving about the line. | |
| • Commands For History: | Getting at previous lines. | |
| • Commands For Text: | Commands for changing text. | |
| • Commands For Killing: | Commands for killing and yanking. | |
| • Numeric Arguments: | Specifying numeric arguments, repeat counts. | |
| • Commands For Completion: | Getting Readline to do the typing for you. | |
| • Keyboard Macros: | Saving and re-executing typed characters | |
| • Miscellaneous Commands: | Other miscellaneous commands. |
This section describes Readline commands that may be bound to key
sequences.
You can list your key bindings by executing
bind -P or, for a more terse format, suitable for an
inputrc file, bind -p. (See Bash Builtins.)
Command names without an accompanying key sequence are unbound by default.
In the following descriptions, point refers to the current cursor
position, and mark refers to a cursor position saved by the
set-mark command.
The text between the point and mark is referred to as the region.
Next: Commands For History, Up: Bindable Readline Commands [Contents][Index]
8.4.1 Commands For Moving
beginning-of-line (C-a)Move to the start of the current line.
end-of-line (C-e)Move to the end of the line.
forward-char (C-f)Move forward a character.
backward-char (C-b)Move back a character.
forward-word (M-f)Move forward to the end of the next word. Words are composed of letters and digits.
backward-word (M-b)Move back to the start of the current or previous word. Words are composed of letters and digits.
shell-forward-word ()Move forward to the end of the next word. Words are delimited by non-quoted shell metacharacters.
shell-backward-word ()Move back to the start of the current or previous word. Words are delimited by non-quoted shell metacharacters.
clear-screen (C-l)Clear the screen and redraw the current line, leaving the current line at the top of the screen.
redraw-current-line ()Refresh the current line. By default, this is unbound.
Next: Commands For Text, Previous: Commands For Moving, Up: Bindable Readline Commands [Contents][Index]
8.4.2 Commands For Manipulating The History
accept-line (Newline or Return)Accept the line regardless of where the cursor is. If this line is non-empty, add it to the history list according to the setting of the
HISTCONTROLandHISTIGNOREvariables. If this line is a modified history line, then restore the history line to its original state.previous-history (C-p)Move ‘back’ through the history list, fetching the previous command.
next-history (C-n)Move ‘forward’ through the history list, fetching the next command.
beginning-of-history (M-<)Move to the first line in the history.
end-of-history (M->)Move to the end of the input history, i.e., the line currently being entered.
reverse-search-history (C-r)Search backward starting at the current line and moving ‘up’ through the history as necessary. This is an incremental search.
forward-search-history (C-s)Search forward starting at the current line and moving ‘down’ through the history as necessary. This is an incremental search.
non-incremental-reverse-search-history (M-p)Search backward starting at the current line and moving ‘up’ through the history as necessary using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.
non-incremental-forward-search-history (M-n)Search forward starting at the current line and moving ‘down’ through the history as necessary using a non-incremental search for a string supplied by the user. The search string may match anywhere in a history line.
history-search-forward ()Search forward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound.
history-search-backward ()Search backward through the history for the string of characters between the start of the current line and the point. The search string must match at the beginning of a history line. This is a non-incremental search. By default, this command is unbound.
history-substr-search-forward ()Search forward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search. By default, this command is unbound.
history-substr-search-backward ()Search backward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. This is a non-incremental search. By default, this command is unbound.
yank-nth-arg (M-C-y)Insert the first argument to the previous command (usually the second word on the previous line) at point. With an argument n, insert the nth word from the previous command (the words in the previous command begin with word 0). A negative argument inserts the nth word from the end of the previous command. Once the argument n is computed, the argument is extracted as if the ‘!n’ history expansion had been specified.
yank-last-arg (M-. or M-_)Insert last argument to the previous command (the last word of the previous history entry). With a numeric argument, behave exactly like
yank-nth-arg. Successive calls toyank-last-argmove back through the history list, inserting the last word (or the word specified by the argument to the first call) of each line in turn. Any numeric argument supplied to these successive calls determines the direction to move through the history. A negative argument switches the direction through the history (back or forward). The history expansion facilities are used to extract the last argument, as if the ‘!$’ history expansion had been specified.
Next: Commands For Killing, Previous: Commands For History, Up: Bindable Readline Commands [Contents][Index]
8.4.3 Commands For Changing Text
end-of-file (usually C-d)The character indicating end-of-file as set, for example, by
stty. If this character is read when there are no characters on the line, and point is at the beginning of the line, Readline interprets it as the end of input and returns EOF.delete-char (C-d)Delete the character at point. If this function is bound to the same character as the tty EOF character, as C-d commonly is, see above for the effects.
backward-delete-char (Rubout)Delete the character behind the cursor. A numeric argument means to kill the characters instead of deleting them.
forward-backward-delete-char ()Delete the character under the cursor, unless the cursor is at the end of the line, in which case the character behind the cursor is deleted. By default, this is not bound to a key.
quoted-insert (C-q or C-v)Add the next character typed to the line verbatim. This is how to insert key sequences like C-q, for example.
self-insert (a, b, A, 1, !, …)Insert yourself.
bracketed-paste-begin ()This function is intended to be bound to the "bracketed paste" escape sequence sent by some terminals, and such a binding is assigned by default. It allows Readline to insert the pasted text as a single unit without treating each character as if it had been read from the keyboard. The characters are inserted as if each one was bound to
self-insert) instead of executing any editing commands.transpose-chars (C-t)Drag the character before the cursor forward over the character at the cursor, moving the cursor forward as well. If the insertion point is at the end of the line, then this transposes the last two characters of the line. Negative arguments have no effect.
transpose-words (M-t)Drag the word before point past the word after point, moving point past that word as well. If the insertion point is at the end of the line, this transposes the last two words on the line.
upcase-word (M-u)Uppercase the current (or following) word. With a negative argument, uppercase the previous word, but do not move the cursor.
downcase-word (M-l)Lowercase the current (or following) word. With a negative argument, lowercase the previous word, but do not move the cursor.
capitalize-word (M-c)Capitalize the current (or following) word. With a negative argument, capitalize the previous word, but do not move the cursor.
overwrite-mode ()Toggle overwrite mode. With an explicit positive numeric argument, switches to overwrite mode. With an explicit non-positive numeric argument, switches to insert mode. This command affects only
emacsmode;vimode does overwrite differently. Each call toreadline()starts in insert mode.In overwrite mode, characters bound to
self-insertreplace the text at point rather than pushing the text to the right. Characters bound tobackward-delete-charreplace the character before point with a space.By default, this command is unbound.
Next: Numeric Arguments, Previous: Commands For Text, Up: Bindable Readline Commands [Contents][Index]
8.4.4 Killing And Yanking
kill-line (C-k)Kill the text from point to the end of the line.
backward-kill-line (C-x Rubout)Kill backward from the cursor to the beginning of the current line.
unix-line-discard (C-u)Kill backward from the cursor to the beginning of the current line.
kill-whole-line ()Kill all characters on the current line, no matter where point is. By default, this is unbound.
kill-word (M-d)Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as
forward-word.backward-kill-word (M-DEL)Kill the word behind point. Word boundaries are the same as
backward-word.shell-kill-word ()Kill from point to the end of the current word, or if between words, to the end of the next word. Word boundaries are the same as
shell-forward-word.shell-backward-kill-word ()Kill the word behind point. Word boundaries are the same as
shell-backward-word.unix-word-rubout (C-w)Kill the word behind point, using white space as a word boundary. The killed text is saved on the kill-ring.
unix-filename-rubout ()Kill the word behind point, using white space and the slash character as the word boundaries. The killed text is saved on the kill-ring.
delete-horizontal-space ()Delete all spaces and tabs around point. By default, this is unbound.
kill-region ()Kill the text in the current region. By default, this command is unbound.
copy-region-as-kill ()Copy the text in the region to the kill buffer, so it can be yanked right away. By default, this command is unbound.
copy-backward-word ()Copy the word before point to the kill buffer. The word boundaries are the same as
backward-word. By default, this command is unbound.copy-forward-word ()Copy the word following point to the kill buffer. The word boundaries are the same as
forward-word. By default, this command is unbound.yank (C-y)Yank the top of the kill ring into the buffer at point.
yank-pop (M-y)Rotate the kill-ring, and yank the new top. You can only do this if the prior command is
yankoryank-pop.
Next: Commands For Completion, Previous: Commands For Killing, Up: Bindable Readline Commands [Contents][Index]
8.4.5 Specifying Numeric Arguments
digit-argument (M-0, M-1, … M--)Add this digit to the argument already accumulating, or start a new argument. M-- starts a negative argument.
universal-argument ()This is another way to specify an argument. If this command is followed by one or more digits, optionally with a leading minus sign, those digits define the argument. If the command is followed by digits, executing
universal-argumentagain ends the numeric argument, but is otherwise ignored. As a special case, if this command is immediately followed by a character that is neither a digit nor minus sign, the argument count for the next command is multiplied by four. The argument count is initially one, so executing this function the first time makes the argument count four, a second time makes the argument count sixteen, and so on. By default, this is not bound to a key.
