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[](bash4) \~~DISCUSSION:off\~~
# The Bash Hackers Wiki
![](keywords>bash shell linux scripting)
\<WRAP center round box 90%\> This wiki is intended to hold
documentation of any kind about GNU Bash. The main motivation was to
provide *human-readable documentation* and information so users aren't
forced to read every bit of the Bash manpage - which can be difficult to
understand. However, the docs here are **not** meant as a newbie
tutorial.
This wiki and any programs found in this wiki are free software: you can
redistribute it and/or modify it under the terms of the GNU General
Public License as published by the Free Software Foundation, either
version 3 of the License, or (at your option) any later version.
This wiki and its programs are distributed in the hope that it will be
useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.
You should have received a copy of the GNU General Public License along
with this program. If not, see \<<http://www.gnu.org/licenses/>\>.
[What would YOU like to see here?](wishes) (outdated and locked, please
use the discussions)
**Stranger!** [Feel free to
register](http://wiki.bash-hackers.org/start&do=register) and comment or
edit the contents. There is a [Bash Hackers Wiki needs love
page](/meta/need_love) that lists some things to do. The registration is
only there to prevent SPAM. \</WRAP\>
## Scripting and general information
- [Bash v4 - a rough overview](bash4) (unmaintained, since Bash 4 is
more or less standard)
- [style](/scripting/style) -- an assorted collection of style and optic
hints
- [basics](/scripting/basics)
- [newbie_traps](/scripting/newbie_traps)
- [bashbehaviour](/scripting/bashbehaviour)
- [posparams](/scripting/posparams)
- [processtree](/scripting/processtree)
- [obsolete](/scripting/obsolete)
- [nonportable](/scripting/nonportable)
- [debuggingtips](/scripting/debuggingtips)
- [terminalcodes](/scripting/terminalcodes)
- [tutoriallist](/scripting/tutoriallist)
## Code snippets
There is a [section that holds small code snippets](/snipplets/start).
See also [some Bash source code excerpts](/misc/readthesourceluke).
## How to....
[Doing specific tasks: concepts, methods, ideas](/howto/start):
- [Simple locking (against parallel run)](/howto/mutex)
- [Rudimentary config files for your scripts](/howto/conffile)
- [Editing files with ed(1)](/howto/edit-ed)
- [Collapsing Functions](/howto/collapsing_functions)
- [Illustrated Redirection Tutorial](/howto/redirection_tutorial)
- [Calculate with dc(1)](/howto/calculate-dc)
- [Introduction to pax - the POSIX archiver](/howto/pax)
- [getopts_tutorial](/howto/getopts_tutorial) (*under construction!*)
- [dissectabadoneliner](/howto/dissectabadoneliner) An example of a bad
oneliner, breakdown and fix (by `kojoro`)
- [Write tests for ./your-script.sh](/howto/testing-your-scripts) by
using bashtest util
## Bash syntax and operations
- [Bash features overview by version](/scripting/bashchanges)
- [Basic grammar rules](/syntax/basicgrammar)
- [Quoting and character escaping](/syntax/quoting)
- [Parsing and execution](/syntax/grammar/parser_exec)
- [Some words about words...](/syntax/words)
- [Patterns and pattern matching](/syntax/pattern)
- [Arithmetic expressions](/syntax/arith_expr)
- [List of shell options](/internals/shell_options)
- [Redirection](/syntax/redirection)
- [Special parameters and shell variables](/syntax/shellvars)
- [Arrays](/syntax/arrays)
\<WRAP column 40%\> ***<u>Compound commands</u>***
| **[Compound commands overview](/syntax/ccmd/intro)** | |
|:-----------------------------------------------------|------------------------------------------------------------------|
| Grouping | |
| `{ ...; }` | [command grouping](/syntax/ccmd/grouping_plain) |
| `( ... )` | [command grouping in a subshell](/syntax/ccmd/grouping_subshell) |
| Conditionals | |
| `[[ ... ]]` | [conditional expression](/syntax/ccmd/conditional_expression) |
| `if ...; then ...; fi` | [conditional branching](/syntax/ccmd/if_clause) |
| `case ... esac` | [pattern-based branching](/syntax/ccmd/case) |
| Loops | |
| `for word in ...; do ...; done` | [classic for-loop](/syntax/ccmd/classic_for) |
| `for ((x=1; x<=10; x++)); do ...; done` | [C-style for-loop](/syntax/ccmd/c_for) |
| `while ...; do ...; done` | [while loop](/syntax/ccmd/while_loop) |
| `until ...; do ...; done` | [until loop](/syntax/ccmd/until_loop) |
| Misc | |
| `(( ... ))` | [arithmetic evaluation](/syntax/ccmd/arithmetic_eval) |
| `select word in ...; do ...; done` | [user selections](/syntax/ccmd/user_select) |
