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{{Short description|GNU replacement for the Bourne shell}}
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{{More refs|date=August 2025}}
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{{Infobox software
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| url = https://www.gnu.org/software/bash/
| url-status = live
| website =
}}
</ref>
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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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}}</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:"''
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: {{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.
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</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"''
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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 = 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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| 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, 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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| 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 = 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 = 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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| 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>
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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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| 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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| 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 = 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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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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=== 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.
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