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{{Short description|GNU replacement for the Bourne shell}}
{{multiple issues|
{{
{{More refs|date=August 2025}}
{{Over-quotation|date=August 2025}}
}}
{{Use dmy dates|date=March 2014}}
{{Infobox software
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| url = https://www.gnu.org/software/bash/
| url-status = live
| website =
}}
</ref>
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| title = GNU in a Nutshell
| url = https://www.gnu.org/gnu/about-gnu.html
| website =
}}
|
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| title = GNU Software
| url = https://www.gnu.org/software/software.en.html
| website =
}}
}}
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| title = GNU's Bulletin, vol 1 no 7, June, 1989 :: GNU Project Status Report
| url = https://www.gnu.org/bulletins/bull7.html#SEC14
| website =
}}
</ref><ref>
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| url = https://www.gnu.org/gnu/thegnuproject.html
| url-status = live
| website =
}}
|
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| url = https://developer.mozilla.org/en-US/docs/Glossary/Character
| website = mozilla.org
| date = 11 July 2025
}}
</ref>
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| quote = UNIX is case sensitive. Because UNIX is case sensitive, our shell scripts are also case sensitive.
| title = Mastering Unix Shell Scripting, 2e
| url = https://books.google.com/books?
}}
</ref>
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BCPL is a precursor of the [[C (programming language) |C programming language]], in which Bash is written.
In [[C++]], a descendant of C, it's also true that "arrays start indexing from element 0."<ref>
{{Cite
| access-date = 25 August 2025
| author = Graham M. Seed
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| publisher = [[Springer Science+Business Media]]
| title = An Introduction to Object-Oriented Programming in C++, 7.4 Array Indexing
| url = https://books.google.com/books?id=_lqj98AsnGAC&q=zeroth+element&pg=PA195
| isbn = 978-1-85233-450-5
}}
</ref>
The array's name and index number are a synonym for the data's ___location in memory.<ref>
{{Cite
| access-date = 25 August 2025
| author = Graham M. Seed
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| quote = The name of an array is a synonym for the memory ___location of the array.
| title = An Introduction to Object-Oriented Programming in C++, 7.12.1 One-Dimensional Arrays
| url = https://books.google.com/books?id=_lqj98AsnGAC&q=zeroth+element&pg=PA210
| isbn = 978-1-85233-450-5
}}
</ref>
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| title = GNU Bash Manual: 8.2.1 Readline Bare Essentials
| url = https://www.gnu.org/software/bash/manual/html_node/Readline-Bare-Essentials.html
| website =
}}
</ref>
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| title = GNU Bash Manual: 8.5 Readline vi mode
| url = https://www.gnu.org/software/bash/manual/html_node/Readline-vi-Mode.html
| website =
}}</ref><ref>
{{Cite web
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}}</ref>
===
<!-- Important concepts:
[] stream of characters
[] delineate full commandlines (newline, semi-colon)
[] division into commands and parts of commands (optargs)
[] uses metacharacters
-->
<syntaxhighlight lang = text>
"Tokens"
\_ "Blanks"
\_ "Operators" (Ops)
\_ "Control Ops"
\_ "Redirection Ops"
\_ "Words"
\_ "Reserved Words"
\_ "Names"
</syntaxhighlight>
{{Blockquote
| syn{{dot}}tax '''1 a :''' the way in which linguistic elements (such as words) are put together to form constituents (such as phrases or clauses) '''b :''' the part of grammar dealing with this<ref>
{{Cite web
| access-date = 25 August 2025
| title = syntax, noun
| url = https://www.merriam-webster.com/dictionary/syntax
| website = merriam-webster.com
}}
</ref>
| author = Merriam-Webster Dictionary
}}
When Bash reads a ''full commandline,'' the complete string is broken down according to a certain set of rules into individual units called ''"tokens."''
"Tokens" are identified using, and separated from each other using ''"metacharacters."'' (As of version 5.3:)
* The ten ''"metacharacters:"''
: {{Pre |‘space’ ‘tab’ ‘newline’ ‘|’ ‘&’ ‘;’ ‘(’ ‘)’ ‘<’ ‘>’}}
''"Blanks"'' are composed entirely of unquoted metacharacters, ''"operators"'' each contain at least one unquoted metacharacter and ''"words"'' may not include any unquoted metacharacters.
In practice, Bash breaks down ''full command strings'' into tokens or groups of tokens that ''do'' contain metacharacters
From there it further breaks ''words'' down into more specific, meaningful pieces like command names, variable assignment statements, etc.
