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GNU replacement for the Bourne shell From Wikipedia, the free encyclopedia
In computing, Bash (short for "Bourne Again SHell,")[6] is an interactive command interpreter and command programming language developed for UNIX-like operating systems.[7] Created in 1989[8] by Brian Fox for the GNU Project, it is supported by the Free Software Foundation and designed as a 100% free alternative for the Bourne shell (sh) and other proprietary Unix shells.
[9]
 | |
Screenshot of a Bash session | |
| Original author(s) | Brian Fox |
|---|---|
| Developer(s) | Chet Ramey |
| Initial release | 8 June 1989 |
| Stable release | |
| Repository | |
| Written in | C |
| Operating system | |
| Platform | GNU |
| Available in | Multilingual (gettext) |
| Type | Shell (computing), Unix shell, command language |
| License |
|
| Website | www |
Since its inception, Bash has gained widespread adoption and is commonly used as the default login shell for numerous Linux distributions.[10] It holds historical significance as one of the earliest programs ported to Linux by Linus Torvalds, alongside the GNU Compiler (GCC).[11] It is available on nearly all modern operating systems, making it a versatile tool in various computing environments.
As a command-line interface (CLI), Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features. The keywords, syntax, dynamically scoped variables, and other basic features of the language are all copied from the Bourne shell, sh. Other features, e.g., history, are copied from the C shell, csh, and the Korn Shell, ksh. It is a POSIX-compliant shell with extensions.
While Bash was developed for UNIX and UNIX-like operating systems, such as GNU/Linux, it is also available on Android, macOS, Windows, and numerous other current and historical operating systems.[12] "Although there have been attempts to create specialized shells, the Bourne shell derivatives continue to be the primary shells in use."[13]
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runcom for "run commands", and the file began to be called "a runcom". rc in Unix is a fossil from that usage."[15][16]"Christopher Strachey, a British scientist, had visited MIT about that time, and his macro-generator design appeared to me a very solid base for a command language, in particular the techniques for quoting and passing arguments. Without being invited on the subject, I wrote a paper explaining how the Multics command language could be designed with this objective. And I coined the word "shell" to name it."[14]
if...fi, case...esac), and completion of substitutions anywhere.[18] These closed forms and the for...done and while...done loop were defined as "Guarded Commands" in 1976 by Dijkstra.[19]glob, for pattern matching, and if, for conditional expressions. "The shell introduced a compact syntax for redirection (< > and >>) and piping (| or ^) that has survived into modern shells. You can also find support for invoking sequential commands (with ;) and asynchronous commands (with &)."[13] It also incorporated a goto command. "However, the Thompson shell was still distributed with the system as osh."[22] |
As a command processor, Bash operates within a text window where users input commands to execute various tasks. It also supports the execution of commands from files, known as shell scripts, facilitating automation. In keeping with Unix shell conventions, Bash incorporates a rich set of features, including:
info bash, and a technical manual available at man bash.if, case, select,&&) and OR (||), andfor, while, until loops, andfor ((;test command,[, [[ ... ]], which includes advanced features:
< and >;|;( ... );trap builtin;job_spec & where job_spec can be one of:
<command_name> <options> <operands> <arguments> &, or%1 &;# hashtag comments, and: colon comments;\ escapes are also honored at the ends of lines;$'...';kernel{-headers},~,$foo or ${bar}, including
declare -A, and${foo//x/y} for sed 's/x/y/g',${bar##[a-zA-Z0-9]*} for cut -c8-,${!array[@]}, and${var:?error message},$( ... ),<() or >(), when a system supports it:(( ... )) or $(( ... )), including
bc and awk, among others),*, ?, [...], and
[...], and the use of brackets in the testing commands, [ and [[ ... ]], are each one different things.)>, and appending, >>,<<,<<<, which allow parameters to be used as input, and>|, which can force overwriting of a file when a shell's "noclobber" setting is enabled;ls or ln, andBash also offers...
pushd and popd built-ins),cdpwdThe Bash command syntax is a superset of the Bourne shell command syntax. Bash supports brace expansion,[80] command line completion (Programmable Completion),[81] basic debugging[82][83] and signal handling (using trap) since bash 2.05a[84][85] among other features. Bash can execute the vast majority of Bourne shell scripts without modification, with the exception of Bourne shell scripts stumbling into fringe syntax behavior interpreted differently in Bash or attempting to run a system command matching a newer Bash builtin, etc. Bash command syntax includes ideas drawn from the Korn Shell (ksh) and the C shell (csh) such as command line editing, command history (history command),[86] the directory stack, the $RANDOM and $PPID variables, and POSIX command substitution syntax $(...).