Next: Keyboard Macros, Previous: Numeric Arguments, Up: Bindable Readline Commands [Contents][Index]
8.4.6 Letting Readline Type For You
complete (TAB)Attempt to perform completion on the text before point. The actual completion performed is application-specific. Bash attempts completion treating the text as a variable (if the text begins with ‘$’), username (if the text begins with ‘~’), hostname (if the text begins with ‘@’), or command (including aliases and functions) in turn. If none of these produces a match, filename completion is attempted.
possible-completions (M-?)List the possible completions of the text before point. When displaying completions, Readline sets the number of columns used for display to the value of
completion-display-width, the value of the environment variableCOLUMNS, or the screen width, in that order.insert-completions (M-*)Insert all completions of the text before point that would have been generated by
possible-completions.menu-complete ()Similar to
complete, but replaces the word to be completed with a single match from the list of possible completions. Repeated execution ofmenu-completesteps through the list of possible completions, inserting each match in turn. At the end of the list of completions, the bell is rung (subject to the setting ofbell-style) and the original text is restored. An argument of n moves n positions forward in the list of matches; a negative argument may be used to move backward through the list. This command is intended to be bound to TAB, but is unbound by default.menu-complete-backward ()Identical to
menu-complete, but moves backward through the list of possible completions, as ifmenu-completehad been given a negative argument.delete-char-or-list ()Deletes the character under the cursor if not at the beginning or end of the line (like
delete-char). If at the end of the line, behaves identically topossible-completions. This command is unbound by default.complete-filename (M-/)Attempt filename completion on the text before point.
possible-filename-completions (C-x /)List the possible completions of the text before point, treating it as a filename.
complete-username (M-~)Attempt completion on the text before point, treating it as a username.
possible-username-completions (C-x ~)List the possible completions of the text before point, treating it as a username.
complete-variable (M-$)Attempt completion on the text before point, treating it as a shell variable.
possible-variable-completions (C-x $)List the possible completions of the text before point, treating it as a shell variable.
complete-hostname (M-@)Attempt completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @)List the possible completions of the text before point, treating it as a hostname.
complete-command (M-!)Attempt completion on the text before point, treating it as a command name. Command completion attempts to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames, in that order.
possible-command-completions (C-x !)List the possible completions of the text before point, treating it as a command name.
dynamic-complete-history (M-TAB)Attempt completion on the text before point, comparing the text against lines from the history list for possible completion matches.
dabbrev-expand ()Attempt menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.
complete-into-braces (M-{)Perform filename completion and insert the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion).
Next: Miscellaneous Commands, Previous: Commands For Completion, Up: Bindable Readline Commands [Contents][Index]
8.4.7 Keyboard Macros
start-kbd-macro (C-x ()Begin saving the characters typed into the current keyboard macro.
end-kbd-macro (C-x ))Stop saving the characters typed into the current keyboard macro and save the definition.
call-last-kbd-macro (C-x e)Re-execute the last keyboard macro defined, by making the characters in the macro appear as if typed at the keyboard.
print-last-kbd-macro ()Print the last keboard macro defined in a format suitable for the inputrc file.
Previous: Keyboard Macros, Up: Bindable Readline Commands [Contents][Index]
8.4.8 Some Miscellaneous Commands
re-read-init-file (C-x C-r)Read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there.
abort (C-g)Abort the current editing command and ring the terminal’s bell (subject to the setting of
bell-style).do-uppercase-version (M-a, M-b, M-x, …)If the metafied character x is lowercase, run the command that is bound to the corresponding uppercase character.
prefix-meta (ESC)Metafy the next character typed. This is for keyboards without a meta key. Typing ‘ESC f’ is equivalent to typing M-f.
undo (C-_ or C-x C-u)Incremental undo, separately remembered for each line.
revert-line (M-r)Undo all changes made to this line. This is like executing the
undocommand enough times to get back to the beginning.tilde-expand (M-&)Perform tilde expansion on the current word.
set-mark (C-@)Set the mark to the point. If a numeric argument is supplied, the mark is set to that position.
exchange-point-and-mark (C-x C-x)Swap the point with the mark. The current cursor position is set to the saved position, and the old cursor position is saved as the mark.
character-search (C-])A character is read and point is moved to the next occurrence of that character. A negative count searches for previous occurrences.
character-search-backward (M-C-])A character is read and point is moved to the previous occurrence of that character. A negative count searches for subsequent occurrences.
skip-csi-sequence ()Read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. Such sequences begin with a Control Sequence Indicator (CSI), usually ESC-[. If this sequence is bound to "\e[", keys producing such sequences will have no effect unless explicitly bound to a readline command, instead of inserting stray characters into the editing buffer. This is unbound by default, but usually bound to ESC-[.
insert-comment (M-#)Without a numeric argument, the value of the
comment-beginvariable is inserted at the beginning of the current line. If a numeric argument is supplied, this command acts as a toggle: if the characters at the beginning of the line do not match the value ofcomment-begin, the value is inserted, otherwise the characters incomment-beginare deleted from the beginning of the line. In either case, the line is accepted as if a newline had been typed. The default value ofcomment-begincauses this command to make the current line a shell comment. If a numeric argument causes the comment character to be removed, the line will be executed by the shell.dump-functions ()Print all of the functions and their key bindings to the Readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.
dump-variables ()Print all of the settable variables and their values to the Readline output stream. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.
dump-macros ()Print all of the Readline key sequences bound to macros and the strings they output. If a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file. This command is unbound by default.
glob-complete-word (M-g)The word before point is treated as a pattern for pathname expansion, with an asterisk implicitly appended. This pattern is used to generate a list of matching file names for possible completions.
glob-expand-word (C-x *)The word before point is treated as a pattern for pathname expansion, and the list of matching file names is inserted, replacing the word. If a numeric argument is supplied, a ‘*’ is appended before pathname expansion.
glob-list-expansions (C-x g)The list of expansions that would have been generated by
glob-expand-wordis displayed, and the line is redrawn. If a numeric argument is supplied, a ‘*’ is appended before pathname expansion.display-shell-version (C-x C-v)Display version information about the current instance of Bash.
shell-expand-line (M-C-e)Expand the line as the shell does. This performs alias and history expansion as well as all of the shell word expansions (see Shell Expansions).
history-expand-line (M-^)Perform history expansion on the current line.
magic-space ()Perform history expansion on the current line and insert a space (see History Interaction).
alias-expand-line ()Perform alias expansion on the current line (see Aliases).
history-and-alias-expand-line ()Perform history and alias expansion on the current line.
insert-last-argument (M-. or M-_)A synonym for
yank-last-arg.operate-and-get-next (C-o)Accept the current line for execution and fetch the next line relative to the current line from the history for editing. Any argument is ignored.
edit-and-execute-command (C-xC-e)Invoke an editor on the current command line, and execute the result as shell commands. Bash attempts to invoke
$VISUAL,$EDITOR, andemacsas the editor, in that order.
Next: Programmable Completion, Previous: Bindable Readline Commands, Up: Command Line Editing [Contents][Index]
8.5 Readline vi Mode
While the Readline library does not have a full set of vi
editing functions, it does contain enough to allow simple editing
of the line. The Readline vi mode behaves as specified in
the POSIX standard.
In order to switch interactively between emacs and vi
editing modes, use the ‘set -o emacs’ and ‘set -o vi’
commands (see The Set Builtin).
The Readline default is emacs mode.
When you enter a line in vi mode, you are already placed in
‘insertion’ mode, as if you had typed an ‘i’. Pressing ESC
switches you into ‘command’ mode, where you can edit the text of the
line with the standard vi movement keys, move to previous
history lines with ‘k’ and subsequent lines with ‘j’, and
so forth.
Next: Programmable Completion Builtins, Previous: Readline vi Mode, Up: Command Line Editing [Contents][Index]
8.6 Programmable Completion
When word completion is attempted for an argument to a command for
which a completion specification (a compspec) has been defined
using the complete builtin (see Programmable Completion Builtins),
the programmable completion facilities are invoked.
First, the command name is identified.
If a compspec has been defined for that command, the
compspec is used to generate the list of possible completions for the word.
If the command word is the empty string (completion attempted at the
beginning of an empty line), any compspec defined with
the -E option to complete is used.
If the command word is a full pathname, a compspec for the full
pathname is searched for first.
If no compspec is found for the full pathname, an attempt is made to
find a compspec for the portion following the final slash.
If those searches do not result in a compspec, any compspec defined with
the -D option to complete is used as the default.
Once a compspec has been found, it is used to generate the list of matching words. If a compspec is not found, the default Bash completion described above (see Commands For Completion) is performed.
First, the actions specified by the compspec are used.
Only matches which are prefixed by the word being completed are
returned.
When the -f or -d option is used for filename or
directory name completion, the shell variable FIGNORE is
used to filter the matches.
See Bash Variables, for a description of FIGNORE.
Any completions specified by a filename expansion pattern to the
-G option are generated next.
The words generated by the pattern need not match the word being completed.
The GLOBIGNORE shell variable is not used to filter the matches,
but the FIGNORE shell variable is used.
Next, the string specified as the argument to the -W option
is considered.
The string is first split using the characters in the IFS
special variable as delimiters.
Shell quoting is honored.