* The two ''
: {{Pre |‘space’ ‘tab’}}
* ''"Operators"''
:* Eight of the twelve ''"control operators:"''
:: <syntaxhighlight lang = text>
‘newline’
</syntaxhighlight>
:* Five of the fourteen ''"redirection operators:"''
:: <syntaxhighlight lang = text>
‘<’ ‘
</syntaxhighlight>
A ''"word"'' is a sequence of (non-meta-) characters treated as a single unit by the shell.
A ''"reserved word"'' is a kind of a ''"word"'' that has a special meaning to the shell.<ref>
{{Cite web
| access-date = 18 August 2025
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}}
</ref>
A ''"name"'' is a kind of a ''"word,"'' separate from ''reserved words'', which consists solely of letters, underscores and numbers; which begins with either a letter or an underscore; which, however, may not begin with a number.
''"Names,"'' also called ''"identifiers,"'' may be used for naming variables and functions.
* ''"Words"''
:* Sixteen of the twenty-two ''"reserved words,"'' which may be characters or words:
:: <syntaxhighlight lang = bash>
‘!’ ‘[[’ ‘{’ ‘]]’ ‘}’
</syntaxhighlight>
:* ''"Names"'' may contain only these characters (in the English language):
:: <syntaxhighlight lang = text>
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</syntaxhighlight>
In
: <syntaxhighlight lang = console>
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$ # ^^ ```` ``` ^^^^ ``````` ^^
</syntaxhighlight>
=== Subshells <span class="anchor" id="Subshells"></span> ===
<!-- Important concepts:
[x] Subshells
[x] Process and environment info, ie, parent/child, exact copy, etc
-->
A "subshell" is an additional instance of the shell which has been intitialized by a current instance of the shell.
When a "parent" shell creates a subshell, or a "child" shell, an exact copy of the parent's environment information is re-created and becomes the environment of the subshell.
In Bash, in non-arithmetic contexts, one can force the use of a subshell by enclosing a full command string in ''single parentheses''.
: <syntaxhighlight lang = console>
$ echo foo
foo
$ ( echo foo )
foo
$
</syntaxhighlight>
For this simple case, the preceding two commands are equivalent, however, use of subshells can have certain unexpected side effects.
There are numerous different forms of syntax which can cause the initialization of a subshell.
=== Expansion <span class="anchor" id="Expansion"></span> ===
<!-- Important concepts:
[]
-->
"Expansion" is a crucial concept in Unix-like shells. See [[String interpolation]].
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| title = Bash Reference Manual, 3.4 Shell Parameters
| url = https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameters
| website =
}}</ref>
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| title = Bash reference manual, 3.4.1 Positional Parameters
| url = https://www.gnu.org/software/bash/manual/bash.html#Positional-Parameters
| website =
}}</ref>
*** "$1" "$2" "$3" ... "${10}" "${11}" "${12}" - "The parentheses are required when there are two or more digits."<ref name = multics_ec>
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| title = Bash reference manual, 3.4.2 Special Parameters
| url = https://www.gnu.org/software/bash/manual/bash.html#Special-Parameters
| website =
}}</ref><ref>
{{Cite web
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| title = Bash reference manual, 5.1 Bourne Shell Variables
| url = https://www.gnu.org/software/bash/manual/bash.html#Bourne-Shell-Variables
| website =
}}</ref>
***** "$HOME"
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| title = Bash reference manual, 5.2 Bash Variables
| url = https://www.gnu.org/software/bash/manual/bash.html#Bash-Variables
| website =
}}</ref>
** Scripting variables: signified by all lower case letters or by CamelCase
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| title = Bash reference manual, 6.7 Arrays
| url = https://www.gnu.org/software/bash/manual/bash.html#Arrays
| website =
}}</ref><ref>
{{Cite web
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| title = Introduction to Linux, Ch. 3 About files and the filesystem, 3.1. General overview of the Linux file system, 3.1.1 Files
| url = https://tldp.org/LDP/intro-linux/html/sect_03_02.html#sect_03_02_02
| website =
}}</ref>{{efn
| This description does not apply to [[Microsoft Windows |Windows]]-based operating systems.
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| title = Introduction to Linux, Ch. 3 About files and the filesystem, 3.4 File security, 3.4.2.3. The file mask
| url = https://tldp.org/LDP/intro-linux/html/sect_03_04.html
| website =
}}</ref>
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| title = Introduction to Linux, Ch. 3 About files and the filesystem, 3.2 Orientation in the filesystem, 3.2.2 Absolute and relative paths
| url = https://tldp.org/LDP/intro-linux/html/sect_03_02.html#sect_03_02_02
| website =
}}</ref>
When the kernel searches for a directory, the starting point is the leftmost character of directory name.
