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[](bash4) \~\~DISCUSSION:off\~\~
# The Bash Hackers Wiki
![](keywords>bash shell linux scripting)
Come visit us at <ircs://irc.libera.chat:6697> #bash ! \~ormaaj
\<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[What would YOU like to see here?](wishes) (outdated and locked, please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%\> ***[Compound commands]{.underline}***
**[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%\> ***[Expansions and substitutions]{.underline}***
**[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%\>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Declaration commands\ Alt Type
\<wrap center round lo todo box 80%\>Commands that set and query attributes/types, and manipulate simple datastructures.\</wrap\>
--------------------------------------------------------------------------------------------------------------------------------------- ------------------------------------------------------------------------------------------------------ -------------- -----------------
[declare](/commands/builtin/declare) Display or set shell variables or functions along with attributes. `typeset` builtin
[export](/commands/builtin/export) Display or set shell variables, also giving them the export attribute. `typeset -x` special builtin
[eval](/commands/builtin/eval) Evaluate arguments as shell code. \- special builtin
[local](/commands/builtin/local) Declare variables as having function local scope. \- builtin
[readonly](/commands/builtin/readonly) Mark variables or functions as read-only. `typeset -r` special builtin
[unset](/commands/builtin/unset) Unset variables and functions. \- special builtin
[shift](/commands/builtin/shift) Shift positional parameters \- special builtin
I/O\ Alt Type
\<wrap center round lo todo box 80%\>Commands for reading/parsing input, or producing/formatting output of standard streams.\</wrap\>
[coproc](/syntax/keywords/coproc) Co-processes: Run a command in the background with pipes for reading / writing its standard streams. \- keyword
[echo](/commands/builtin/echo) Create output from arguments. \- builtin
[mapfile](/commands/builtin/mapfile) Read lines of input into an array. `readarray` builtin
[printf](/commands/builtin/printf) \"advanced `echo`.\" \- builtin
[read](/commands/builtin/read) Read input into variables or arrays, or split strings into fields using delimiters. \- builtin
Configuration and Debugging\ Alt Type
\<wrap center round lo todo box 80%\>Commands that modify shell behavior, change special options, assist in debugging.\</wrap\>
[caller](/commands/builtin/caller) Identify/print execution frames. \- builtin
[set](/commands/builtin/set) Set the positional parameters and/or set options that affect shell behaviour. \- special builtin
[shopt](/commands/builtin/shopt) set/get some bash-specific shell options. \- builtin
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\</WRAP\>
\<WRAP column 46%\>
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Control flow and data processing\ Alt Type
\<wrap center round lo todo box 80%\>Commands that operate on data and/or affect control flow.\</wrap\>
------------------------------------------------------------------------------------------------------------- -------------------------------------------------------- -------- -----------------
[colon](/commands/builtin/true) \"true\" null command. true special builtin
[dot](/commands/builtin/source) Source external files. source special builtin
[false](/commands/builtin/false) Fail at doing nothing. \- builtin
[continue / break](/commands/builtin/continueBreak) continue with or break out of loops. \- special builtin
[let](/commands/builtin/let) Arithmetic evaluation simple command. \- builtin
[return](/commands/builtin/return) Return from a function with a specified exit status. \- special builtin
[\[](/commands/classictest) The classic `test` simple command. test builtin
Process and Job control\ Alt Type
\<wrap center round lo todo box 80%\>Commands related to jobs, signals, process groups, subshells.\</wrap\>
[exec](/commands/builtin/exec) Replace the current shell process or set redirections. \- special builtin
[exit](/commands/builtin/exit) Exit the shell. \- special builtin
[kill](/commands/builtin/kill) Send a signal to specified process(es) \- builtin
[trap](/commands/builtin/trap) Set signal handlers or output the current handlers. \- special builtin
[times](/commands/builtin/times) Display process times. \- special builtin
[wait](/commands/builtin/wait) Wait for background jobs and asynchronous lists. \- builtin
Alt Type
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\</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
\<note tip\>A list of expressions, words, and their meanings is
[here](/dict/index).\</note\>
## 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 <ircs://irc.libera.chat:6697>, 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\>
\<WRAP clear\>\</WRAP\>