Each word is then expanded using
brace expansion, tilde expansion, parameter and variable expansion,
command substitution, and arithmetic expansion,
as described above (see Shell Expansions).
The results are split using the rules described above
(see Word Splitting).
The results of the expansion are prefix-matched against the word being
completed, and the matching words become the possible completions.
After these matches have been generated, any shell function or command
specified with the -F and -C options is invoked.
When the command or function is invoked, the COMP_LINE,
COMP_POINT, COMP_KEY, and COMP_TYPE variables are
assigned values as described above (see Bash Variables).
If a shell function is being invoked, the COMP_WORDS and
COMP_CWORD variables are also set.
When the function or command is invoked, the first argument ($1) is the
name of the command whose arguments are being completed, the
second argument ($2) is the word being completed, and the third argument
($3) is the word preceding the word being completed on the current command
line.
No filtering of the generated completions against the word being completed
is performed; the function or command has complete freedom in generating
the matches.
Any function specified with -F is invoked first.
The function may use any of the shell facilities, including the
compgen and compopt builtins described below
(see Programmable Completion Builtins), to generate the matches.
It must put the possible completions in the COMPREPLY array
variable, one per array element.
Next, any command specified with the -C option is invoked in an environment equivalent to command substitution. It should print a list of completions, one per line, to the standard output. Backslash may be used to escape a newline, if necessary.
After all of the possible completions are generated, any filter
specified with the -X option is applied to the list.
The filter is a pattern as used for pathname expansion; a ‘&’
in the pattern is replaced with the text of the word being completed.
A literal ‘&’ may be escaped with a backslash; the backslash
is removed before attempting a match.
Any completion that matches the pattern will be removed from the list.
A leading ‘!’ negates the pattern; in this case any completion
not matching the pattern will be removed.
If the nocasematch shell option
(see the description of shopt in The Shopt Builtin)
is enabled, the match is performed without regard to the case
of alphabetic characters.
Finally, any prefix and suffix specified with the -P and -S options are added to each member of the completion list, and the result is returned to the Readline completion code as the list of possible completions.
If the previously-applied actions do not generate any matches, and the
-o dirnames option was supplied to complete when the
compspec was defined, directory name completion is attempted.
If the -o plusdirs option was supplied to complete when
the compspec was defined, directory name completion is attempted and any
matches are added to the results of the other actions.
By default, if a compspec is found, whatever it generates is returned to
the completion code as the full set of possible completions.
The default Bash completions are not attempted, and the Readline default
of filename completion is disabled.
If the -o bashdefault option was supplied to complete when
the compspec was defined, the default Bash completions are attempted
if the compspec generates no matches.
If the -o default option was supplied to complete when the
compspec was defined, Readline’s default completion will be performed
if the compspec (and, if attempted, the default Bash completions)
generate no matches.
When a compspec indicates that directory name completion is desired, the programmable completion functions force Readline to append a slash to completed names which are symbolic links to directories, subject to the value of the mark-directories Readline variable, regardless of the setting of the mark-symlinked-directories Readline variable.
There is some support for dynamically modifying completions. This is most useful when used in combination with a default completion specified with -D. It’s possible for shell functions executed as completion handlers to indicate that completion should be retried by returning an exit status of 124. If a shell function returns 124, and changes the compspec associated with the command on which completion is being attempted (supplied as the first argument when the function is executed), programmable completion restarts from the beginning, with an attempt to find a new compspec for that command. This allows a set of completions to be built dynamically as completion is attempted, rather than being loaded all at once.
For instance, assuming that there is a library of compspecs, each kept in a file corresponding to the name of the command, the following default completion function would load completions dynamically:
_completion_loader()
{
. "/etc/bash_completion.d/$1.sh" >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader -o bashdefault -o default
Next: A Programmable Completion Example, Previous: Programmable Completion, Up: Command Line Editing [Contents][Index]
8.7 Programmable Completion Builtins
Three builtin commands are available to manipulate the programmable completion facilities: one to specify how the arguments to a particular command are to be completed, and two to modify the completion as it is happening.
compgen-
compgen [option] [word]Generate possible completion matches for word according to the options, which may be any option accepted by the
completebuiltin with the exception of -p and -r, and write the matches to the standard output. When using the -F or -C options, the various shell variables set by the programmable completion facilities, while available, will not have useful values.The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed.
The return value is true unless an invalid option is supplied, or no matches were generated.
complete-
complete [-abcdefgjksuv] [-o comp-option] [-DE] [-A action] [-G globpat] [-W wordlist] [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] name [name …]complete -pr [-DE] [name …]Specify how arguments to each name should be completed. If the -p option is supplied, or if no options are supplied, existing completion specifications are printed in a way that allows them to be reused as input. The -r option removes a completion specification for each name, or, if no names are supplied, all completion specifications. The -D option indicates that the remaining options and actions should apply to the “default” command completion; that is, completion attempted on a command for which no completion has previously been defined. The -E option indicates that the remaining options and actions should apply to “empty” command completion; that is, completion attempted on a blank line.
The process of applying these completion specifications when word completion is attempted is described above (see Programmable Completion). The -D option takes precedence over -E.
Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X options (and, if necessary, the -P and -S options) should be quoted to protect them from expansion before the
completebuiltin is invoked.-o comp-optionThe comp-option controls several aspects of the compspec’s behavior beyond the simple generation of completions. comp-option may be one of:
bashdefaultPerform the rest of the default Bash completions if the compspec generates no matches.
defaultUse Readline’s default filename completion if the compspec generates no matches.
dirnamesPerform directory name completion if the compspec generates no matches.
filenamesTell Readline that the compspec generates filenames, so it can perform any filename-specific processing (like adding a slash to directory names quoting special characters, or suppressing trailing spaces). This option is intended to be used with shell functions specified with -F.
noquoteTell Readline not to quote the completed words if they are filenames (quoting filenames is the default).
nosortTell Readline not to sort the list of possible completions alphabetically.
nospaceTell Readline not to append a space (the default) to words completed at the end of the line.
plusdirsAfter any matches defined by the compspec are generated, directory name completion is attempted and any matches are added to the results of the other actions.
-A actionThe action may be one of the following to generate a list of possible completions:
aliasAlias names. May also be specified as -a.
arrayvarArray variable names.
bindingReadline key binding names (see Bindable Readline Commands).
builtinNames of shell builtin commands. May also be specified as -b.
commandCommand names. May also be specified as -c.
directoryDirectory names. May also be specified as -d.
disabledNames of disabled shell builtins.
enabledNames of enabled shell builtins.
exportNames of exported shell variables. May also be specified as -e.
fileFile names. May also be specified as -f.
functionNames of shell functions.
groupGroup names. May also be specified as -g.
helptopicHelp topics as accepted by the
helpbuiltin (see Bash Builtins).hostnameHostnames, as taken from the file specified by the
HOSTFILEshell variable (see Bash Variables).jobJob names, if job control is active. May also be specified as -j.
keywordShell reserved words. May also be specified as -k.
runningNames of running jobs, if job control is active.
serviceService names. May also be specified as -s.
setoptValid arguments for the -o option to the
setbuiltin (see The Set Builtin).shoptShell option names as accepted by the
shoptbuiltin (see Bash Builtins).signalSignal names.
stoppedNames of stopped jobs, if job control is active.
userUser names. May also be specified as -u.
variableNames of all shell variables. May also be specified as -v.
-C commandcommand is executed in a subshell environment, and its output is used as the possible completions.
-F functionThe shell function function is executed in the current shell environment. When it is executed, $1 is the name of the command whose arguments are being completed, $2 is the word being completed, and $3 is the word preceding the word being completed, as described above (see Programmable Completion). When it finishes, the possible completions are retrieved from the value of the
COMPREPLYarray variable.-G globpatThe filename expansion pattern globpat is expanded to generate the possible completions.
-P prefixprefix is added at the beginning of each possible completion after all other options have been applied.
-S suffixsuffix is appended to each possible completion after all other options have been applied.
-W wordlistThe wordlist is split using the characters in the
IFSspecial variable as delimiters, and each resultant word is expanded. The possible completions are the members of the resultant list which match the word being completed.-X filterpatfilterpat is a pattern as used for filename expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ‘!’ in filterpat negates the pattern; in this case, any completion not matching filterpat is removed.
The return value is true unless an invalid option is supplied, an option other than -p or -r is supplied without a name argument, an attempt is made to remove a completion specification for a name for which no specification exists, or an error occurs adding a completion specification.
compopt-
compopt[-o option] [-DE] [+o option] [name]Modify completion options for each name according to the options, or for the currently-executing completion if no names are supplied. If no options are given, display the completion options for each name or the current completion. The possible values of option are those valid for the
completebuiltin described above. The -D option indicates that the remaining options should apply to the “default” command completion; that is, completion attempted on a command for which no completion has previously been defined. The -E option indicates that the remaining options should apply to “empty” command completion; that is, completion attempted on a blank line.The -D option takes precedence over -E.
The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a name for which no completion specification exists, or an output error occurs.
Previous: Programmable Completion Builtins, Up: Command Line Editing [Contents][Index]
8.8 A Programmable Completion Example
The most common way to obtain additional completion functionality beyond
the default actions complete and compgen provide is to use
a shell function and bind it to a particular command using complete -F.