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| title = Introduction to Linux, Ch. 3 About files and the filesystem, 3.2 Orientation in the filesystem, 3.2.1 The path
| url = https://tldp.org/LDP/intro-linux/html/sect_03_02.html#sect_03_02_01
| website =
}}</ref>
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[ ] scripts execute the contents of a file in a subshell
-->
With the {{code| source}}, or synonymous {{code| .}} command, Bash reads and executes shell commands from any text file by name.<ref>
{{Cite web
| access-date = 26 August 2025
| publisher = [[Free Software Foundation, Inc.]]
| title = 4.1 Bourne Shell Builtins
| url = https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html
| website = gnu.org
}}
</ref>
=== Login and non-login shells <span class="anchor" id="Login and non-login shells"></span><span class="anchor" id="Login shells"></span><span class="anchor" id="Non-login shells"></span> ===
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| title = Bash reference manual, 6.2 Bash startup files
| url = https://www.gnu.org/software/bash/manual/bash.html#Bash-Startup-Files-1
| website =
}}</ref>
* How bash is called
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| url = https://tldp.org/LDP/abs/html/portabilityissues.html
| url-status = live
| website =
}}</ref>
To cause a script to be initialized in POSIX mode, one would use the either the hashbang {{Code| #! /bin/env sh}} or the less portable {{Code| #!/bin/sh}}.
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| url = https://www.gnu.org/software/autoconf/manual/html_node/Portable-Shell.html
| url-status = live
| website =
}}</ref>
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| title = Bash Reference Manual, B.1 Implementation Differences from the SVR4.2 Shell
| url = https://www.gnu.org/software/bash/manual/bash.html#Implementation-Differences-From-The-SVR4_002e2-Shell
| website =
}}</ref> It can be enabled with {{Code| set -p| bash}} and disabled with {{Code| set +p| bash}}.<ref>
{{Cite web
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| title = Bash Reference Manual, 4.3.1 The Set Builtin
| url = https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
| website =
}}</ref> When privileged mode is enabled, the {{Code| $SHELLOPTS| bash}} shell variables includes the string, "privileged."
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| title = Bash Reference Manual, 6.12 Shell Compatability Mode
| url = https://www.gnu.org/software/bash/manual/bash.html#Shell-Compatibility-Mode
| website =
}}</ref>
| author = Bash Reference Manual, 6.12 Shell Compatability Mode
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| url = https://www.gnu.org/software/bash/manual/bash.html#index-background
| url-status = live
| website =
}}</ref>
A list of all processes, both in the background and stopped, can be achieved by running {{mono|jobs}}:
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| quote = 19 :: SIGSTOP :: Stop, usually Ctrl + z
| title = Mastering Unix Shell Scripting, 2e
| url = https://books.google.com/books?
}}</ref>
When a process receives a SIGKILL, the process terminates immediately and messily.
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| quote = Use KILL only as a last resort!
| title = Learning the bash Shell: Unix Shell Programming
| isbn = 978-0-596-55500-9
| url = https://books.google.com/books?id=dzBCH3x6fYEC
}}</ref>
The SIGKILL signal cannot be blocked or handled.
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| quote = In Korn shell the {{Mono |echo}} command recognizes these command options by default. In Bash shell we must add the {{Mono |-e}} switch to the {{Mono |echo}} command, {{Code| echo -e "\n"| bash}} for one new line.
| title = Mastering Unix Shell Scripting, 2e
| url = https://books.google.com/books?
}}</ref>
The list of options is not uniform across implementations, though {{mono|echo}} and {{mono|printf}} are both specified by POSIX.
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| title = Bash Reference Manual: 3.7.3: Command Execution Environment
| url = https://www.gnu.org/software/bash/manual/html_node/Command-Execution-Environment.html
| website =
}}</ref>
* Shell and [[Session (computer science)|session]] startup files such as {{Code| ~/.bashrc}} and {{Code| ~/.profile}} (i.e., [[Hidden file and hidden directory|dotfiles]]);
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| title = Bash Reference Manual, 6.6 Aliases
| url = https://www.gnu.org/software/bash/manual/html_node/Aliases.html
| website =
}}</ref><ref>
{{Cite web
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| title = Advanced Bash Scripting Guide, Ch 25. Aliases
| url = https://tldp.org/LDP/abs/html/abs-guide.html#ALIASES
| website =
}}</ref><ref>
{{Cite web
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| title = Bash Reference Manual, 3.3 Shell Functions
| url = https://www.gnu.org/software/bash/manual/html_node/Shell-Functions.html
| website =
}}</ref><ref>
{{Cite web
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| title = Advanced Bash Scripting Guide, Ch 24. Functions
| url = https://tldp.org/LDP/abs/html/abs-guide.html#FUNCTIONS
| website =
}}</ref><ref>
{{Cite web
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| title = Bash Reference Manual: 4.1: Bourne Shell Builtins
