Built in commands linux

What is a built-in command in Linux?

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:

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:

]$ 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.

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.

]$ 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)

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

]$ 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

$ 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:

]$ 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

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

]$ help -d cd
cd – Change the shell working directory.

To get simple command options use -s option.

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

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A – Z Linux Commands – Overview with Examples

Last Updated On: 7th October 2019

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There are a countless number of commands in Linux. We are bound to use a number of them on a daily routine or numerous times to perform common tasks than others. It is important to note that certain commands are “distro-based” – they can only be found in specific distros. While others are generic Unix/Linux commands that you’ll find in all if not most mainstream distros.

In this article, we will introduce you a list of most frequently used Linux commands with their examples for easy learning. You can find the actual description of each Linux command in their manual page which you can access like this:

adduser/addgroup Command

The adduser and addgroup commands are used to add a user and group to the system respectively according to the default configuration specified in /etc/adduser.conf file.

For more adduser and addgroup commands: 15 Practical Examples on adduser Command in Linux

agetty Command

agetty is a program which manages physical or virtual terminals and is invoked by init. Once it detects a connection, it opens a tty port, asks for a user’s login name and calls up the /bin/login command. Agetty is a substitute of Linux getty:

alias Command

alias is a useful shell built-in command for creating aliases (shortcut) to a Linux command on a system. It is helpful for creating new/custom commands from existing Shell/Linux commands (including options):

The above command will create an alias called home for /home/tecmint/public_html directory, so whenever you type home in the terminal prompt, it will put you in the /home/tecmint/public_html directory.

anacron Command

anacron is a Linux facility used to run commands periodically with a frequency defined in days, weeks and months.

Unlike its sister cron; it assumes that a system will not run continuously, therefore if a scheduled job is due when the system is off, it’s run once the machine is powered on.

For more information about anacron and cron read: Cron Vs Anacron: How to Schedule Jobs Using Anacron on Linux

apropos Command

apropos command is used to search and display a short man page description of a command/program as follows.

apt Command

apt tool is a relatively new higher-level package manager for Debian/Ubuntu systems:

apt-get Command

apt-get is a powerful and free front-end package manager for Debian/Ubuntu systems. It is used to install new software packages, remove available software packages, upgrade existing software packages as well as upgrade entire operating system.

aptitude Command

aptitude is a powerful text-based interface to the Debian GNU/Linux package management system. Like apt-get and apt; it can be used to install, remove or upgrade software packages on a system.

arch Command

arch is a simple command for displaying machine architecture or hardware name (similar to uname -m):

arp Command

ARP (Address Resolution Protocol) is a protocol that maps IP network addresses of a network neighbor with the hardware (MAC) addresses in an IPv4 network.

You can use it as below to find all alive hosts on a network:

at Command

at command is used to schedule tasks to run in a future time. It’s an alternative to cron and anacron, however, it runs a task once at a given future time without editing any config files:

For example, to shutdown the system at 23:55 today, run:

atq Command

atq command is used to view jobs in at command queue:

atrm Command

atrm command is used to remove/deletes jobs (identified by their job number) from at command queue:

For more usage about at command, read: How to Use ‘at’ Command to Schedule a Task in Linux

awk Command

Awk is a powerful programming language created for text processing and generally used as a data extraction and reporting tool.

For more Awk concepts (basic and advance) with simple and easy-to-understand, we’ve created a book that contains 13 chapters with a total of 41 pages, which covers all Awk basic and advance usage with practical examples: Awk Getting Started Guide for Beginners

batch Command

batch is also used to schedule tasks to run a future time, similar to the at command.

basename Command

basename command helps to print the name of a file stripping of directories in the absolute path:

bc Command

bc is a simple yet powerful and arbitrary precision CLI calculator language which can be used like this:

bg Command

bg is a command used to send a process to the background.

bzip2 Command

bzip2 command is used to compress or decompress file(s).

cal Command

The cal command print a calendar on the standard output.

cat Command

cat command is used to view contents of a file or concatenate files, or data provided on standard input, and display it on the standard output.

To learn more about cat command, read: 13 Useful Cat Command Examples on Linux

chgrp Command

chgrp command is used to change the group ownership of a file. Provide the new group name as its first argument and the name of file as the second argument like this:

chmod Command

chmod command is used to change/update file access permissions like this.

chown Command

chown command changes/updates the user and group ownership of a file/directory like this.

To learn more about chgrp, chmod and chwon commands, read: Managing Users & Groups, File Permissions & Attributes in Linux

cksum Command

cksum command is used to display the CRC checksum and byte count of an input file.

clear Command

clear command lets you clear the terminal screen, simply type.

cmp Command

cmp performs a byte-by-byte comparison of two files like this.

comm Command

comm command is used to compare two sorted files line-by-line as shown below.

cp Command

cp command is used for copying files and directories from one location to another.