When a user presses the tab key within an interactive command-shell, Bash automatically uses command line completion, since beta version 2.04,[87] to match partly typed program names, filenames and variable names. The Bash command-line completion system is very flexible and customizable, and is often packaged with functions that complete arguments and filenames for specific programs and tasks.
Bash's syntax has many extensions lacking in the Bourne shell. Bash can perform integer calculations ("arithmetic evaluation") without spawning external processes. It uses the ((...)) command and the $((...)) variable syntax for this purpose. Its syntax simplifies I/O redirection. For example, it can redirect standard output (stdout) and standard error (stderr) at the same time using the &> operator. This is simpler to type than the Bourne shell equivalent 'command > file 2>&1'. Bash supports process substitution using the <(command) and >(command)syntax, which substitutes the output of (or input to) a command where a filename is normally used. (This is implemented through /proc/fd/ unnamed pipes on systems that support that, or via temporary named pipes where necessary).
When using the 'function' keyword, Bash function declarations are not compatible with Bourne/Korn/POSIX scripts (the KornShell has the same problem when using 'function'), but Bash accepts the same function declaration syntax as the Bourne and Korn shells, and is POSIX-conformant. Because of these and other differences, Bash shell scripts are rarely runnable under the Bourne or Korn shell interpreters unless deliberately written with that compatibility in mind, which is becoming less common as Linux becomes more widespread. But in POSIX mode, Bash conforms with POSIX more closely.[88]
Bash supports here documents. Since version 2.05b Bash can redirect standard input (stdin) from a "here string" using the <<< operator.
Bash 3.0 supports in-process regular expression matching using a syntax reminiscent of Perl.[89]
In February 2009,[90] Bash 4.0 introduced support for associative arrays.[4] Associative array indices are strings, in a manner similar to AWK or Tcl.[91] They can be used to emulate multidimensional arrays. Bash 4 also switches its license to GPL-3.0-or-later.[92]
Bash supplies "conditional execution" command separators that make execution of a command contingent on the exit code set by a precedent command. For example:
cd "$SOMEWHERE" && ./do_something || echo "An error occurred" >&2
Where ./do_something is only executed if the cd (change directory) command was "successful" (returned an exit status of zero) and the echo command would only be executed if either the cd or the ./do_something command return an "error" (non-zero exit status).
For all commands the exit status is stored in the special variable $?. Bash also supports if ...;then ...;else ...;fi and case $VARIABLE in $pattern)...;;$other_pattern)...;; esac forms of conditional command evaluation.
The Bash shell has two modes of execution for commands: batch (asynchronous), and concurrent (synchronous).
To execute commands in batch mode (i.e., in sequence) they must be separated by the character ";", or on separate lines:
command1; command2
command3
In this example, when command1 is finished, command2 is executed, and when command2 has completed, command3 will execute.
A background execution of command1 can occur using (symbol &) at the end of an execution command, and process will be executed in background while immediately returning control to the shell and allowing continued execution of commands.
command1 &
Or to have a concurrent execution of command1 and command2, they must be executed in the Bash shell in the following way:
command1 & command2
In this case command1 is executed in the background & symbol, returning immediately control to the shell that executes command2 in the foreground.
A process can be stopped and control returned to bash by typing Ctrl+z while the process is running in the foreground.[93]
A list of all processes, both in the background and stopped, can be achieved by running jobs:
$ jobs
[1]- Running command1 &
[2]+ Stopped command2
In the output, the number in brackets refers to the job id. The plus sign signifies the default process for bg and fg. The text "Running" and "Stopped" refer to the process state. The last string is the command that started the process.