\</WRAP\>
\<WRAP column 40%\> ***<u>Expansions and substitutions</u>***
| **[Introduction to expansions and substitutions](/syntax/expansion/intro)** | |
|:----------------------------------------------------------------------------|------------------------------------------------------|
| `{A,B,C} {A..C}` | [Brace expansion](/syntax/expansion/brace) |
| `~/ ~root/` | [Tilde expansion](/syntax/expansion/tilde) |
| `$FOO ${BAR%.mp3}` | [Parameter expansion](/syntax/pe) |
| `` `command` $(command) `` | [Command substitution](/syntax/expansion/cmdsubst) |
| `<(command) >(command)` | [Process substitution](/syntax/expansion/proc_subst) |
| `$((1 + 2 + 3)) $[4 + 5 + 6]` | [Arithmetic expansion](/syntax/expansion/arith) |
| `Hello <---> Word!` | [Word splitting](/syntax/expansion/wordsplit) |
| `/data/*-av/*.mp?` | [Pathname expansion](/syntax/expansion/globs) |
\</WRAP\> \<WRAP clear\>\</WRAP\>
## Builtin Commands
This is a selection of builtin commands and command-like keywords,
loosely arranged by their common uses. These are provided directly by
the shell, rather than invoked as standalone external commands.
\<WRAP column 46%\>
<table>
<thead>
<tr class="header">
<th style="text-align: center;">Declaration commands<br />
&lt;wrap center round lo todo box 80%&gt;Commands that set and query
attributes/types, and manipulate simple
datastructures.&lt;/wrap&gt;</th>
<th></th>
<th style="text-align: center;">Alt</th>
<th style="text-align: center;">Type</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/declare">declare</a></td>
<td>Display or set shell variables or functions along with
attributes.</td>
<td style="text-align: center;"><code>typeset</code></td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/export">export</a></td>
<td>Display or set shell variables, also giving them the export
attribute.</td>
<td style="text-align: center;"><code>typeset -x</code></td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/eval">eval</a></td>
<td>Evaluate arguments as shell code.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/local">local</a></td>
<td>Declare variables as having function local scope.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/readonly">readonly</a></td>
<td>Mark variables or functions as read-only.</td>
<td style="text-align: center;"><code>typeset -r</code></td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/unset">unset</a></td>
<td>Unset variables and functions.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/shift">shift</a></td>
<td>Shift positional parameters</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;">I/O<br />
&lt;wrap center round lo todo box 80%&gt;Commands for reading/parsing
input, or producing/formatting output of standard
streams.&lt;/wrap&gt;</td>
<td></td>
<td style="text-align: center;">Alt</td>
<td style="text-align: center;">Type</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/syntax/keywords/coproc">coproc</a></td>
<td>Co-processes: Run a command in the background with pipes for reading
/ writing its standard streams.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">keyword</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/echo">echo</a></td>
<td>Create output from arguments.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/mapfile">mapfile</a></td>
<td>Read lines of input into an array.</td>
<td style="text-align: center;"><code>readarray</code></td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/printf">printf</a></td>
<td>"advanced <code>echo</code>."</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/read">read</a></td>
<td>Read input into variables or arrays, or split strings into fields
using delimiters.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;">Configuration and Debugging<br />
&lt;wrap center round lo todo box 80%&gt;Commands that modify shell
behavior, change special options, assist in debugging.&lt;/wrap&gt;</td>
<td></td>
<td style="text-align: center;">Alt</td>
<td style="text-align: center;">Type</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/caller">caller</a></td>
<td>Identify/print execution frames.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/set">set</a></td>
<td>Set the positional parameters and/or set options that affect shell
behaviour.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/shopt">shopt</a></td>
<td>set/get some bash-specific shell options.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
</tbody>
</table>
\</WRAP\>
\<WRAP column 46%\>
<table>
<thead>
<tr class="header">
