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Unix Dweeb

Using the Linux compgen bash builtin

Nov 01, 20237 mins

The Linux compgen command is a bash builtin that's used to provide different varieties of information.

linux code binary

Linux’s compgen command is not actually a Linux command. In other words, it’s not implemented as an executable file, but is instead a bash builtin. That means that it’s part of the bash executable. So, if you were to type “which compgen”, your shell would run through all of the locations included in your $PATH variable, but it just wouldn’t find it.

$ which compgen
/usr/bin/which: no compgen in (.:/home/shs/.local/bin:/home/shs/bin:/usr/local/bin:

Obviously, the which command had no luck in finding it.

If, on the other hand, you type “man compgen”, you’ll end up looking at the man page for the bash shell. From that man page, you can scroll down to this explanation if you’re patient enough to look for it.

Here’s what you should look for …

compgen [option] [word]
       Generate possible completion matches for  word  according  to  the  options,
       which  may be any option accepted by the complete builtin with the exception
       of -p and -r, and write the matches to the standard output.  When using  the
       -F  or  -C options, the various shell variables set by the programmable com‐
       pletion facilities, while available, will not have useful values.

       The matches will be generated in the same way as if the programmable comple‐
       tion  code  had generated them directly from a completion specification with
       the same flags.  If word is specified, only those completions matching  word
       will be displayed.

       The return value is true unless an invalid option is supplied, or no matches
       were generated.

Typing “help compgen” will give you a brief description of just the compgen command:

$ help compgen
compgen: compgen [-abcdefgjksuv] [-o option] [-A action] [-G globpat] [-W wordlist] [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] [word]
    Display possible completions depending on the options.

    Intended to be used from within a shell function generating possible
    completions.  If the optional WORD argument is supplied, matches against
    WORD are generated.

    Exit Status:
    Returns success unless an invalid option is supplied or an error occurs.

Listing commands with compgen

Basically, compgen is a bash builtin which is used to provide different varieties of information. For example, it can list all the commands that could be executed on the Linux system. To get a list of all commands available for direct execution, use a command like the one shown below – but expect a lot of output than what you see here when you do! The output below is seriously truncated.

$ comgen -c | column
big5                                            pkaction
c                                               pkcheck
egrep                                           pkexec
fgrep                                           pkttyagent
grep                                            virt-admin
install                                         virt-host-validate
l.                                              virt-ssh-helper
ll                                              nm-online
ls                                              nmcli
myprocs                                         udisksctl
myps                                            cd-create-profile
rec                                             cd-fix-profile
update                                          cd-iccdump
which                                           cd-it8
xzegrep                                         colormgr

The command below illustrates how many lines of output would be generated by the command shown above if not truncated:

$ compgen -c | wc -l

Listing bash builtins with compgen

You can also use compgen to list all of the bash builtins. I like to pass the output to the column command to make it easier to review.

$ compgen -b | column
.               cd              echo            getopts         mapfile         set             type
:               command         enable          hash            popd            shift           typeset
[               compgen         eval            help            printf          shopt           ulimit
alias           complete        exec            history         pushd           source          umask
bg              compopt         exit            jobs            pwd             suspend         unalias
bind            continue        export          kill            read            test            unset
break           declare         false           let             readarray       times           wait
builtin         dirs            fc              local           readonly        trap
caller          disown          fg              logout          return          true

You can also use the help command to list bash builtins like this:

$ help
GNU bash, version 5.2.15(1)-release (x86_64-redhat-linux-gnu)
These shell commands are defined internally.  Type `help' to see this list.
Type `help name' to find out more about the function `name'.
Use `info bash' to find out more about the shell in general.
Use `man -k' or `info' to find out more about commands not in this list.