| url = https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html
| website =
}}</ref>
*** {{mono|cd}}, {{mono|pwd}}, etc.
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| publisher = [[GNU Project]]
| title = Bash Reference Manual: 4.3.1: The Set Builtin
| url = https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
| website = [[Free Software Foundation, Inc.]]
}}</ref>
*** Xtrace: [ {{Code| set -x| bash}} | {{Code| set -o xtrace| bash}} ]. The shell's primary means of debugging. Both xtrace and verbose can be turned off at the same time with the command {{Code| set -| bash}}.
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-->
{{Blockquote
| ITERATION: Sometimes programs are repeated indefinitely or until a specific outcome is reached. Each execution of the instructions is an
{{Cite web
| access-date = 15 August 2025
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| url = https://onlinegrad.syracuse.edu/blog/coding-terms-for-beginners/
| website = syracuse.edu
| date = 13 January 2020
}}</ref>
| author =
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| url = https://www.tldp.org/LDP/abs/html/bashver3.html#BASH3REF
| url-status = live
| website =
}}</ref>
Regexp matching is limited to strings on the right side of the <CODE>=~</CODE> operator in the {{Code| [[..]]}} extended test construct.<ref>
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| title = GNU Bash Manual: 3.2.5.2: Conditional Constructs
| url = https://www.gnu.org/software/bash/manual/bash.html#index-_005b_005b
| website =
}}</ref>
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| title = Bash Reference Manual, 3.2.6 Coprocesses
| url = https://www.gnu.org/software/bash/manual/bash.html#Coprocesses
| website =
}}</ref>
| author = Bash Reference Manual, 3.2.6 Coprocesses
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| quote = Learning this now can save us a lot of pain and heartache later, especially....
| title = Mastering Linux Shell Scripting
| isbn = 978-1-78439-759-3
| url = https://books.google.com/books?id=ITjlCwAAQBAJ
}}</ref>
| author = Mastering Linux Shell Scripting, by Andrew Mallett
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| url = https://www.gnu.org/software/bash/manual/html_node/Brace-Expansion.html
| url-status = live
| website =
}}</ref>
Generated results need not exist as files.
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=== Tilde Expansion ===
{{Empty section|date=August 2025}}
=== Parameter and variable expansion ===
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| url = https://tldp.org/LDP/abs/html/bashver4.html
| url-status = live
| website =
}}</ref>
Bash 4.0{{efn|Bash 4 also switches its license to [[GNU General Public License#Version 3|GPL-3.0-or-later]].
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| url = https://www.gnu.org/software/bash/manual/html_node/Arrays.html
| url-status = live
| website =
}}</ref> They can be used to emulate multidimensional arrays.}}
* Parameter Expansion
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| title = Bash Reference Manual, 3.1.2.5 Locale-Specific Translation
| url = https://www.gnu.org/software/bash/manual/bash.html#Locale-Translation
| website =
}}</ref>
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| url = https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Bash-History-Builtins
| url-status = live
| website =
}}</ref>
This feature is available in interactive mode only.
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| title = Bash Reference Manual: 8.6: Programmable Completion
| url = https://www.gnu.org/software/bash/manual/bash.html#Programmable-Completion
| website =
}}</ref>
<ref name =bashfaq061/>
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| title = Advanced Bash Scripting Guide: Appendix J: An Introduction to Programmable Completion
| url = https://tldp.org/LDP/abs/html/tabexpansion.html
| website =
}}</ref>
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| title = Bash Reference Manual, 6.10 The Restricted Shell
| url = https://www.gnu.org/software/bash/manual/bash.html#The-Restricted-Shell-1
| website =
}}</ref>
}}
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| title = Bash
| url = https://www.gnu.org/savannah-checkouts/gnu/bash/bash.html
| website =
}}
</ref>
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| title = GNU Bash manual
| url = https://www.gnu.org/software/bash/manual/index.html
| website =
}}
</ref><ref>
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| title = git: index : bash.git
| url = https://git.savannah.gnu.org/cgit/bash.git/tree/doc/bash.0
| website =
}}
</ref>
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| title = GNU Coreutils manual v.9.7, 15.2 printf: Format and print data
| url = https://www.gnu.org/software/coreutils/manual/coreutils.html#printf_003a-Format-and-print-data
| website =
}}</ref>
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| title = GNU Bash Manual: 1.1: What is Bash?
| url = https://www.gnu.org/software/bash/manual/bash.html#What-is-Bash_003f
| website =
}}
</ref>
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| title = POSIX 2024
| url = https://pubs.opengroup.org/onlinepubs/9799919799/
| website =
}}
</ref>
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It is recommended to use {{mono|sudo}} on a per-command basis instead.