<th style="text-align: center;">Control flow and data processing<br />
&lt;wrap center round lo todo box 80%&gt;Commands that operate on data
and/or affect control flow.&lt;/wrap&gt;</th>
<th></th>
<th style="text-align: center;">Alt</th>
<th style="text-align: center;">Type</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/true">colon</a></td>
<td>"true" null command.</td>
<td style="text-align: center;">true</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/source">dot</a></td>
<td>Source external files.</td>
<td style="text-align: center;">source</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/false">false</a></td>
<td>Fail at doing nothing.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/continueBreak">continue / break</a></td>
<td>continue with or break out of loops.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/let">let</a></td>
<td>Arithmetic evaluation simple command.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/return">return</a></td>
<td>Return from a function with a specified exit status.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/classictest">[</a></td>
<td>The classic <code>test</code> simple command.</td>
<td style="text-align: center;">test</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;">Process and Job control<br />
&lt;wrap center round lo todo box 80%&gt;Commands related to jobs,
signals, process groups, subshells.&lt;/wrap&gt;</td>
<td></td>
<td style="text-align: center;">Alt</td>
<td style="text-align: center;">Type</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/exec">exec</a></td>
<td>Replace the current shell process or set redirections.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/exit">exit</a></td>
<td>Exit the shell.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/kill">kill</a></td>
<td>Send a signal to specified process(es)</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/trap">trap</a></td>
<td>Set signal handlers or output the current handlers.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"><a
href="/commands/builtin/times">times</a></td>
<td>Display process times.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">special builtin</td>
</tr>
<tr class="even">
<td style="text-align: center;"><a
href="/commands/builtin/wait">wait</a></td>
<td>Wait for background jobs and asynchronous lists.</td>
<td style="text-align: center;">-</td>
<td style="text-align: center;">builtin</td>
</tr>
<tr class="odd">
<td style="text-align: center;"></td>
<td></td>
<td style="text-align: center;">Alt</td>
<td style="text-align: center;">Type</td>
</tr>
</tbody>
</table>
\</WRAP\> \<WRAP clear\>\</WRAP\> \<WRAP hide\> previous alphabetical
version \<WRAP column 40%\>
| A-G | |
|--------------------------------------|------------------------------------------------------------------------------------------------------|
| [caller](/commands/builtin/caller) | identify/print execution frames (Bash builtin) |
| [coproc](/syntax/keywords/coproc) | Co-processes (Bash keyword) |
| [declare](/commands/builtin/declare) | display or set shell variables or functions along with attributes (Bash builtin, synonym: `typeset`) |
| [exec](/commands/builtin/exec) | replace the shell, set redirections (Bash builtin) |
| I-N | |
| [let](/commands/builtin/let) | arithmetic evaluation - an old fashioned way (Bash builtin) |
| [mapfile](/commands/builtin/mapfile) | Mapping lines of input to an array, also named `readarray` (Bash builtin) |
\</WRAP\>
\<WRAP column 40%\>
| O-T | |
|----------------------------------------|------------------------------------------------------------------|
| [printf](/commands/builtin/printf) | "advanced `echo`" (Bash builtin) |
| [read](/commands/builtin/read) | getting input from `stdin` (Bash builtin) |
| [readonly](/commands/builtin/readonly) | mark variables or functions as read-only (Bash builtin) |
| [set](/commands/builtin/set) | control positional parameters and shell behaviour (Bash builtin) |
| [shopt](/commands/builtin/shopt) | set/get shell options (Bash builtin) |
| [test](/commands/classictest) | the classic `test` command (Bash builtin) |
| U-Z | |
| [unset](/commands/builtin/unset) | unset variables and functions (Bash builtin) |
\</WRAP\> \<WRAP clear\>\</WRAP\> \</WRAP\>
## Dictionary
| :bulb: A list of expressions, words, and their meanings is [here](/dict/index). |
| --- |
## Links
### Official Bash links
- [Chet Ramey's Bash
page](http://tiswww.case.edu/php/chet/bash/bashtop.html) and its
[FAQ](http://tiswww.case.edu/php/chet/bash/FAQ).