The following function provides completions for the cd builtin.
It is a reasonably good example of what shell functions must do when
used for completion. This function uses the word passsed as $2
to determine the directory name to complete. You can also use the
COMP_WORDS array variable; the current word is indexed by the
COMP_CWORD variable.
The function relies on the complete and compgen builtins
to do much of the work, adding only the things that the Bash cd
does beyond accepting basic directory names:
tilde expansion (see Tilde Expansion),
searching directories in $CDPATH, which is described above
(see Bourne Shell Builtins),
and basic support for the cdable_vars shell option
(see The Shopt Builtin).
_comp_cd modifies the value of IFS so that it contains only
a newline to accommodate file names containing spaces and tabs –
compgen prints the possible completions it generates one per line.
Possible completions go into the COMPREPLY array variable, one completion per array element. The programmable completion system retrieves the completions from there when the function returns.
# A completion function for the cd builtin
# based on the cd completion function from the bash_completion package
_comp_cd()
{
local IFS=$' \t\n' # normalize IFS
local cur _skipdot _cdpath
local i j k
# Tilde expansion, with side effect of expanding tilde to full pathname
case "$2" in
\~*) eval cur="$2" ;;
*) cur=$2 ;;
esac
# no cdpath or absolute pathname -- straight directory completion
if [[ -z "${CDPATH:-}" ]] || [[ "$cur" == @(./*|../*|/*) ]]; then
# compgen prints paths one per line; could also use while loop
IFS=$'\n'
COMPREPLY=( $(compgen -d -- "$cur") )
IFS=$' \t\n'
# CDPATH+directories in the current directory if not in CDPATH
else
IFS=$'\n'
_skipdot=false
# preprocess CDPATH to convert null directory names to .
_cdpath=${CDPATH/#:/.:}
_cdpath=${_cdpath//::/:.:}
_cdpath=${_cdpath/%:/:.}
for i in ${_cdpath//:/$'\n'}; do
if [[ $i -ef . ]]; then _skipdot=true; fi
k="${#COMPREPLY[@]}"
for j in $( compgen -d -- "$i/$cur" ); do
COMPREPLY[k++]=${j#$i/} # cut off directory
done
done
$_skipdot || COMPREPLY+=( $(compgen -d -- "$cur") )
IFS=$' \t\n'
fi
# variable names if appropriate shell option set and no completions
if shopt -q cdable_vars && [[ ${#COMPREPLY[@]} -eq 0 ]]; then
COMPREPLY=( $(compgen -v -- "$cur") )
fi
return 0
}
We install the completion function using the -F option to
complete:
# Tell readline to quote appropriate and append slashes to directories; # use the bash default completion for other arguments complete -o filenames -o nospace -o bashdefault -F _comp_cd cd
Since we’d like Bash and Readline to take care of some
of the other details for us, we use several other options to tell Bash
and Readline what to do. The -o filenames option tells Readline
that the possible completions should be treated as filenames, and quoted
appropriately. That option will also cause Readline to append a slash to
filenames it can determine are directories (which is why we might want to
extend _comp_cd to append a slash if we’re using directories found
via CDPATH: Readline can’t tell those completions are directories).
The -o nospace option tells Readline to not append a space
character to the directory name, in case we want to append to it.
The -o bashdefault option brings in the rest of the "Bash default"
completions – possible completion that Bash adds to the default Readline
set. These include things like command name completion, variable completion
for words beginning with ‘{’, completions containing pathname
expansion patterns (see Filename Expansion), and so on.
Once installed using complete, _comp_cd will be called every
time we attempt word completion for a cd command.
Many more examples – an extensive collection of completions for most of the common GNU, Unix, and Linux commands – are available as part of the bash_completion project. This is installed by default on many GNU/Linux distributions. Originally written by Ian Macdonald, the project now lives at http://bash-completion.alioth.debian.org/. There are ports for other systems such as Solaris and Mac OS X.
An older version of the bash_completion package is distributed with bash in the examples/complete subdirectory.
Next: Installing Bash, Previous: Command Line Editing, Up: Top [Contents][Index]
9 Using History Interactively
This chapter describes how to use the GNU History Library interactively, from a user’s standpoint. It should be considered a user’s guide. For information on using the GNU History Library in other programs, see the GNU Readline Library Manual.
| • Bash History Facilities: | How Bash lets you manipulate your command history. | |
| • Bash History Builtins: | The Bash builtin commands that manipulate the command history. | |
| • History Interaction: | What it feels like using History as a user. |
Next: Bash History Builtins, Up: Using History Interactively [Contents][Index]
9.1 Bash History Facilities
When the -o history option to the set builtin
is enabled (see The Set Builtin),
the shell provides access to the command history,
the list of commands previously typed.
The value of the HISTSIZE shell variable is used as the
number of commands to save in a history list.
The text of the last $HISTSIZE
commands (default 500) is saved.
The shell stores each command in the history list prior to
parameter and variable expansion
but after history expansion is performed, subject to the
values of the shell variables
HISTIGNORE and HISTCONTROL.
When the shell starts up, the history is initialized from the
file named by the HISTFILE variable (default ~/.bash_history).
The file named by the value of HISTFILE is truncated, if
necessary, to contain no more than the number of lines specified by
the value of the HISTFILESIZE variable.
When a shell with history enabled exits, the last
$HISTSIZE lines are copied from the history list to the file
named by $HISTFILE.
If the histappend shell option is set (see Bash Builtins),
the lines are appended to the history file,
otherwise the history file is overwritten.
If HISTFILE
is unset, or if the history file is unwritable, the history is not saved.
After saving the history, the history file is truncated
to contain no more than $HISTFILESIZE lines.
If HISTFILESIZE is unset, or set to null, a non-numeric value, or
a numeric value less than zero, the history file is not truncated.
If the HISTTIMEFORMAT is set, the time stamp information
associated with each history entry is written to the history file,
marked with the history comment character.
When the history file is read, lines beginning with the history
comment character followed immediately by a digit are interpreted
as timestamps for the following history entry.
The builtin command fc may be used to list or edit and re-execute
a portion of the history list.
The history builtin may be used to display or modify the history
list and manipulate the history file.
When using command-line editing, search commands
are available in each editing mode that provide access to the
history list (see Commands For History).
The shell allows control over which commands are saved on the history
list. The HISTCONTROL and HISTIGNORE
variables may be set to cause the shell to save only a subset of the
commands entered.
The cmdhist
shell option, if enabled, causes the shell to attempt to save each
line of a multi-line command in the same history entry, adding
semicolons where necessary to preserve syntactic correctness.
The lithist
shell option causes the shell to save the command with embedded newlines
instead of semicolons.
The shopt builtin is used to set these options.
See Bash Builtins, for a description of shopt.
Next: History Interaction, Previous: Bash History Facilities, Up: Using History Interactively [Contents][Index]
9.2 Bash History Builtins
Bash provides two builtin commands which manipulate the history list and history file.
fc-
fc [-e ename] [-lnr] [first] [last]fc -s [pat=rep] [command]The first form selects a range of commands from first to last from the history list and displays or edits and re-executes them. Both first and last may be specified as a string (to locate the most recent command beginning with that string) or as a number (an index into the history list, where a negative number is used as an offset from the current command number). If last is not specified it is set to first. If first is not specified it is set to the previous command for editing and -16 for listing. If the -l flag is given, the commands are listed on standard output. The -n flag suppresses the command numbers when listing. The -r flag reverses the order of the listing. Otherwise, the editor given by ename is invoked on a file containing those commands. If ename is not given, the value of the following variable expansion is used:
${FCEDIT:-${EDITOR:-vi}}. This says to use the value of theFCEDITvariable if set, or the value of theEDITORvariable if that is set, orviif neither is set. When editing is complete, the edited commands are echoed and executed.In the second form, command is re-executed after each instance of pat in the selected command is replaced by rep. command is intepreted the same as first above.
A useful alias to use with the
fccommand isr='fc -s', so that typing ‘r cc’ runs the last command beginning withccand typing ‘r’ re-executes the last command (see Aliases). history-
history [n] history -c history -d offset history [-anrw] [filename] history -ps arg
With no options, display the history list with line numbers. Lines prefixed with a ‘*’ have been modified. An argument of n lists only the last n lines. If the shell variable
HISTTIMEFORMATis set and not null, it is used as a format string for strftime to display the time stamp associated with each displayed history entry. No intervening blank is printed between the formatted time stamp and the history line.Options, if supplied, have the following meanings:
-cClear the history list. This may be combined with the other options to replace the history list completely.
-d offsetDelete the history entry at position offset. offset should be specified as it appears when the history is displayed.
-aAppend the new history lines to the history file. These are history lines entered since the beginning of the current Bash session, but not already appended to the history file.
-nAppend the history lines not already read from the history file to the current history list. These are lines appended to the history file since the beginning of the current Bash session.
-rRead the history file and append its contents to the history list.
-wWrite out the current history list to the history file.
-pPerform history substitution on the args and display the result on the standard output, without storing the results in the history list.
-sThe args are added to the end of the history list as a single entry.
