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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://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
| website = books.google.com▼
}}
</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
| website = books.google.com▼
}}
</ref>
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<!-- Important concepts:
[] stream of characters
[] delineate full commandlines (newline, semi-colon)
[] division into commands and parts of commands (optargs)
[] uses metacharacters
-->
"Tokens"
\_ "Blanks"
\_ "Operators" (Ops)
\_ "Control Ops"
\_ "Redirection Ops"
\_ "Words"
\_ "Reserved Words"
\_ "Names"
</syntaxhighlight>
{{Blockquote
When Bash reads a full commandline, the complete string is broken down according to a certain set of rules into individual units called ''tokens.''▼
| 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>
''Tokens'' are identified using, and separated from each other using ''metacharacters.'' (As of version 5.3:)▼
{{Cite web
| access-date = 25 August 2025
| title = syntax, noun
| url = https://www.merriam-webster.com/dictionary/syntax
}}
</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."''
▲
* 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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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> ===
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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
}}
</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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| 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?
}}</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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| 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
▲/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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| 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?
}}</ref>
| author = Mastering Linux Shell Scripting, by Andrew Mallett
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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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