The state of a process can be changed using various commands. The fg command brings a process to the foreground, while bg sets a stopped process running in the background. bg and fg can take a job id as their first argument, to specify the process to act on. Without one, they use the default process, identified by a plus sign in the output of jobs. The kill command can be used to end a process prematurely, by sending it a signal. The job id must be specified after a percent sign:
kill %1
Invoking Bash with the --posix option or stating set -o posix in a script causes Bash to conform very closely with the POSIX 1003.2 standard.[94] Bash shell scripts intended for portability should take into account at least the POSIX shell standard. Some bash features not found in POSIX are:[94][95]
[[...]] extended test construct and its regex matching(( ... )); $(( ... )) is POSIX)local for scoped variablesIf a piece of code uses such a feature, it is called a "bashism" – a problem for portable use. Debian's checkbashisms and Vidar Holen's shellcheck can be used to make sure that a script does not contain these parts.[97][98] The list varies depending on the actual target shell: Debian's policy allows some extensions in their scripts (as they are in the dash shell),[95] while a script intending to support pre-POSIX Bourne shells, like autoconf's configure, are even more limited in the features they can use.[99]
Brace expansion, also called alternation, is a feature copied from the C shell. It generates a set of alternative combinations. Generated results need not exist as files. The results of each expanded string are not sorted and left to right order is preserved:
$ echo a{p,c,d,b}e
ape ace ade abe
$ echo {a,b,c}{d,e,f}
ad ae af bd be bf cd ce cf
Users should not use brace expansions in portable shell scripts, because the Bourne shell does not produce the same output.
$ # bash shell
$/bin/bash -c 'echo a{p,c,d,b}e'
ape ace ade abe
$ # A traditional shell does not produce the same output
$ /bin/sh -c 'echo a{p,c,d,b}e'
a{p,c,d,b}e
When brace expansion is combined with wildcards, the braces are expanded first, and then the resulting wildcards are substituted normally. Hence, a listing of JPEG and PNG images in the current directory could be obtained using:
ls *.{jpg,jpeg,png} # expands to *.jpg *.jpeg *.png - after which,
# the wildcards are processed
echo *.{png,jp{e,}g} # echo just shows the expansions -
# and braces in braces are possible.
In addition to alternation, brace expansion can be used for sequential ranges between two integers or characters separated by double dots. Newer versions of Bash allow a third integer to specify the increment.
$ echo {1..10}
1 2 3 4 5 6 7 8 9 10
$ echo {01..10}
01 02 03 04 05 06 07 08 09 10
$ echo file{1..4}.txt
file1.txt file2.txt file3.txt file4.txt
$ echo {a..e}
a b c d e
$ echo {1..10..3}
1 4 7 10
$ echo {a..j..3}
a d g j
When brace expansion is combined with variable expansion (A.K.A. parameter expansion and parameter substitution) the variable expansion is performed after the brace expansion, which in some cases may necessitate the use of the eval built-in, thus:
$ start=1; end=10
$ echo {$start..$end} # fails to expand due to the evaluation order
{1..10}
$ eval echo {$start..$end} # variable expansion occurs then resulting string is evaluated
1 2 3 4 5 6 7 8 9 10
| This section contains instructions, advice, or how-to content. (January 2019) |
When Bash starts, it executes the commands in a variety of dot files.[100] Unlike Bash shell scripts, dot files do typically have neither the execute permission enabled nor an interpreter directive like #!/bin/bash.
The example ~/.bash_profile below is compatible with the Bourne shell and gives semantics similar to csh for the ~/.bashrc and ~/.bash_login. The [ -r filename ] && cmd is a short-circuit evaluation that tests if filename exists and is readable, skipping the part after the && if it is not.
[ -r ~/.profile ] && . ~/.profile # set up environment, once, Bourne-sh syntax only
if [ -n "$PS1" ] ; then # are we interactive?