For more information on how to copy files in Linux, read:

date Command

date command displays/sets the system date and time like this.

To learn more about how to set date in Linux, read: How to Set System Date in Linux

dd Command

dd command is used for copying files, converting and formatting according to flags provided on the command line. It can strip headers, extracting parts of binary files and so on.

The example below shows creating a boot-able USB device:

df Command

df command is used to show file system disk space usage as follows.

diff Command

diff command is used to compare two files line by line. It can also be used to find the difference between two directories in Linux like this:

dir Command

dir command works like Linux ls command, it lists the contents of a directory.

dmidecode Command

dmidecode command is a tool for retrieving hardware information of any Linux system. It dumps a computer’s DMI (a.k.a SMBIOS) table contents in a human-readable format for easy retrieval.

To view your system hardware info, you can type:

Some useful tools to find out Linux system hardware info: 10 Useful Commands to Get Linux Hardware Information

du Command

du command is used to show disk space usage of files present in a directory as well as its sub-directories as follows.

echo Command

echo command prints a text of line provided to it.

eject Command

eject command is used to eject removable media such as DVD/CD ROM or floppy disk from the system.

env Command

env command lists all the current environment variables and used to set them as well.

exit Command

exit command is used to exit a shell like so.

expr Command

expr command is used to calculate an expression as shown below.

factor Command

factor command is used to show the prime factors of a number.

find Command

find command lets you search for files in a directory as well as its sub-directories. It searches for files by attributes such as permissions, users, groups, file type, date, size and other possible criteria.

Learn more on how to find files in Linux:

free Command

free command shows the system memory usage (free, used, swapped, cached, etc.) in the system including swap space. Use the -h option to display output in human friendly format.

Learn more on how to find memory usage in Linux.

grep Command

grep command searches for a specified pattern in a file (or files) and displays in output lines containing that pattern as follows.

Learn more about grep command usage in Linux.

groups Command

groups command displays all the names of groups a user is a part of like this.

gzip Command

Gzip helps to compress a file, replaces it with one having a .gz extension as shown below:

gunzip Command

gunzip expands or restores files compressed with gzip command like this.

head Command

head command is used to show first lines (10 lines by default) of the specified file or stdin to the screen:

history Command

history command is used to show previously used commands or to get info about command executed by a user.

Learn more about Linux history command.

hostname Command

hostname command is used to print or set system hostname in Linux.

hostnamectl Command

hostnamectl command controls the system hostname under systemd. It is used to print or modify the system hostname and any related settings:

hwclock

hwclock is a tool for managing the system hardware clock; read or set the hardware clock (RTC).

hwinfo Command

hwinfo is used to probe for the hardware present in a Linux system like this.

Learn more about how to get Linux hardware info.

id Command

id command shows user and group information for the current user or specified username as shown below.

ifconfig Command

ifconfig command is used to configure a Linux systems network interfaces. It is used to configure, view and control network interfaces.

ionice Command

ionice command is used to set or view process I/O scheduling class and priority of the specified process.

If invoked without any options, it will query the current I/O scheduling class and priority for that process:

To understand how it works, read this article: How to Delete HUGE (100-200GB) Files in Linux

iostat Command

iostat is used to show CPU and input/output statistics for devices and partitions. It produces useful reports for updating system configurations to help balance the input/output load between physical disks.

ip Command

ip command is used to display or manage routing, devices, policy routing and tunnels. It also works as a replacement for well known ifconfig command.

This command will assign an IP address to a specific interface (eth1 in this case).

iptables Command

iptables is a terminal based firewall for managing incoming and outgoing traffic via a set of configurable table rules.

The command below is used to check existing rules on a system (using it may require root privileges).

Learn more about iptables firewall in Linux.

iw Command

iw command is used to manage wireless devices and their configuration.

iwlist Command

iwlist command displays detailed wireless information from a wireless interface. The command below enables you to get detailed information about the wlp1s0 interface.

kill Command

kill command is used to kill a process using its PID by sending a signal to it (default signal for kill is TERM).

killall Command

killall command is used to kill a process by its name.

Learn more about kill and killall command in Linux.

kmod Command

kmod command is used to manage Linux kernel modules. To list all currently loaded modules, type.

last Command

last command display a listing of last logged in users.

ln Command

ln command is used to create a soft link between files using the -s flag like this.

locate Command

locate command is used to find a file by name. The locate utility works better and faster than it’s find counterpart.

The command below will search for a file by its exact name (not *name*):

login Command

login command is used to create a new session with the system. You’ll be asked to provide a username and a password to login as below.

ls Command

ls command is used to list contents of a directory. It works more or less like dir command.

The -l option enables long listing format like this.