When any of the -w, -r, -a, or -n options is used, if filename is given, then it is used as the history file. If not, then the value of the
HISTFILEvariable is used.
Previous: Bash History Builtins, Up: Using History Interactively [Contents][Index]
9.3 History Expansion
The History library provides a history expansion feature that is similar
to the history expansion provided by csh. This section
describes the syntax used to manipulate the history information.
History expansions introduce words from the history list into the input stream, making it easy to repeat commands, insert the arguments to a previous command into the current input line, or fix errors in previous commands quickly.
History expansion is performed immediately after a complete line is read, before the shell breaks it into words.
History expansion takes place in two parts. The first is to determine which line from the history list should be used during substitution. The second is to select portions of that line for inclusion into the current one. The line selected from the history is called the event, and the portions of that line that are acted upon are called words. Various modifiers are available to manipulate the selected words. The line is broken into words in the same fashion that Bash does, so that several words surrounded by quotes are considered one word. History expansions are introduced by the appearance of the history expansion character, which is ‘!’ by default. Only ‘\’ and ‘'’ may be used to escape the history expansion character, but the history expansion character is also treated as quoted if it immediately precedes the closing double quote in a double-quoted string.
Several shell options settable with the shopt
builtin (see Bash Builtins) may be used to tailor
the behavior of history expansion. If the
histverify shell option is enabled, and Readline
is being used, history substitutions are not immediately passed to
the shell parser.
Instead, the expanded line is reloaded into the Readline
editing buffer for further modification.
If Readline is being used, and the histreedit
shell option is enabled, a failed history expansion will be
reloaded into the Readline editing buffer for correction.
The -p option to the history builtin command
may be used to see what a history expansion will do before using it.
The -s option to the history builtin may be used to
add commands to the end of the history list without actually executing
them, so that they are available for subsequent recall.
This is most useful in conjunction with Readline.
The shell allows control of the various characters used by the
history expansion mechanism with the histchars variable,
as explained above (see Bash Variables). The shell uses
the history comment character to mark history timestamps when
writing the history file.
| • Event Designators: | How to specify which history line to use. | |
| • Word Designators: | Specifying which words are of interest. | |
| • Modifiers: | Modifying the results of substitution. |
Next: Word Designators, Up: History Interaction [Contents][Index]
9.3.1 Event Designators
An event designator is a reference to a command line entry in the history list. Unless the reference is absolute, events are relative to the current position in the history list.
!Start a history substitution, except when followed by a space, tab, the end of the line, ‘=’ or ‘(’ (when the
extglobshell option is enabled using theshoptbuiltin).!nRefer to command line n.
!-nRefer to the command n lines back.
!!Refer to the previous command. This is a synonym for ‘!-1’.
!stringRefer to the most recent command preceding the current position in the history list starting with string.
!?string[?]Refer to the most recent command preceding the current position in the history list containing string. The trailing ‘?’ may be omitted if the string is followed immediately by a newline.
^string1^string2^Quick Substitution. Repeat the last command, replacing string1 with string2. Equivalent to
!!:s/string1/string2/.!#The entire command line typed so far.
Next: Modifiers, Previous: Event Designators, Up: History Interaction [Contents][Index]
9.3.2 Word Designators
Word designators are used to select desired words from the event. A ‘:’ separates the event specification from the word designator. It may be omitted if the word designator begins with a ‘^’, ‘$’, ‘*’, ‘-’, or ‘%’. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces.
For example,
!!designates the preceding command. When you type this, the preceding command is repeated in toto.
!!:$designates the last argument of the preceding command. This may be shortened to
!$.!fi:2designates the second argument of the most recent command starting with the letters
fi.
Here are the word designators:
0 (zero)The
0th word. For many applications, this is the command word.nThe nth word.
^The first argument; that is, word 1.
$The last argument.
%The word matched by the most recent ‘?string?’ search.
x-yA range of words; ‘-y’ abbreviates ‘0-y’.
*All of the words, except the
0th. This is a synonym for ‘1-$’. It is not an error to use ‘*’ if there is just one word in the event; the empty string is returned in that case.x*Abbreviates ‘x-$’
x-Abbreviates ‘x-$’ like ‘x*’, but omits the last word.
If a word designator is supplied without an event specification, the previous command is used as the event.
Previous: Word Designators, Up: History Interaction [Contents][Index]
9.3.3 Modifiers
After the optional word designator, you can add a sequence of one or more of the following modifiers, each preceded by a ‘:’.
hRemove a trailing pathname component, leaving only the head.
tRemove all leading pathname components, leaving the tail.
rRemove a trailing suffix of the form ‘.suffix’, leaving the basename.
eRemove all but the trailing suffix.
pPrint the new command but do not execute it.
qQuote the substituted words, escaping further substitutions.
xQuote the substituted words as with ‘q’, but break into words at spaces, tabs, and newlines.
s/old/new/Substitute new for the first occurrence of old in the event line. Any delimiter may be used in place of ‘/’. The delimiter may be quoted in old and new with a single backslash. If ‘&’ appears in new, it is replaced by old. A single backslash will quote the ‘&’. The final delimiter is optional if it is the last character on the input line.
&Repeat the previous substitution.
gaCause changes to be applied over the entire event line. Used in conjunction with ‘s’, as in
gs/old/new/, or with ‘&’.GApply the following ‘s’ modifier once to each word in the event.
Next: Reporting Bugs, Previous: Using History Interactively, Up: Top [Contents][Index]
10 Installing Bash
This chapter provides basic instructions for installing Bash on the various supported platforms. The distribution supports the GNU operating systems, nearly every version of Unix, and several non-Unix systems such as BeOS and Interix. Other independent ports exist for MS-DOS, OS/2, and Windows platforms.
| • Basic Installation: | Installation instructions. | |
| • Compilers and Options: | How to set special options for various systems. | |
| • Compiling For Multiple Architectures: | How to compile Bash for more than one kind of system from the same source tree. | |
| • Installation Names: | How to set the various paths used by the installation. | |
| • Specifying the System Type: | How to configure Bash for a particular system. | |
| • Sharing Defaults: | How to share default configuration values among GNU programs. | |
| • Operation Controls: | Options recognized by the configuration program. | |
| • Optional Features: | How to enable and disable optional features when building Bash. |
Next: Compilers and Options, Up: Installing Bash [Contents][Index]
10.1 Basic Installation
These are installation instructions for Bash.
The simplest way to compile Bash is:
-
cdto the directory containing the source code and type ‘./configure’ to configure Bash for your system. If you’re usingcshon an old version of System V, you might need to type ‘sh ./configure’ instead to preventcshfrom trying to executeconfigureitself.Running
configuretakes some time. While running, it prints messages telling which features it is checking for. - Type ‘make’ to compile Bash and build the
bashbugbug reporting script. - Optionally, type ‘make tests’ to run the Bash test suite.
- Type ‘make install’ to install
bashandbashbug. This will also install the manual pages and Info file.
The configure shell script attempts to guess correct
values for various system-dependent variables used during
compilation. It uses those values to create a Makefile in
each directory of the package (the top directory, the
builtins, doc, and support directories,
each directory under lib, and several others). It also creates a
config.h file containing system-dependent definitions.
Finally, it creates a shell script named config.status that you
can run in the future to recreate the current configuration, a
file config.cache that saves the results of its tests to
speed up reconfiguring, and a file config.log containing
compiler output (useful mainly for debugging configure).
If at some point
config.cache contains results you don’t want to keep, you
may remove or edit it.
To find out more about the options and arguments that the
configure script understands, type
bash-2.04$ ./configure --help
at the Bash prompt in your Bash source directory.
If you need to do unusual things to compile Bash, please
try to figure out how configure could check whether or not
to do them, and mail diffs or instructions to
bash-maintainers@gnu.org so they can be
considered for the next release.
The file configure.ac is used to create configure
by a program called Autoconf. You only need
configure.ac if you want to change it or regenerate
configure using a newer version of Autoconf. If
you do this, make sure you are using Autoconf version 2.50 or
newer.
You can remove the program binaries and object files from the
source code directory by typing ‘make clean’. To also remove the
files that configure created (so you can compile Bash for
a different kind of computer), type ‘make distclean’.
Next: Compiling For Multiple Architectures, Previous: Basic Installation, Up: Installing Bash [Contents][Index]
10.2 Compilers and Options
Some systems require unusual options for compilation or linking
that the configure script does not know about. You can
give configure initial values for variables by setting
them in the environment. Using a Bourne-compatible shell, you
can do that on the command line like this:
CC=c89 CFLAGS=-O2 LIBS=-lposix ./configure
On systems that have the env program, you can do it like this:
env CPPFLAGS=-I/usr/local/include LDFLAGS=-s ./configure
The configuration process uses GCC to build Bash if it is available.
Next: Installation Names, Previous: Compilers and Options, Up: Installing Bash [Contents][Index]
10.3 Compiling For Multiple Architectures
You can compile Bash for more than one kind of computer at the
same time, by placing the object files for each architecture in their
own directory. To do this, you must use a version of make that
supports the VPATH variable, such as GNU make.
cd to the
directory where you want the object files and executables to go and run
the configure script from the source directory. You may need to
supply the --srcdir=PATH argument to tell configure where the
source files are. configure automatically checks for the
source code in the directory that configure is in and in ‘..’.