[ -r ~/.bashrc ] && . ~/.bashrc # tty/prompt/function setup for interactive shells
[ -r ~/.bash_login ] && . ~/.bash_login # any at-login tasks for login shell only
fi # End of "if" block
Some versions of Unix and Linux contain Bash system startup scripts, generally under the /etc directory. Bash executes these files as part of its standard initialization, but other startup files can read them in a different order than the documented Bash startup sequence. The default content of the root user's files may also have issues, as well as the skeleton files the system provides to new user accounts upon setup. The startup scripts that launch the X window system may also do surprising things with the user's Bash startup scripts in an attempt to set up user-environment variables before launching the window manager. These issues can often be addressed using a ~/.xsession or ~/.xprofile file to read the ~/.profile — which provides the environment variables that Bash shell windows spawned from the window manager need, such as xterm or Gnome Terminal.
set Built-inset -x | set -o xtrace ]The shell's primary means of debugging. Both xtrace and verbose can be turned off at the same time with the command set -.
set -v | set -o verbose ]Prints a command to the terminal as Bash reads it. Bash reads constructs all at once, such as compound commands which include if-fi and case-esac blocks. If a set -v is included within a compound command, then "verbose" will be enabled the next time Bash reads code as input, ie, after the end of the currently executing construct.[101] Both xtrace and verbose can be turned off at the same time with the command set -.
shopt Built-inOn by default in interactive shells. Some developers discourage its use in scripts.
Bash supports programmable completion via built-in complete, compopt, and compgen commands.[102] The feature has been available since the beta version of 2.04 released in 2000.[103][104] These commands enable complex and intelligent completion specification for commands (i.e. installed programs), functions, variables, and filenames.[105]
The complete and compopt two commands specify how arguments of some available commands or options are going to be listed in the readline input. As of version 5.1 completion of the command or the option is usually activated by the Tab ↹ keystroke after typing its name.[105]
Bash uses GNU Readline to provide keyboard shortcuts for command line editing using the default ( Emacs ) key bindings. Vi-bindings can be enabled by running set -o vi.[106]
As the standard upon which bash is based, the POSIX Standard, or IEEE Std 1003.1,[107] et seq, is especially informative.
The Linux "man page"[108][109] is intended to be the authoritative explanatory technical document for the understanding of how bash operates. It is usually available by running man bash.
The GNU manual is sometimes considered more user-friendly for reading. "You may also find information about Bash by running info bash ... or by looking at /usr/share/doc/bash/, /usr/local/share/doc/bash/, or similar directories on your system. A brief summary is available by running bash --help.[110]
" If a user invoke RUNCOM without any arguments it prints some instructions on how to use it and stops, returning the user to the supervisor's (system's) command line.(RUNCOM)"
On modern Linuxes, information on shell built-in commands can be found by executing help, help [built-in name] or man builtins at a terminal prompt where bash is installed. Some commands, such as echo, false, kill, printf, test or true, depending on your system and on your locally installed version of bash, can refer to either a shell built-in or a system binary executable file. When one of these command name collisions occurs, bash will by default execute a given command line using the shell built-in. Specifying a binary executable's absolute path (i.e., /bin/printf) is one way of ensuring that the shell uses a system binary. This name collision issue also effects any "help summaries" viewed with kill --help and /bin/kill --help. Shell built-ins and system binary executable files of the same name often have differing options.
"The project maintainer also has a Bash page which includes Frequently Asked Questions",[111][110] this FAQ is current as of bash version 5.1 and is no longer updated.
Running any shell scripts as the root user has, for years, been widely criticized as poor security practice. One commonly given reason is that, when a script is executed as root, the negative effects of any bugs in a script would be magnified by root's elevated privileges.
One common example: a script contains the command, rm -rf ${dir}/, but the variable $dir is left undefined. In Linux, if the script was executed by a regular user, the shell would attempt to execute the command rm -rf / as a regular user, and the command would fail. However, if the script was executed by the root user, then the command would likely succeed and the filesystem would be erased.