- [GNU Bash software page](http://www.gnu.org/software/bash/)
- Official Bash mailing lists:
- **Bug reports**: <bug-bash@gnu.org>
([archives](http://mail.gnu.org/pipermail/bug-bash))
- **General questions**: <help-bash@gnu.org>
([archives](http://mail.gnu.org/pipermail/help-bash))
- Official Bash git repository:
- **Browse**: [cgit](http://git.savannah.gnu.org/cgit/bash.git)
- **Clone**: //<git://> ssh://git.sv.gnu.org/srv/git/bash.git •
//<ssh://> ssh://git.sv.gnu.org/srv/git/bash.git • //<http://>
http://git.savannah.gnu.org/r/bash.git
### Recommended Shell resources
- [Greg's wiki](http://mywiki.wooledge.org/) - Greg Wooledge's (aka
"greycat") wiki -- with **MASSIVE** information about Bash and UNIX(r)
in general.
- [BashFAQ](http://mywiki.wooledge.org/BashFAQ) •
[BashGuide](http://mywiki.wooledge.org/BashGuide) •
[BashPitfalls](http://mywiki.wooledge.org/BashPitfalls) •
[BashSheet](http://mywiki.wooledge.org/BashSheet)
- [Sven Mascheck's pages](http://www.in-ulm.de/~mascheck/) - A goldmine
of information. A must-read.
- [\#ksh channel page](https://www.mirbsd.org/ksh-chan.htm) - \#ksh
Freenode channel page maintains a solid collection of recommended
links.
- [The Grymoire Unix pages](http://www.grymoire.com/Unix/) - Good
scripting information, especially read the
[quoting](http://www.grymoire.com/Unix/Quote.html) guide.
- [Heiner's "Shell Dorado"](http://www.shelldorado.com) - Tips, tricks,
links - for every situation.
- [The Single Unix Specification (version 4, aka
POSIX-2008)](http://pubs.opengroup.org/onlinepubs/9699919799/)
- [The Austin Group](http://www.opengroup.org/austin/) - [List
archives](http://dir.gmane.org/gmane.comp.standards.posix.austin.general),
[Bug tracker](http://austingroupbugs.net/main_page.php)
- [comp.unix.shell FAQ](http://cfajohnson.com/shell/cus-faq.html)
- [Advanced Bash-Scripting
Guide](https://tldp.org/LDP/abs/html/index.html) - last update: 10 Mar
2014, but still very useful guide.
#### Documentation / Reference
- **Bash**: [man page](http://tiswww.case.edu/php/chet/bash/bash.html)
[info page](http://tiswww.case.edu/php/chet/bash/bashref.html)
- **AT&T ksh**:
[ksh88](http://www2.research.att.com/sw/download/man/man1/ksh88.html)
[ksh93](http://www2.research.att.com/sw/download/man/man1/ksh.html)
- [mksh](https://www.mirbsd.org/htman/i386/man1/mksh.htm) (pdksh
successor)
- [zsh](http://zsh.sourceforge.net/Doc/)
- [dash](http://man7.org/linux/man-pages/man1/dash.1.html)
- [Heirloom Bourne shell](http://heirloom.sourceforge.net/man/sh.1.html)
- [Thompson shell](http://v6shell.org/man/osh.1.html)
### Assorted interesting links
- [History and development of the traditional Bourne shell
family](http://www.in-ulm.de/~mascheck/bourne/) - very interesting and
nice to read!