If you have to use a make that does not supports the VPATH
variable, you can compile Bash for one architecture at a
time in the source code directory. After you have installed
Bash for one architecture, use ‘make distclean’ before
reconfiguring for another architecture.
Alternatively, if your system supports symbolic links, you can use the support/mkclone script to create a build tree which has symbolic links back to each file in the source directory. Here’s an example that creates a build directory in the current directory from a source directory /usr/gnu/src/bash-2.0:
bash /usr/gnu/src/bash-2.0/support/mkclone -s /usr/gnu/src/bash-2.0 .
The mkclone script requires Bash, so you must have already built
Bash for at least one architecture before you can create build
directories for other architectures.
Next: Specifying the System Type, Previous: Compiling For Multiple Architectures, Up: Installing Bash [Contents][Index]
10.4 Installation Names
By default, ‘make install’ will install into
/usr/local/bin, /usr/local/man, etc. You can
specify an installation prefix other than /usr/local by
giving configure the option --prefix=PATH,
or by specifying a value for the DESTDIR ‘make’
variable when running ‘make install’.
You can specify separate installation prefixes for
architecture-specific files and architecture-independent files.
If you give configure the option
--exec-prefix=PATH, ‘make install’ will use
PATH as the prefix for installing programs and libraries.
Documentation and other data files will still use the regular prefix.
Next: Sharing Defaults, Previous: Installation Names, Up: Installing Bash [Contents][Index]
10.5 Specifying the System Type
There may be some features configure can not figure out
automatically, but need to determine by the type of host Bash
will run on. Usually configure can figure that
out, but if it prints a message saying it can not guess the host
type, give it the --host=TYPE option. ‘TYPE’ can
either be a short name for the system type, such as ‘sun4’,
or a canonical name with three fields: ‘CPU-COMPANY-SYSTEM’
(e.g., ‘i386-unknown-freebsd4.2’).
See the file support/config.sub for the possible values of each field.
Next: Operation Controls, Previous: Specifying the System Type, Up: Installing Bash [Contents][Index]
10.6 Sharing Defaults
If you want to set default values for configure scripts to
share, you can create a site shell script called
config.site that gives default values for variables like
CC, cache_file, and prefix. configure
looks for PREFIX/share/config.site if it exists, then
PREFIX/etc/config.site if it exists. Or, you can set the
CONFIG_SITE environment variable to the location of the site
script. A warning: the Bash configure looks for a site script,
but not all configure scripts do.
Next: Optional Features, Previous: Sharing Defaults, Up: Installing Bash [Contents][Index]
10.7 Operation Controls
configure recognizes the following options to control how it
operates.
--cache-file=fileUse and save the results of the tests in file instead of ./config.cache. Set file to /dev/null to disable caching, for debugging
configure.--helpPrint a summary of the options to
configure, and exit.--quiet--silent-qDo not print messages saying which checks are being made.
--srcdir=dirLook for the Bash source code in directory dir. Usually
configurecan determine that directory automatically.--versionPrint the version of Autoconf used to generate the
configurescript, and exit.
configure also accepts some other, not widely used, boilerplate
options. ‘configure --help’ prints the complete list.
Previous: Operation Controls, Up: Installing Bash [Contents][Index]
10.8 Optional Features
The Bash configure has a number of --enable-feature
options, where feature indicates an optional part of Bash.
There are also several --with-package options,
where package is something like ‘bash-malloc’ or ‘purify’.
To turn off the default use of a package, use
--without-package. To configure Bash without a feature
that is enabled by default, use --disable-feature.
Here is a complete list of the --enable- and
--with- options that the Bash configure recognizes.
--with-afsDefine if you are using the Andrew File System from Transarc.
--with-bash-mallocUse the Bash version of
mallocin the directory lib/malloc. This is not the samemallocthat appears in GNU libc, but an older version originally derived from the 4.2 BSDmalloc. Thismallocis very fast, but wastes some space on each allocation. This option is enabled by default. The NOTES file contains a list of systems for which this should be turned off, andconfiguredisables this option automatically for a number of systems.--with-cursesUse the curses library instead of the termcap library. This should be supplied if your system has an inadequate or incomplete termcap database.
--with-gnu-mallocA synonym for
--with-bash-malloc.--with-installed-readline[=PREFIX]Define this to make Bash link with a locally-installed version of Readline rather than the version in lib/readline. This works only with Readline 5.0 and later versions. If PREFIX is
yesor not supplied,configureuses the values of the make variablesincludedirandlibdir, which are subdirectories ofprefixby default, to find the installed version of Readline if it is not in the standard system include and library directories. If PREFIX isno, Bash links with the version in lib/readline. If PREFIX is set to any other value,configuretreats it as a directory pathname and looks for the installed version of Readline in subdirectories of that directory (include files in PREFIX/includeand the library in PREFIX/lib).--with-purifyDefine this to use the Purify memory allocation checker from Rational Software.
--enable-minimal-configThis produces a shell with minimal features, close to the historical Bourne shell.
There are several --enable- options that alter how Bash is compiled and linked, rather than changing run-time features.
--enable-largefileEnable support for large files if the operating system requires special compiler options to build programs which can access large files. This is enabled by default, if the operating system provides large file support.
--enable-profilingThis builds a Bash binary that produces profiling information to be processed by
gprofeach time it is executed.--enable-static-linkThis causes Bash to be linked statically, if
gccis being used. This could be used to build a version to use as root’s shell.
The ‘minimal-config’ option can be used to disable all of the following options, but it is processed first, so individual options may be enabled using ‘enable-feature’.
All of the following options except for ‘disabled-builtins’, ‘direxpand-default’, and ‘xpg-echo-default’ are enabled by default, unless the operating system does not provide the necessary support.
--enable-aliasAllow alias expansion and include the
aliasandunaliasbuiltins (see Aliases).--enable-arith-for-commandInclude support for the alternate form of the
forcommand that behaves like the C languageforstatement (see Looping Constructs).--enable-array-variablesInclude support for one-dimensional array shell variables (see Arrays).
--enable-bang-historyInclude support for
csh-like history substitution (see History Interaction).--enable-brace-expansionInclude
csh-like brace expansion (b{a,b}c→bac bbc). See Brace Expansion, for a complete description.--enable-casemod-attributesInclude support for case-modifying attributes in the
declarebuiltin and assignment statements. Variables with the uppercase attribute, for example, will have their values converted to uppercase upon assignment.--enable-casemod-expansionInclude support for case-modifying word expansions.
--enable-command-timingInclude support for recognizing
timeas a reserved word and for displaying timing statistics for the pipeline followingtime(see Pipelines). This allows pipelines as well as shell builtins and functions to be timed.--enable-cond-commandInclude support for the
[[conditional command. (see Conditional Constructs).--enable-cond-regexpInclude support for matching POSIX regular expressions using the ‘=~’ binary operator in the
[[conditional command. (see Conditional Constructs).--enable-coprocessesInclude support for coprocesses and the
coprocreserved word (see Pipelines).--enable-debuggerInclude support for the bash debugger (distributed separately).
--enable-direxpand-defaultCause the
direxpandshell option (see The Shopt Builtin) to be enabled by default when the shell starts. It is normally disabled by default.--enable-directory-stackInclude support for a
csh-like directory stack and thepushd,popd, anddirsbuiltins (see The Directory Stack).--enable-disabled-builtinsAllow builtin commands to be invoked via ‘builtin xxx’ even after
xxxhas been disabled using ‘enable -n xxx’. See Bash Builtins, for details of thebuiltinandenablebuiltin commands.--enable-dparen-arithmeticInclude support for the
((…))command (see Conditional Constructs).--enable-extended-globInclude support for the extended pattern matching features described above under Pattern Matching.
--enable-extended-glob-defaultSet the default value of the extglob shell option described above under The Shopt Builtin to be enabled.
--enable-function-importInclude support for importing function definitions exported by another instance of the shell from the environment. This option is enabled by default.
--enable-glob-asciirange-defaultSet the default value of the globasciiranges shell option described above under The Shopt Builtin to be enabled. This controls the behavior of character ranges when used in pattern matching bracket expressions.
--enable-help-builtinInclude the
helpbuiltin, which displays help on shell builtins and variables (see Bash Builtins).--enable-historyInclude command history and the
fcandhistorybuiltin commands (see Bash History Facilities).--enable-job-controlThis enables the job control features (see Job Control), if the operating system supports them.
--enable-multibyteThis enables support for multibyte characters if the operating system provides the necessary support.
--enable-net-redirectionsThis enables the special handling of filenames of the form
/dev/tcp/host/portand/dev/udp/host/portwhen used in redirections (see Redirections).--enable-process-substitutionThis enables process substitution (see Process Substitution) if the operating system provides the necessary support.
--enable-progcompEnable the programmable completion facilities (see Programmable Completion). If Readline is not enabled, this option has no effect.
--enable-prompt-string-decodingTurn on the interpretation of a number of backslash-escaped characters in the
$PS1,$PS2,$PS3, and$PS4prompt strings. See Controlling the Prompt, for a complete list of prompt string escape sequences.--enable-readlineInclude support for command-line editing and history with the Bash version of the Readline library (see Command Line Editing).