It is recommended to use sudo on a per-command basis instead.
| Feature | POSIX 2024 | Description | Bash ver. | ||
|---|---|---|---|---|---|
| Grammar type | Formal name | Syntax | |||
| Parameter Expansions | Indicate Null or Unset | "${parameter:?[word]}"
| Yes | "Where the expansion of [word], perhaps an error message or a line number, is written to STDERR and the shell exits with a non-zero exit code." | ? |
| Special Parameters | Exit Status | "$?"
| Yes | "Expands to the shortest representation of the decimal exit status." | ? |
| Special Parameters | PID of Invoked Shell | "$$"
| Yes | "Expands to the shortest representation of the decimal process ID of the invoked shell." | ? |
| Special Built-In Utility | set :: xtrace | set -x
| Yes | The shell's primary means of debugging. It "writes to standard error a trace for each command after it expands the command and before it executes it." | ? |
| Special Built-In Utility | set :: verbose | set -v
| Yes | "Writes its input to standard error as it is read." | ? |
| Special Built-In Utility | set :: pipefail | set -o pipefail
| Yes | "Derive the exit status of a pipeline from the exit statuses of all of the commands in the pipeline, not just the last (rightmost) command." | ? |
| Special Built-In Utility | set :: nounset | set -u
| Yes | When enabled, will cause the shell to exit with an error message when it encounters an unset variable expansion. Its use has a number of counter-intuitive pitfalls. | ? |
| Special Built-In Utility | set :: errexit | set -e
| Yes | ErrExit, is a setting that, when enabled, will, under certain very specific conditions, cause the shell to exit without an error message whenever the shell receives a non-zero exit code. Its use is somewhat controversial, to the extent that any somewhat obscure computer program can be considered controversial. Adherents claim that ErrExit provides an assurance of verifiability in situations where shell scripts "must not fail." However, opponents claim that its use is unreliable, deceptively simple, highly counter-intuitive, rife with gotchas and pitfalls, and in essence "security theater." Numerous developers of Bash have strongly discouraged the use of this particular setting. | ? |
| Special Built-In Utility | trap :: EXIT | trap '[arg]' EXIT
| Yes | "If a [sigspec] (signal specifier) is 0 or EXIT, [arg] is executed when the shell exits." If [arg] contains expansions, then [arg] should be in single quotes. | ? |
| Utility | printf | printf '<%s>\n' "${var}"
| Yes | A means of reliably printing the contents of a variable. | ? |
| Bash Variables | BASHPID | "${BASHPID}"
| No | "Expands to the process ID of the current bash process."[112] | ? |
| Bash Variables | BASH_ARGC | "${BASH_ARGC[@]}"
| No | "An array variable whose values are the number of parameters in each frame of the current bash execution call stack."[113] | ? |
| Bash Variables | BASH_ARGV | "${BASH_ARGV[@]}"
| No | "An array variable containing all of the parameters in the current bash execution call stack."[114] | ? |
| Bash Variables | BASH_LINENO | "${BASH_LINENO[@]}"
| No | "An array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked."[115] | ? |
| Bash Variables | BASH_REMATCH | "${BASH_REMATCH[@]}"
| No | "An array variable whose members are assigned by the =~ binary operator to the [[ conditional command."[116] | ? |
| Bash Variables | BASH_SOURCE | "${BASH_SOURCE}"
| No | "An array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined."[117] | ? |
| Bash Variables | BASH_XTRACEFD | "${BASH_XTRACEFD}"
| No | "If set to an integer corresponding to a valid file descriptor, Bash will write the trace output generated when ‘set -x’ is enabled to that file descriptor."[118] | ? |
| Bash Variables | EPOCHREALTIME | "${EPOCHREALTIME}"
| No | "Each time this parameter is referenced, it expands to the number of seconds since the Unix Epoch (see time(3)) as a floating point value with micro-second granularity."[119] | ? |
| Bash Variables | FUNCNAME | "${FUNCNAME[@]}"
| No | "An array variable containing the names of all shell functions currently in the execution call stack."[120] | ? |
| Bash Variables | LINENO | "${LINENO}"