A star (*) next to a name means that the command is disabled.

 job_spec [&]                                  history [-c] [-d offset] [n] or history -a>
 (( expression ))                              if COMMANDS; then COMMANDS; [ elif COMMAND>
 . filename [arguments]                        jobs [-lnprs] [jobspec ...] or jobs -x com>
 :                                             kill [-s sigspec | -n signum | -sigspec] p>
 [ arg... ]                                    let arg [arg ...]
 [[ expression ]]                              local [option] name[=value] ...
 alias [-p] [name[=value] ... ]                logout [n]
 bg [job_spec ...]                             mapfile [-d delim] [-n count] [-O origin] >
 bind [-lpsvPSVX] [-m keymap] [-f filename] >  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]...) COMMA>  read [-ers] [-a array] [-d delim] [-i text>
 cd [-L|[-P [-e]] [-@]] [dir]                  readarray [-d delim] [-n count] [-O origin>
 command [-pVv] command [arg ...]              readonly [-aAf] [name[=value] ...] or read>
 compgen [-abcdefgjksuv] [-o option] [-A act>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DEI] [-o o>  select NAME [in WORDS ... ;] do COMMANDS; >
 compopt [-o|+o option] [-DEI] [name ...]      set [-abefhkmnptuvxBCEHPT] [-o option-name>
 continue [n]                                  shift [n]
 coproc [NAME] command [redirections]          shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgiIlnrtux] [name[=value] ...]>  source filename [arguments]
 dirs [-clpv] [+N] [-N]                        suspend [-f]
 disown [-h] [-ar] [jobspec ... | pid ...]     test [expr]
 echo [-neE] [arg ...]                         time [-p] pipeline
 enable [-a] [-dnps] [-f filename] [name ...>  times
 eval [arg ...]                                trap [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [argument ...>  true
 exit [n]                                      type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or export ->  typeset [-aAfFgiIlnrtux] name[=value] ... >
 false                                         ulimit [-SHabcdefiklmnpqrstuvxPRT] [limit]
 fc [-e ename] [-lnr] [first] [last] or fc ->  umask [-p] [-S] [mode]
 fg [job_spec]                                 unalias [-a] name [name ...]
 for NAME [in WORDS ... ] ; do COMMANDS; don>  unset [-f] [-v] [-n] [name ...]
 for (( exp1; exp2; exp3 )); do COMMANDS; do>  until COMMANDS; do COMMANDS-2; done
 function name { COMMANDS ; } or name () { C>  variables - Names and meanings of some she>
 getopts optstring name [arg ...]              wait [-fn] [-p var] [id ...]
 hash [-lr] [-p pathname] [-dt] [name ...]     while COMMANDS; do COMMANDS-2; done
 help [-dms] [pattern ...]                     { COMMANDS ; }

Listing your aliases with compgen

To get compgen to list your aliases, use the -a option like this:

$ compgen -a | column
big5    egrep   grep    l.      ls      myps    update  xzegrep xzgrep  zfgrep
c       fgrep   install ll      myprocs rec     which   xzfgrep zegrep  zgrep

Of course, you could also use the alias command – probably easier to remember — and see the command that each alias is associated with.

$ alias
alias big5='du -h | sort -h | tail -5'
alias c='clear'
alias egrep='egrep --color=auto'
alias fgrep='fgrep --color=auto'
alias grep='grep --color=auto'
alias install='sudo dnf install'
alias l.='ls -d .* --color=auto'
alias ll='ls -l --color=auto'
alias ls='ls --color=auto'
alias myprocs='ps -ef | grep shs'
alias myps='ps -ef | grep shs | awk '''{print $2}''' '
alias rec='ls -ltr | tail -3'
alias update='sudo dnf upgrade –refresh'
alias which='(alias; declare -f) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot'
alias xzegrep='xzegrep --color=auto'
alias xzfgrep='xzfgrep --color=auto'
alias xzgrep='xzgrep --color=auto'
alias zegrep='zegrep --color=auto'
alias zfgrep='zfgrep --color=auto'
alias zgrep='zgrep --color=auto'


The compgen command can come in very handy, but it has a lot of options to use and determine which are the most useful ones. With very brief explanations, they are listed below:

a == aliases
b == builtins
c == commands
d == directories in current file system position
e == exported shell variables
f == files
g == groups
j == jobs
k == shell reserved words
s == services
u == user names
v == shell variables
Unix Dweeb

Sandra Henry-Stocker has been administering Unix systems for more than 30 years. She describes herself as "USL" (Unix as a second language) but remembers enough English to write books and buy groceries. She lives in the mountains in Virginia where, when not working with or writing about Unix, she's chasing the bears away from her bird feeders.

The opinions expressed in this blog are those of Sandra Henry-Stocker and do not necessarily represent those of IDG Communications, Inc., its parent, subsidiary or affiliated companies.

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