Exploring bash builtins on Linux

You most likely use some bash builtins pretty typically whether or not or not you consider them as builtins or just as instructions. In spite of everything, bash builtins are instructions, however not applied as separate executables. As an alternative, they’re a part of the bash executable. In different phrases, they’re “constructed into” bash, thus the time period “bash builtins”.

If you happen to’re in search of a specific builtin, the which command is not going to search out it for you as a result of it solely appears to be like via a set of executables. This consists of system instructions like /bin/echo in addition to scripts for which you could have execute permission. Here is an instance of which not discovering something:

$ which assist
/usr/bin/which: no assist in (/usr/bin:/usr/sbin:/usr/native/bin:/usr/native/sbin:/house/shs/bin:/choose/pash:/house/shs)

As you’ll be able to see, the which command appears to be like down your search path ($PATH) and studies again when it could possibly’t discover a match.

Ask for a person web page on any specific builtin, alternatively, and a person web page describing all the builtins will likely be displayed together with explanations for each. Simply be ready for lots of scrolling as you scan via the output! Here is a line depend for my generated output:

$ man popd | wc -l
2109

NOTE: It does not matter which specific builtin you ask about. You’ll get the identical huge serving to of descriptions for all of them.

$ man shift | wc -l
2109

Ask a few builtin with the bash builtin referred to as “assist” and you are going to get a reasonably thorough clarification of what that builtin will do for you. Since assist is a builtin, it could possibly inform you about itself and some other bash builtin.

$ assist assist
assist: assist [-dms] [pattern ...]
    Show details about builtin instructions.
    
    Shows transient summaries of builtin instructions.  If PATTERN is
    specified, offers detailed assistance on all instructions matching PATTERN,
    in any other case the listing of assist matters is printed.
    
    Choices:
      -d	output brief description for every subject
      -m	show utilization in pseudo-manpage format
      -s	output solely a brief utilization synopsis for every subject matching
    		PATTERN
    
    Arguments:
      PATTERN	Sample specifying a assist subject
    
    Exit Standing:
    Returns success until PATTERN is just not discovered or an invalid choice is given.

To get a listing of all the bash builtins, simply kind “assist” on the command line and you need to get a two-column itemizing like that proven beneath.

$ assist
GNU bash, model 5.2.15(1)-release (x86_64-redhat-linux-gnu)
These shell instructions are outlined internally.  Kind `assist' to see this listing.
Kind `assist title' to search out out extra concerning the operate `title'.
Use `information bash' to search out out extra concerning the shell normally.
Use `man -k' or `information' to search out out extra about instructions not on this listing.

A star (*) subsequent to a reputation implies that the command is disabled.

 job_spec [&]                           historical past [-c] [-d offset] [n] or his>
 (( expression ))                       if COMMANDS; then COMMANDS; [ elif >
 . filename [arguments]                 jobs [-lnprs] [jobspec ...] or jobs>
 :                                      kill [-s sigspec | -n signum | -sig>
 [ arg... ]                             let arg [arg ...]
 [[ expression ]]                       native [option] title[=value] ...
 alias [-p] [name[=value] ... ]         logout [n]
 bg [job_spec ...]                      mapfile [-d delim] [-n count] [-O o>
 bind [-lpsvPSVX] [-m keymap] [-f fil>  popd [-n] [+N | -N]
 break [n]                              printf [-v var] format [arguments]
 builtin [shell-builtin [arg ...]]      pushd [-n] [+N | -N | dir]
 caller [expr]                          pwd [-LP]
 case WORD in [PATTERN [| PATTERN]...>  learn [-ers] [-a array] [-d delim] [>
 cd [-L|[-P [-e]] [-@]] [dir]           readarray [-d delim] [-n count] [-O>
 command [-pVv] command [arg ...]       readonly [-aAf] [name[=value] ...] >
 compgen [-abcdefgjksuv] [-o option] >  return [n]
 full [-abcdefgjksuv] [-pr] [-DEI>  select NAME [in WORDS ... ;] do COM>
 compopt [-o|+o option] [-DEI] [name >  set [-abefhkmnptuvxBCEHPT] [-o opti>
 continue [n]                           shift [n]
 coproc [NAME] command [redirections>   shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgiIlnrtux] [name[=valu>  source filename [arguments]
 dirs [-clpv] [+N] [-N]                 droop [-f]
 disown [-h] [-ar] [jobspec ... | pid>  test [expr]
 echo [-neE] [arg ...]                  time [-p] pipeline
 allow [-a] [-dnps] [-f filename] [n>  times
 eval [arg ...]                         lure [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [argum>  true
 exit [n]                               kind [-afptP] title [name ...]
 export [-fn] [name[=value] ...] or e>  typeset [-aAfFgiIlnrtux] title[=valu>
 false                                  ulimit [-SHabcdefiklmnpqrstuvxPRT] >
 fc [-e ename] [-lnr] [first] [last] >  umask [-p] [-S] [mode]
 fg [job_spec]                          unalias [-a] title [name ...]
 for NAME [in WORDS ... ] ; do COMMAN>  unset [-f] [-v] [-n] [name ...]
 for (( exp1; exp2; exp3 )); do COMMA>  till COMMANDS; do COMMANDS-2; achieved
 operate title { COMMANDS ; } or title>  variables - Names and meanings of s>
 getopts optstring title [arg ...]       wait [-fn] [-p var] [id ...]
 hash [-lr] [-p pathname] [-dt] [name>  while COMMANDS; do COMMANDS-2; done
 help [-dms] [pattern ...]              { COMMANDS ; }

Wrap-Up

There are possible many bash builtins that you just use pretty typically on the command line, a wholesome handful that you just use in scripts, and quantity that you’ve by no means tried. Possibly it is time to discover and study some new methods!