| No | "Each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function."[121] | ? |
| Bash Variables | PIPESTATUS | "${PIPESTATUS[@]}"
| No | "An array variable containing a list of exit status values from the processes in the most-recently-executed foreground pipeline (which may contain only a single command)."[122] | ? |
| Bash Variables | PPID | "${PPID}"
| No | "The process ID of the shell's parent."[123] | ? |
| Bash Variables | PS4 | "${PS4}"
| No | "The value of this parameter is expanded as with PS1 and the value is printed before each command bash displays during an execution trace."[124] | ? |
| Shell Builtin | set :: restricted | set -r
| No | Restricted mode is intended to improve the security of an individual shell instance from a malicious human with physical access to a machine. As threat models have changed, it has become less commonly used now than it once was. | ? |
| Shell Builtin | shopt :: extdebug | shopt -s extdebug
| No | "Behavior intended for use by debuggers." | ? |
| Shell Builtin | trap :: DEBUG | trap '[arg]' DEBUG
| No | "If a sigspec is DEBUG, the command arg is executed before" certain kinds of commands. | ? |
| Shell Builtin | trap :: ERR | trap '[arg]' ERR
| No | "If a sigspec is ERR, the command arg is executed whenever..." certain kinds of commands "return a non-zero exit status," subject to similar restrictions as with ErrExit. | ? |
| Shell Builtin | trap :: RETURN | trap '[arg]' RETURN
| No | "If a sigspec is RETURN, the command arg is executed each time a shell function or a script executed with the . or source builtins finishes executing." | ? |
With the :? parameter expansion, an unset or null variable can halt a script.
#!/bin/bash
bar="foo is not defined"
echo "${foo:?$bar}"
echo this message doesn't print
$ ./ex.sh
./ex.sh: line 3: foo: foo is not defined
Reliably printing the contents of an array that contains spaces and newlines first in a portable syntax, and then the same thing in Bash. Note that in Bash, the number of spaces before the newline is made clear.
$ # In POSIX shell:
$ array=( "a " " b" "
> c " )
$ printf ',%s,\n' "${array[@]}"
,a ,
, b,
,
c ,
# In Bash:
declare -p array
declare -a array=([0]="a " [1]=" b" [2]=$' \n c ')
Printing an error message when there's a problem.
if ! lsblk | grep sdb
then
echo Error, line $LINENO
fi
$ ./error.sh
Error, line 130
Using xtrace. If errexit had been enabled, then echo quux would not have been executed.
#!/bin/bash
set -x
foo=bar; echo $foo
false
echo quux
$ ./test.sh
+ foo=bar
+ echo bar
bar
+ false
+ echo quux
quux
`...` is deprecated in favor of $(...);test/[/[[ commands,
[ -r ./file -a ! -l ./file ] is deprecated in favor of [ -r ./file ] && ! [ -l ./file ];$[...] is deprecated in favor of $((...)) or ((...)), as appropriate;^ as a pipeline is deprecated in favor of |;expr or let.In September 2014, a security bug was discovered[139] in the program. It was dubbed "Shellshock." Public disclosure quickly led to a range of attacks across the Internet.[140][141][142]
Exploitation of the vulnerability could enable arbitrary code execution in CGI scripts executable by certain versions of Bash. The bug involved how Bash passed function definitions to subshells through environment variables.[143] The bug had been present in the source code since August 1989 (version 1.03)[144] and was patched in September 2014 (version 4.3).
Patches to fix the bugs were made available soon after the bugs were identified. Upgrading to a current version is strongly advised.
It was assigned the Common Vulnerability identifiers CVE-2014-6271, CVE-2014-6277 and CVE-2014-7169, among others. Under CVSS Metrics 2.x and 3.x, the bug is regarded as "high" and "critical," respectively.
An external command called bashbug reports Bash shell bugs. When the command is invoked, it brings up the user's default editor with a form to fill in. The form is mailed to the Bash maintainers (or optionally to other email addresses).[145][146]
This book describes the latest release of bash 2.0 (version 2.01, dated June 1997).
Copyright © 2003, 2014 Apple Inc. All Rights Reserved. ... Updated: 2014-03-10
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