--enable-restrictedInclude support for a restricted shell. If this is enabled, Bash, when called as
rbash, enters a restricted mode. See The Restricted Shell, for a description of restricted mode.--enable-selectInclude the
selectcompound command, which allows the generation of simple menus (see Conditional Constructs).--enable-separate-helpfilesUse external files for the documentation displayed by the
helpbuiltin instead of storing the text internally.--enable-single-help-stringsStore the text displayed by the
helpbuiltin as a single string for each help topic. This aids in translating the text to different languages. You may need to disable this if your compiler cannot handle very long string literals.--enable-strict-posix-defaultMake Bash POSIX-conformant by default (see Bash POSIX Mode).
--enable-usg-echo-defaultA synonym for
--enable-xpg-echo-default.--enable-xpg-echo-defaultMake the
echobuiltin expand backslash-escaped characters by default, without requiring the -e option. This sets the default value of thexpg_echoshell option toon, which makes the Bashechobehave more like the version specified in the Single Unix Specification, version 3. See Bash Builtins, for a description of the escape sequences thatechorecognizes.
The file config-top.h contains C Preprocessor
‘#define’ statements for options which are not settable from
configure.
Some of these are not meant to be changed; beware of the consequences if
you do.
Read the comments associated with each definition for more
information about its effect.
Next: Major Differences From The Bourne Shell, Previous: Installing Bash, Up: Top [Contents][Index]
Appendix A Reporting Bugs
Please report all bugs you find in Bash. But first, you should make sure that it really is a bug, and that it appears in the latest version of Bash. The latest version of Bash is always available for FTP from ftp://ftp.gnu.org/pub/gnu/bash/.
Once you have determined that a bug actually exists, use the
bashbug command to submit a bug report.
If you have a fix, you are encouraged to mail that as well!
Suggestions and ‘philosophical’ bug reports may be mailed
to bug-bash@gnu.org or posted to the Usenet
newsgroup gnu.bash.bug.
All bug reports should include:
- The version number of Bash.
- The hardware and operating system.
- The compiler used to compile Bash.
- A description of the bug behaviour.
- A short script or ‘recipe’ which exercises the bug and may be used to reproduce it.
bashbug inserts the first three items automatically into
the template it provides for filing a bug report.
Please send all reports concerning this manual to bug-bash@gnu.org.
Next: GNU Free Documentation License, Previous: Reporting Bugs, Up: Top [Contents][Index]
Appendix B Major Differences From The Bourne Shell
Bash implements essentially the same grammar, parameter and
variable expansion, redirection, and quoting as the Bourne Shell.
Bash uses the POSIX standard as the specification of
how these features are to be implemented. There are some
differences between the traditional Bourne shell and Bash; this
section quickly details the differences of significance. A
number of these differences are explained in greater depth in
previous sections.
This section uses the version of sh included in SVR4.2 (the
last version of the historical Bourne shell) as the baseline reference.
- Bash is POSIX-conformant, even where the POSIX specification
differs from traditional
shbehavior (see Bash POSIX Mode). - Bash has multi-character invocation options (see Invoking Bash).
- Bash has command-line editing (see Command Line Editing) and
the
bindbuiltin. - Bash provides a programmable word completion mechanism
(see Programmable Completion), and builtin commands
complete,compgen, andcompopt, to manipulate it. - Bash has command history (see Bash History Facilities) and the
historyandfcbuiltins to manipulate it. The Bash history list maintains timestamp information and uses the value of theHISTTIMEFORMATvariable to display it. - Bash implements
csh-like history expansion (see History Interaction). - Bash has one-dimensional array variables (see Arrays), and the appropriate variable expansions and assignment syntax to use them. Several of the Bash builtins take options to act on arrays. Bash provides a number of built-in array variables.
- The
$'…'quoting syntax, which expands ANSI-C backslash-escaped characters in the text between the single quotes, is supported (see ANSI-C Quoting). - Bash supports the
$"…"quoting syntax to do locale-specific translation of the characters between the double quotes. The -D, --dump-strings, and --dump-po-strings invocation options list the translatable strings found in a script (see Locale Translation). - Bash implements the
!keyword to negate the return value of a pipeline (see Pipelines). Very useful when anifstatement needs to act only if a test fails. The Bash ‘-o pipefail’ option tosetwill cause a pipeline to return a failure status if any command fails. - Bash has the
timereserved word and command timing (see Pipelines). The display of the timing statistics may be controlled with theTIMEFORMATvariable. - Bash implements the
for (( expr1 ; expr2 ; expr3 ))arithmetic for command, similar to the C language (see Looping Constructs). - Bash includes the
selectcompound command, which allows the generation of simple menus (see Conditional Constructs). - Bash includes the
[[compound command, which makes conditional testing part of the shell grammar (see Conditional Constructs), including optional regular expression matching. - Bash provides optional case-insensitive matching for the
caseand[[constructs. - Bash includes brace expansion (see Brace Expansion) and tilde expansion (see Tilde Expansion).
- Bash implements command aliases and the
aliasandunaliasbuiltins (see Aliases). - Bash provides shell arithmetic, the
((compound command (see Conditional Constructs), and arithmetic expansion (see Shell Arithmetic). - Variables present in the shell’s initial environment are automatically
exported to child processes. The Bourne shell does not normally do
this unless the variables are explicitly marked using the
exportcommand. - Bash supports the ‘+=’ assignment operator, which appends to the value of the variable named on the left hand side.
- Bash includes the POSIX pattern removal ‘%’, ‘#’, ‘%%’ and ‘##’ expansions to remove leading or trailing substrings from variable values (see Shell Parameter Expansion).
- The expansion
${#xx}, which returns the length of${xx}, is supported (see Shell Parameter Expansion). - The expansion
${var:offset[:length]}, which expands to the substring ofvar’s value of length length, beginning at offset, is present (see Shell Parameter Expansion). - The expansion
${var/[/]pattern[/replacement]}, which matches pattern and replaces it with replacement in the value ofvar, is available (see Shell Parameter Expansion). - The expansion
${!prefix*}expansion, which expands to the names of all shell variables whose names begin with prefix, is available (see Shell Parameter Expansion). - Bash has indirect variable expansion using
${!word}(see Shell Parameter Expansion). - Bash can expand positional parameters beyond
$9using${num}. - The POSIX
$()form of command substitution is implemented (see Command Substitution), and preferred to the Bourne shell’s``(which is also implemented for backwards compatibility). - Bash has process substitution (see Process Substitution).
- Bash automatically assigns variables that provide information about the
current user (
UID,EUID, andGROUPS), the current host (HOSTTYPE,OSTYPE,MACHTYPE, andHOSTNAME), and the instance of Bash that is running (BASH,BASH_VERSION, andBASH_VERSINFO). See Bash Variables, for details. - The
IFSvariable is used to split only the results of expansion, not all words (see Word Splitting). This closes a longstanding shell security hole. - The filename expansion bracket expression code uses ‘!’ and ‘^’ to negate the set of characters between the brackets. The Bourne shell uses only ‘!’.
- Bash implements the full set of POSIX filename expansion operators, including character classes, equivalence classes, and collating symbols (see Filename Expansion).
- Bash implements extended pattern matching features when the
extglobshell option is enabled (see Pattern Matching). - It is possible to have a variable and a function with the same name;
shdoes not separate the two name spaces. - Bash functions are permitted to have local variables using the
localbuiltin, and thus useful recursive functions may be written (see Bash Builtins). - Variable assignments preceding commands affect only that command, even
builtins and functions (see Environment).
In
sh, all variable assignments preceding commands are global unless the command is executed from the file system. - Bash performs filename expansion on filenames specified as operands to input and output redirection operators (see Redirections).
- Bash contains the ‘<>’ redirection operator, allowing a file to be opened for both reading and writing, and the ‘&>’ redirection operator, for directing standard output and standard error to the same file (see Redirections).
- Bash includes the ‘<<<’ redirection operator, allowing a string to be used as the standard input to a command.
- Bash implements the ‘[n]<&word’ and ‘[n]>&word’ redirection operators, which move one file descriptor to another.
- Bash treats a number of filenames specially when they are used in redirection operators (see Redirections).
- Bash can open network connections to arbitrary machines and services with the redirection operators (see Redirections).