- [Interview with Chet
Ramey](http://www.computerworld.com.au/article/222764/-z_programming_languages_bash_bourne-again_shell)
- [Interview with Steve
Bourne](http://www.computerworld.com.au/article/279011/a-z_programming_languages_sh)
• [Stephen Bourne - BSDCan 2015
keynote](https://www.youtube.com/watch?v=2kEJoWfobpA)
- [Interview with David
Korn](http://news.slashdot.org/story/01/02/06/2030205/david-korn-tells-all)
- [Kernighan on the Unix pipeline (computerphile
video)](https://www.youtube.com/watch?v=bKzonnwoR2I)
- Linux in general, with some shell related stuff: [nixCraft: Linux
Tips, Hacks, Tutorials and Ideas](http://www.cyberciti.biz/)
- Linux tutorials, guides and how-tos: [RoseHosting
Blog](https://www.rosehosting.com/blog/), [bash script for installing
WordPress](https://www.rosehosting.com/blog/script-install-wordpress-on-a-debianubuntu-vps/)
and some [basic shell
commands](https://www.rosehosting.com/blog/basic-shell-commands-after-putty-ssh-logon/)
- [Bashphorism list from the Bash IRC channel on
Freenode](/misc/bashphorisms)
- [Some more or less funny commandline stuff](/misc/shell_humor)
- [How to Enable SSH on Ubuntu
Tutorial](https://thishosting.rocks/how-to-enable-ssh-on-ubuntu/)
- [How To Make an Awesome Custom Shell with
ZSH](https://linuxstans.com/how-to-make-an-awesome-custom-shell-with-zsh/)
### Bash Libraries (needs review)
- [An oo-style bash library for bash
4](http://sourceforge.net/projects/oobash/) - provides tools for rapid
script development and huge libraries.
- [General purpose shell framework for bash
4](https://github.com/hornos/shf3) - in development.
- [General purpose bash library for bash
4](https://github.com/chilicuil/learn/blob/master/sh/lib) - active
development
\<div hide\>
## Most wanted
| Statistics for Month: **April 2012** | | |
|--------------------------------------|-------|------------------------------------------------------------|
| Rank | Views | Article |
| 1 | 6833 | [getopts_tutorial](/howto/getopts_tutorial) (March: 11663) |
| 2 | 4025 | [printf](/commands/builtin/printf) (March: 7079) |
| 3 | 2861 | [quoting](/syntax/quoting) (March: 5730) |
| 4 | 2854 | [classictest](/commands/classictest) (March: 6145) |
| 5 | 2222 | [pe](/syntax/pe) (March: 4540) |
| 6 | 1760 | [posparams](/scripting/posparams) (March: 3591) |
| 7 | 1249 | [bash4](bash4) (March: --) |
| 8 | 1212 | [edit-ed](/howto/edit-ed) (March: 2727) |
| 9 | 1205 | [read](/commands/builtin/read) (March: --) |
| 10 | 1182 | [mutex](/howto/mutex) (March: --) |
- "Total visits": 35144
- The overall "start" page is not counted
\</div\>
## Contact
\<WRAP column 40%\> Visit us in `irc.freenode.net`, channel `#bash` ;-)
If you have critiques or suggestions, please feel free to send a mail
using the contact form on the right. Note that there is a simple
discussion option below every article.
Please also see the [imprint](/user/thebonsai/imprint) if you have
problems with the site and its contents (legality, ...)!
It also would be nice to drop a line when
- it helped you
- it didn't help you (something missing / unclear)
- you like it
- you don't like it
- you found mistakes / bugs
Simply: Reader's feedback \</WRAP\>
\<WRAP column 40%\> ![](contact>subj=Wiki reader feedback) \</WRAP\>
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