=== CGI
CGI scripts are a significant source of vulnerability.<ref>
{{Cite web
| access-date = 17 August 2025
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| author = OWASP Input Validation Cheat Sheet
}}
=== Shellshock ===
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== Deprecated syntax <span class="anchor" id="Deprecated syntax"></span> ==
{{Unreferenced section|date=August 2025}}
* [[Backtick]] style command substitutions: {{Code| `...`| bash}} is deprecated in favor of
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| url = https://tldp.org/LDP/Bash-Beginners-Guide/html/sect_02_03.html
| url-status = live
| website =
}}</ref>
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASHPID
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fARGC
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fARGV
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fLINENO
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fREMATCH
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fSOURCE
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-BASH_005fXTRACEFD
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-EPOCHREALTIME
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-FUNCNAME
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-LINENO
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-PIPESTATUS
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-PPID
| website =
}}</ref>
| {{dunno}}
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html#index-PS4
| website =
}}</ref>
| {{dunno}}
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| title = GNU Bash Manual, 3.5.3: Shell Parameter Expansion
| url = https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
| website =
}}
|
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| title = Bash Reference Manual: 3.4.2: Special Parameters
| url = https://www.gnu.org/software/bash/manual/html_node/Special-Parameters.html
| website =
}}</ref><ref>
{{Cite web
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| title = GNU Bash Manual, 4.3.1: The Set Builtin
| url = https://www.gnu.org/software/bash/manual/html_node/The-Set-Builtin.html
| website =
}}
|
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| title = Bourne Shell Builtins (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/Bourne-Shell-Builtins.html#index-trap
| website =
}}</ref>
*** POSIX does specify certain uses of the {{mono|trap}} builtin: ...
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| title = Bash Reference Manual: 5.2: Bash Variables
| url = https://www.gnu.org/software/bash/manual/html_node/Bash-Variables.html
| website =
}}</ref><ref>
{{Cite web
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| title = The Shopt Builtin (Bash Reference Manual)
| url = https://www.gnu.org/software/bash/manual/html_node/The-Shopt-Builtin.html
| website =
}}</ref><ref name =case.edu_bash1/>
** Shell Builtin {{mono|trap}}:<ref name =gnu.org_trap/><ref name =case.edu_bash1/>
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=== Timeline ===
<!-- consider moving to dedicated 'Timeline of Bash' article -->
Significant events in Bash history are listed below:
{| class = "wikitable sortable collapsible collapsed"
! Date
! Event
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| url = https://www.gnu.org/gnu/thegnuproject.html
| url-status = live
| website =
}}
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Bash became the default shell on Apple's operating systems (i.e., MacOS) starting with OS X 10.3 Panther.<ref>[https://www.google.com/books/edition/Essential_Mac_OS_X_Panther_Server_Admini/zrI-U0KWj3cC?hl=en&gbpv=1&dq=bash&pg=PA189&printsec=frontcover Essential Mac OS S Panther Server Administration, pg 189]
</ref><ref>
{{Cite
| access-date = 8 August 2025
| archive-date = 2 March 2021
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| url = https://books.google.com/books?id=dwIRERUpQPEC&q=bash+most+popular+unix+shell&pg=PA6
| url-status = live
}}
</ref>
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| url = https://ftp.gnu.org/gnu/bash
| url-status = live
| website =
}}
</ref>
=== Unix shells ===
{{div col|colwidth=24em}}
* [[Almquist shell | Almquist shell (ash)]]
* [[Bourne shell | Bourne shell (sh)]]
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* yash – Yet Another Shell, aims "to be the most POSIX-compliant shell in the world"; available on Arch.
* [[Z shell | Z shell (zsh)]]
{{
<!-- This subsection added from https://ru.wikipedia.org/wiki/Bash on 6 Aug 2025 -->
=== Graphical interface to scripts ===
There are many programs that allow you to create a graphical interface for shell scripts.
| |||