To know more about ls command, read our guides.

lshw Command

lshw command is a minimal tool to get detailed information on the hardware configuration of the machine, invoke it with superuser privileges to get a comprehensive information.

lscpu Command

lscpu command displays system’s CPU architecture information (such as number of CPUs, threads, cores, sockets, and more).

lsof Command

lsof command displays information related to files opened by processes. Files can be of any type, including regular files, directories, block special files, character special files, executing text reference, libraries, and stream/network files.

To view files opened by a specific user’s processes, type the command below.

lsusb Command

lsusb command shows information about USB buses in the system and the devices connected to them like this.

man Command

man command is used to view the on-line reference manual pages for commands/programs like so.

md5sum Command

md5sum command is used to compute and print the MD5 message digest of a file. If run without arguments, debsums checks every file on your system against the stock md5sum files:

mkdir Command

mkdir command is used to create single or more directories, if they do not already exist (this can be overridden with the -p option).

more Command

more command enables you to view through relatively lengthy text files one screenful at a time.

Check difference between more and less command and Learn Why ‘less’ is Faster Than ‘more’ Command

mv Command

mv command is used to rename files or directories. It also moves a file or directory to another location in the directory structure.

nano Command

nano is a popular small, free and friendly text editor for Linux; a clone of Pico, the default editor included in the non-free Pine package.

To open a file using nano, type:

nc/netcat Command

nc (or netcat) is used for performing any operation relating to TCP, UDP, or UNIX-domain sockets. It can handle both IPv4 and IPv6 for opening TCP connections, sending UDP packets, listening on arbitrary TCP and UDP ports, performing port scanning.

The command below will help us see if the port 22 is open on the host 192.168.56.5.

Learn more examples and usage on nc command.

netstat Command

netstat command displays useful information concerning the Linux networking subsystem (network connections, routing tables, interface statistics, masquerade connections, and multicast memberships).

This command will display all open ports on the local system:

nice Command

nice command is used to show or change the nice value of a running program. It runs specified command with an adjusted niceness. When run without any command specified, it prints the current niceness.

The following command starts the process “tar command” setting the “nice” value to 12.

nmap Command

nmap is a popular and powerful open source tool for network scanning and security auditing. It was intended to quickly scan large networks, but it also works fine against single hosts.

The command below will probe open ports on all live hosts on the specified network.

nproc Command

nproc command shows the number of processing units present to the current process. It’s output may be less than the number of online processors on a system.

openssl Command

The openssl is a command line tool for using the different cryptography operations of OpenSSL’s crypto library from the shell. The command below will create an archive of all files in the current directory and encrypt the contents of the archive file:

passwd Command

passwd command is used to create/update passwords for user accounts, it can also change the account or associated password validity period. Note that normal system users may only change the password of their own account, while root may modify the password for any account.

pidof Command

pidof displays the process ID of a running program/command.

ping Command

ping command is used to determine connectivity between hosts on a network (or the Internet):

ps Command

pstree Command

pstree displays running processes as a tree which is rooted at either PID or init if PID is omitted.

pwd Command

pwd command displays the name of current/working directory as below.

rdiff-backup Command

rdiff-backup is a powerful local/remote incremental backup script written in Python. It works on any POSIX operating system such as Linux, Mac OS X.

Note that for remote backups, you must install the same version of rdiff-backup on both the local and remote machines. Below is an example of a local backup command:

reboot Command

reboot command may be used to halt, power-off or reboot a system as follows.

rename Command

rename command is used to rename many files at once. If you’ve a collection of files with “.html” extension and you want to rename all of them with “.php” extension, you can type the command below.

rm command

rm command is used to remove files or directories as shown below.

rmdir Command

rmdir command helps to delete/remove empty directories as follows.

scp Command

scp command enables you to securely copy files between hosts on a network, for example.

shutdown Command

shutdown command schedules a time for the system to be powered down. It may be used to halt, power-off or reboot the machine like this.

sleep Command

sleep command is used to delay or pause (specifically execution of a command) for a specified amount of time.

sort Command

sort command is used to sort lines of text in the specified file(s) or from stdin as shown below

Learn more examples of sort command in Linux.

split Command

split as the name suggests, is used to split a large file into small parts.

ssh Command

ssh (SSH client) is an application for remotely accessing and running commands on a remote machine. It is designed to offer a secure encrypted communications between two untrusted hosts over an insecure network such as the Internet.

Learn more about ssh command and how to use it on Linux.

stat Command

stat is used to show a file or file system status like this ( -f is used to specify a filesystem).

su Command

su command is used to switch to another user ID or become root during a login session. Note that when su is invoked without a username, it defaults to becoming root.

sudo Command

sudo command allows a permitted system user to run a command as root or another user, as defined by the security policy such as sudoers.

In this case, the real (not effective) user ID of the user running sudo is used to determine the user name with which to query the security policy.