- The
noclobberoption is available to avoid overwriting existing files with output redirection (see The Set Builtin). The ‘>|’ redirection operator may be used to overridenoclobber. - The Bash
cdandpwdbuiltins (see Bourne Shell Builtins) each take -L and -P options to switch between logical and physical modes. - Bash allows a function to override a builtin with the same name, and provides
access to that builtin’s functionality within the function via the
builtinandcommandbuiltins (see Bash Builtins). - The
commandbuiltin allows selective disabling of functions when command lookup is performed (see Bash Builtins). - Individual builtins may be enabled or disabled using the
enablebuiltin (see Bash Builtins). - The Bash
execbuiltin takes additional options that allow users to control the contents of the environment passed to the executed command, and what the zeroth argument to the command is to be (see Bourne Shell Builtins). - Shell functions may be exported to children via the environment
using
export -f(see Shell Functions). - The Bash
export,readonly, anddeclarebuiltins can take a -f option to act on shell functions, a -p option to display variables with various attributes set in a format that can be used as shell input, a -n option to remove various variable attributes, and ‘name=value’ arguments to set variable attributes and values simultaneously. - The Bash
hashbuiltin allows a name to be associated with an arbitrary filename, even when that filename cannot be found by searching the$PATH, using ‘hash -p’ (see Bourne Shell Builtins). - Bash includes a
helpbuiltin for quick reference to shell facilities (see Bash Builtins). - The
printfbuiltin is available to display formatted output (see Bash Builtins). - The Bash
readbuiltin (see Bash Builtins) will read a line ending in ‘\’ with the -r option, and will use theREPLYvariable as a default if no non-option arguments are supplied. The Bashreadbuiltin also accepts a prompt string with the -p option and will use Readline to obtain the line when given the -e option. Thereadbuiltin also has additional options to control input: the -s option will turn off echoing of input characters as they are read, the -t option will allowreadto time out if input does not arrive within a specified number of seconds, the -n option will allow reading only a specified number of characters rather than a full line, and the -d option will read until a particular character rather than newline. - The
returnbuiltin may be used to abort execution of scripts executed with the.orsourcebuiltins (see Bourne Shell Builtins). - Bash includes the
shoptbuiltin, for finer control of shell optional capabilities (see The Shopt Builtin), and allows these options to be set and unset at shell invocation (see Invoking Bash). - Bash has much more optional behavior controllable with the
setbuiltin (see The Set Builtin). - The ‘-x’ (xtrace) option displays commands other than simple commands when performing an execution trace (see The Set Builtin).
- The
testbuiltin (see Bourne Shell Builtins) is slightly different, as it implements the POSIX algorithm, which specifies the behavior based on the number of arguments. - Bash includes the
callerbuiltin, which displays the context of any active subroutine call (a shell function or a script executed with the.orsourcebuiltins). This supports the bash debugger. - The
trapbuiltin (see Bourne Shell Builtins) allows aDEBUGpseudo-signal specification, similar toEXIT. Commands specified with aDEBUGtrap are executed before every simple command,forcommand,casecommand,selectcommand, every arithmeticforcommand, and before the first command executes in a shell function. TheDEBUGtrap is not inherited by shell functions unless the function has been given thetraceattribute or thefunctraceoption has been enabled using theshoptbuiltin. Theextdebugshell option has additional effects on theDEBUGtrap.The
trapbuiltin (see Bourne Shell Builtins) allows anERRpseudo-signal specification, similar toEXITandDEBUG. Commands specified with anERRtrap are executed after a simple command fails, with a few exceptions. TheERRtrap is not inherited by shell functions unless the-o errtraceoption to thesetbuiltin is enabled.The
trapbuiltin (see Bourne Shell Builtins) allows aRETURNpseudo-signal specification, similar toEXITandDEBUG. Commands specified with anRETURNtrap are executed before execution resumes after a shell function or a shell script executed with.orsourcereturns. TheRETURNtrap is not inherited by shell functions unless the function has been given thetraceattribute or thefunctraceoption has been enabled using theshoptbuiltin. - The Bash
typebuiltin is more extensive and gives more information about the names it finds (see Bash Builtins). - The Bash
umaskbuiltin permits a -p option to cause the output to be displayed in the form of aumaskcommand that may be reused as input (see Bourne Shell Builtins). - Bash implements a
csh-like directory stack, and provides thepushd,popd, anddirsbuiltins to manipulate it (see The Directory Stack). Bash also makes the directory stack visible as the value of theDIRSTACKshell variable. - Bash interprets special backslash-escaped characters in the prompt strings when interactive (see Controlling the Prompt).
- The Bash restricted mode is more useful (see The Restricted Shell); the SVR4.2 shell restricted mode is too limited.
- The
disownbuiltin can remove a job from the internal shell job table (see Job Control Builtins) or suppress the sending ofSIGHUPto a job when the shell exits as the result of aSIGHUP. - Bash includes a number of features to support a separate debugger for shell scripts.
- The SVR4.2 shell has two privilege-related builtins
(
mldmodeandpriv) not present in Bash. - Bash does not have the
stopornewgrpbuiltins. - Bash does not use the
SHACCTvariable or perform shell accounting. - The SVR4.2
shuses aTIMEOUTvariable like Bash usesTMOUT.
More features unique to Bash may be found in Bash Features.
B.1 Implementation Differences From The SVR4.2 Shell
Since Bash is a completely new implementation, it does not suffer from many of the limitations of the SVR4.2 shell. For instance:
- Bash does not fork a subshell when redirecting into or out of
a shell control structure such as an
iforwhilestatement. - Bash does not allow unbalanced quotes. The SVR4.2 shell will silently
insert a needed closing quote at
EOFunder certain circumstances. This can be the cause of some hard-to-find errors. - The SVR4.2 shell uses a baroque memory management scheme based on
trapping
SIGSEGV. If the shell is started from a process withSIGSEGVblocked (e.g., by using thesystem()C library function call), it misbehaves badly. - In a questionable attempt at security, the SVR4.2 shell, when invoked without the -p option, will alter its real and effective UID and GID if they are less than some magic threshold value, commonly 100. This can lead to unexpected results.
- The SVR4.2 shell does not allow users to trap
SIGSEGV,SIGALRM, orSIGCHLD. - The SVR4.2 shell does not allow the
IFS,MAILCHECK,PATH,PS1, orPS2variables to be unset. - The SVR4.2 shell treats ‘^’ as the undocumented equivalent of ‘|’.
- Bash allows multiple option arguments when it is invoked (
-x -v); the SVR4.2 shell allows only one option argument (-xv). In fact, some versions of the shell dump core if the second argument begins with a ‘-’. - The SVR4.2 shell exits a script if any builtin fails; Bash exits a script only if one of the POSIX special builtins fails, and only for certain failures, as enumerated in the POSIX standard.
- The SVR4.2 shell behaves differently when invoked as
jsh(it turns on job control).
Next: Indexes, Previous: Major Differences From The Bourne Shell, Up: Top [Contents][Index]
Appendix C GNU Free Documentation License
Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/ Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
- PREAMBLE
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
- APPLICABILITY AND DEFINITIONS
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document’s overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work’s title, preceding the beginning of the body of the text.
The “publisher” means any person or entity that distributes copies of the Document to the public.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
- VERBATIM COPYING
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
- COPYING IN QUANTITY
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document’s license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
- MODIFICATIONS
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
- Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
- List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
- State on the Title page the name of the publisher of the Modified Version, as the publisher.
- Preserve all the copyright notices of the Document.
- Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
- Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
- Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document’s license notice.
- Include an unaltered copy of this License.
- Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
- Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
- For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
- Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
- Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
- Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
- Preserve any Warranty Disclaimers.
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version’s license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
- COMBINING DOCUMENTS
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
- COLLECTIONS OF DOCUMENTS
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
- AGGREGATION WITH INDEPENDENT WORKS
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation’s users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document’s Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
- TRANSLATION
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
- TERMINATION
You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.
However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.
Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.
- FUTURE REVISIONS OF THIS LICENSE
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy’s public statement of acceptance of a version permanently authorizes you to choose that version for the Document.
- RELICENSING
“Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.
“CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.
“Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.
An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.
The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.
ADDENDUM: How to use this License for your documents
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with…Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
Previous: GNU Free Documentation License, Up: Top [Contents][Index]
Appendix D Indexes
| • Builtin Index: | Index of Bash builtin commands. | |
| • Reserved Word Index: | Index of Bash reserved words. | |
| • Variable Index: | Quick reference helps you find the variable you want. | |
| • Function Index: | Index of bindable Readline functions. | |
| • Concept Index: | General index for concepts described in this manual. |
Next: Reserved Word Index, Up: Indexes [Contents][Index]
D.1 Index of Shell Builtin Commands
| Jump to: | .
:
[
A B C D E F G H J K L M P R S T U W |
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| Jump to: | .
:
[
A B C D E F G H J K L M P R S T U W |
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Next: Variable Index, Previous: Builtin Index, Up: Indexes [Contents][Index]
D.2 Index of Shell Reserved Words
| Jump to: | !
[
]
{
}
C D E F I S T U W |
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| Jump to: | !
[
]
{
}
C D E F I S T U W |
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Next: Function Index, Previous: Reserved Word Index, Up: Indexes [Contents][Index]
D.3 Parameter and Variable Index
| Jump to: | !
#
$
*
-
0
?
@
_
A B C D E F G H I K L M O P R S T U V |
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| Jump to: | !
#
$
*
-
0
?
@
_
A B C D E F G H I K L M O P R S T U V |
|---|
Next: Concept Index, Previous: Variable Index, Up: Indexes [Contents][Index]
D.4 Function Index
| Jump to: | A B C D E F G H I K M N O P Q R S T U Y |
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| Jump to: | A B C D E F G H I K M N O P Q R S T U Y |
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Previous: Function Index, Up: Indexes [Contents][Index]
D.5 Concept Index
| Jump to: | A B C D E F H I J K L M N O P Q R S T V W Y |
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| Jump to: | A B C D E F H I J K L M N O P Q R S T V W Y |
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:: Command execute :: | |
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