A builtin command is a Linux/Unix command which is "built into a shell interpreter such as sh, ksh, bash, dash, csh etc". Thats where the name came from for these built-in commands. In other words we can say that these commands will always available in RAM so that accessing them is bit fast when compared to external commands which are stored on hard disk. There are many advantages for this type of commands. Below are some advantages.

Increase performance

When we keep frequently execute commands in RAM they will decrease time delay in accessing them from disks and improve performance. Built-in commands will solve this problem by loading them self into RAM when an Interpreter is loaded in to 

Maintain set of commands at minimal shell

Sometimes there is high chance of system crash and you can not access anything on a machine. But if we still have shell access we can execute all builtin commands to do basic troubleshooting and recovery system.

How to list all built-in commands?

There are many ways we can list builtin commands. Commands like compgen and help will help us to list all available builtin commands for a particular shell.
Example:

	[surendra@linuxnix.com ~]$ compgen -b

	 

	Output:

	.

	:

	[

	alias

	bg

	bind

	break

	builtin

	caller

	cd

	command

	compgen

	complete

	compopt

	continue

	declare

	dirs

	disown

	echo

	enable

	eval

	exec

	exit

	export

	false

	fc

	fg

	getopts

	hash

	help

	history

	jobs

	kill

	let

	local

	logout

	mapfile

	popd

	printf

	pushd

	pwd

	read

	readarray

	readonly

	return

	set

	shift

	shopt

	source

	suspend

	test

	times

	trap

	true

	type

	typeset

	ulimit

	umask

	unalias

	unset

	wait

	[surendra@linuxnix.com ~]$

or

[surendra@linuxnix.com ~]$ help

	

		We should make note that "builtin commands varies from shell to shell".

How to check if a Linux command is built-in or not?

To check if a command is external or internal command(built-in) we have to execute type command
Example:

	[surendra@linuxnix.com ~]$ type cd

	Output:

	cd is a shell builtin

Other examples:

[surendra@linuxnix.com ~]$ type pwd
pwd is a shell builtin
[surendra@linuxnix.com ~]$ type ls
ls is aliased to `ls –color=auto'
[surendra@linuxnix.com ~]$ type whoami
whoami is /usr/bin/whoami
If you observe ls and whoami commands are not built-in commands.

	

		How to find Linux built-in command location?

This is bit tricky questions. As said earlier builtin commands are built into the shell and they are not individual files which can be located in the system. So there is no location for built in commands and this is the reason you will get below error for some of the built-in commands when you try to locate them.

[surendra@linuxnix.com ~]$ which cd
/usr/bin/which: no cd in (/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/visplad/bin)
[surendra@linuxnix.com ~]$ which let
/usr/bin/which: no let in (/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/visplad/bin)

	How to find size of built-in commands?

Again this is bit tricky. We can not find the size of individual built-in commands as they are part and parcel of your shell. To know the size of your shell just execute below command


[surendra@linuxnix.com ~]$ du -hs $(which bash)
884K    /bin/bash
[surendra@linuxnix.com ~]$

Replace bash with your desired shell.

How to get help on built-in commands?

So the important question is how to get help on these built-in commands. When you try to execute man on these commands you will get below error

surendra@linuxnix.com:~$ man cd
No manual entry for cd
surendra@linuxnix.com:~$

Then where we can get help on them?

There is special command to help you on this. The command to get more help on Linux/Unix built-in commands is help. To know about a command in detail just execute below command.


help built-in_command

Example

help cd
or
help -m cd

Output:

[surendra@linuxnix.com ~]$ help -m cd
NAME
   cd – Change the shell working directory.


SYNOPSIS
   cd [-L|-P] [dir]


DESCRIPTION
   Change the shell working directory.


   Change the current directory to DIR.  The default DIR is the value of the
   HOME shell variable.


   The variable CDPATH defines the search path for the directory containing
   DIR.  Alternative directory names in CDPATH are separated by a colon (:).
   A null directory name is the same as the current directory.  If DIR begins
   with a slash (/), then CDPATH is not used.


   If the directory is not found, and the shell option `cdable_vars' is set,
   the word is assumed to be  a variable name.  If that variable has a value,
   it’s value is used for DIR.


   Options:
       -L      force symbolic links to be followed
       -P      use the physical directory structure without following symbolic
       links


   The default is to follow symbolic links, as if `-L' were specified.


   Exit Status:
   Returns 0 if the directory is changed; non-zero otherwise.


SEE ALSO
   bash(1)


IMPLEMENTATION
   GNU bash, version 4.1.2(1)-release (x86_64-redhat-linux-gnu)
   Copyright (C) 2009 Free Software Foundation, Inc.
   License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

To get single line answer use -d option with help command.

[surendra@linuxnix.com ~]$ help -d cd
cd – Change the shell working directory.

To get simple command options use -s option.


[surendra@linuxnix.com ~]$ help -s cd
cd: cd [-L|-P] [dir]
 

Hope this helps to understand about built-in commands in Linux.

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Surendra Anne

Mr Surendra Anne is from Vijayawada, Andhra Pradesh, India. He is a Linux/Open source supporter who believes in Hard work, A down to earth person, Likes to share knowledge with others, Loves dogs, Likes photography. He works as Devops Engineer with Taggle systems, an IOT automatic water metering company, Sydney . You can contact him at surendra (@) linuxnix dot com.