Learn more about sudo command and how to use it on Linux.

sum Command

sum command is used to show the checksum and block counts for each each specified file on the command line.

tac Command

tac command concatenates and displays files in reverse. It simply prints each file to standard output, showing last line first.

tail Command

tail command is used to display the last lines (10 lines by default) of each file to standard output.

If there more than one file, precede each with a header giving the file name. Use it as follow (specify more lines to display using -n option).

talk Command

talk command is used to talk to another system/network user. To talk to a user on the same machine, use their login name, however, to talk to a user on another machine use ‘[email protected]’.

tar Command

tar command is a most powerful utility for archiving files in Linux.

Learn more about tar command and its usage on Linux.

tee Command

tee command is used to read from standard input and prints to standard output and files as shown below.

tree Command

The tree command is a tiny, cross-platform command-line program used to recursively list or display the content of a directory in a tree-like format.

time Command

time command runs programs and summarizes system resource usage.

top Command

top program displays all processes on a Linux system in regards to memory and CPU usage and provides a dynamic real-time view of a running system.

touch Command

touch command changes file timestamps, it can also be used to create a file as follows.

tr Command

tr command is a useful utility used to translate (change) or delete characters from stdin, and write the result to stdout or send to a file as follows.

uname Command

uname command displays system information such as operating system, network node hostname kernel name, version and release etc.

Use the -a option to show all the system information:

uniq Command

uniq command displays or omits repeated lines from input (or standard input). To indicate the number of occurrences of a line, use the -c option.

uptime Command

uptime command shows how long the system has been running, number of logged on users and the system load averages as follows.

users Command

users command shows the user names of users currently logged in to the current host like this.

vim/vi Command

vim (Vi Improved) popular text editor on Unix-like operating systems. It can be used to edit all kinds of plain text and program files.

Learn how to use vi/vim editor in Linux along with some tips and tricks.

w Command

w command displays system uptime, load averages and information about the users currently on the machine, and what they are doing (their processes) like this.

wall Command

wall command is used to send/display a message to all users on the system as follows.

watch Command

watch command runs a program repeatedly while displaying its output on fullscreen. It can also be used to watch changes to a file/directory. The example below shows how to watch the contents of a directory change.

wc Command

wc command is used to display newline, word, and byte counts for each file specified, and a total for many files.

wget Command

wget command is a simple utility used to download files from the Web in a non-interactive (can work in the background) way.

whatis Command

whatis command searches and shows a short or one-line manual page descriptions of the provided command name(s) as follows.

which Command

which command displays the absolute path (pathnames) of the files (or possibly links) which would be executed in the current environment.

who Command

who command shows information about users who are currently logged in like this.

whereis Command

whereis command helps us locate the binary, source and manual files for commands.

xargs Command

xargs command is a useful utility for reading items from the standard input, delimited by blanks (protected with double or single quotes or a backslash) or newlines, and executes the entered command.

The example below show xargs being used to copy a file to multiple directories in Linux.

yes Command

yes command is used to display a string repeatedly until when terminated or killed using [ Ctrl + C ] as follows.

youtube-dl Command

youtube-dl is a lightweight command-line program to download videos and also extract MP3 tracks from YouTube.com and a few more sites.

The command below will list available formats for the video in the provided link.

zcmp/zdiff Command

zcmp and zdiff minimal utilities used to compare compressed files as shown in the examples below.

zip Command

zip is a simple and easy-to-use utility used to package and compress (archive) files.

zz Command

zz command is an alias of the fasd commandline tool that offers quick access to files and directories in Linux. It is used to quickly and interactively cd into a previously accessed directory by selecting the directory number from the first field as follows.

That’s it for now! As we mentioned before, there are a countless number of commands in Linux. The list is possibly longer than we can offer. Use the feedback form below to share any useful and frequently used commands missing in this list.

26 thoughts on “A – Z Linux Commands – Overview with Examples”

1. 

A very good list, but you list split but no join command?

Very good articles on the Linux commands, much appreciated the effort put into it…

I don’t know who has invented IT it is more demanding and tough to learn for those who are not passionate about it.

What is the command for print any file??

• 

To print files in Linux from command-line.

Where is the cd command?

• 

You are right! They are not executed cd command!

Really helpful articles on Linux commands, thanks a lot for putting all together..

Okay, let’s say for a second someone has made themselves admin WITHOUT MY PERMISSION… Can I remove them using Linux?

• 

Gosh I hope so. This is a simplistic solution and doesn’t consider the actual situation that you may be in:

Of course you’ll want to figure out how they got admin without your permission (sudo config, shared password, etc.)

Of course you can. As long as you belong to the superuser groups. You need to use the *userdel* command to delete a user account and related files from user account under Linux.

The *userdel* is the way to do it on the command line.

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