How to tell if you’re using a bash builtin in Linux

A built-in is a Linux command that's part of whatever shell you're using. Can you tell what commands are built-ins and which are not?

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If you’re not sure if you’re running a Linux command or you’re using a bash builtin, don’t stress, it isn’t all that obvious. In fact, you can get very used to commands like cd without realizing that they’re part of your shell, unlike commands like date and whoami that invoke executables (/bin/date and /usr/bin/whoami).

Builtins in general are commands that are built into shell interpreters, and bash is especially rich in them, which is a good thing because built-ins by their very nature run a bit faster than commands which have to be loaded into memory when you call them into play.

In other words, some commands are built into the shell because they pretty much have to be. After all, a command like cd needs to change the shell’s view of the world – or at least its perspective on the file system. Others provide the shell with its special ability to loop and evaluate data – like case, for and while commands. In short, these commands make the shell what it is for all of its devoted users. Still others just make commands run a little faster.

To get a list of bash built-ins, all you have to type is “help”.

$ help
GNU bash, version 5.0.3(1)-release (x86_64-pc-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 -an>
 (( expression ))                               if COMMANDS; then COMMANDS; [ elif COMMANDS>
 . filename [arguments]                         jobs [-lnprs] [jobspec ...] or jobs -x comm>
 :                                              kill [-s sigspec | -n signum | -sigspec] pi>
 [ 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]...) COMMAN>  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 reado>
 compgen [-abcdefgjksuv] [-o option] [-A acti>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DEI] [-o op>  select NAME [in WORDS ... ;] do COMMANDS; d>
 compopt [-o|+o option] [-DEI] [name ...]       set [-abefhkmnptuvxBCHP] [-o option-name] [>
 continue [n]                                   shift [n]
 coproc [NAME] command [redirections]           shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgilnrtux] [-p] [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 [arguments ...>  true
 exit [n]                                       type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or export ->   typeset [-aAfFgilnrtux] [-p] name[=value] .>
 false                                          ulimit [-SHabcdefiklmnpqrstuvxPT] [limit]
 fc [-e ename] [-lnr] [first] [last] or fc -s>  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; don>  until COMMANDS; do COMMANDS; done
 function name { COMMANDS ; } or name () { CO>  variables - Names and meanings of some shel>
 getopts optstring name [arg]                   wait [-fn] [id ...]
 hash [-lr] [-p pathname] [-dt] [name ...]      while COMMANDS; do COMMANDS; done
 help [-dms] [pattern ...]                      { COMMANDS ; }

You might notice that some of these built-ins (e.g., echo and kill) also exist as executables.

$ ls -l /bin/echo /bin/kill
-rwxr-xr-x 1 root root 39256 Sep  5 06:38 /bin/echo
-rwxr-xr-x 1 root root 30952 Aug  8 12:46 /bin/kill

One quick way to determine whether the command you are using is a bash built-in or not is to use the command “command”. Yes, the command is called “command”. Try it with a -V (capital V) option like this:

$ command -V command
command is a shell builtin
$ command -V echo
echo is a shell builtin
$ command -V date
date is hashed (/bin/date)

When you see a “command is hashed” message like the one above, that means that the command has been put into a hash table for quicker lookup.

Looking for help in other shells

If you switch shells and try running “help”, you’ll notice that some support this command and others do not. You can run a command like this in bash to see what each of the shells on your system will tell you:

for shell in `ls /bin/*sh`
    echo $shell
    $shell -c "help"
    echo ===============

This loop will try running the help command in each of the shells in /bin. The $shell -c (e.g., zsh -c) syntax will run just the single help command in that shell and then exit.

How to tell what shell you're currently using

If you switch shells you can’t depend on $SHELL to tell you what shell you’re currently using because $SHELL is just an environment variable that is set when you log in and doesn't necessarily reflect your current shell. Try ps -p $$ instead as shown in these examples:

$ ps -p $$
  PID TTY          TIME CMD
18340 pts/0    00:00:00 bash	<==
$ /bin/dash
$ ps -p $$
  PID TTY          TIME CMD
19517 pts/0    00:00:00 dash	<==

Built-ins are extremely useful and give each shell a lot of its character. If you use some particular shell all of the time, it’s easy to lose track of which commands are part of your shell and which are not. Differentiating a shell built-in from a Linux executable requires only a little extra effort.


Copyright © 2019 IDG Communications, Inc.

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