Essential Linux Commands Every Sysadmin Should Know

This article presents a concise reference of common Linux commands, describing their purpose, typical usage, and useful options.

1. uptime command

The uptime command shows how long the system has been running, the number of logged‑in users, and the load averages for the past 1, 5 and 15 minutes.

# uptime
08:16:26 up 22min, 1 user, load average: 0.00, 0.03, 0.22

To check the version:

# uptime -V
procps version 3.2.8

2. w command

The w command displays currently logged‑in users, their processes, and load averages, along with details such as login name, tty, remote host, login time, idle time, JCPU, PCPU, and the command being executed.

# w
08:27:44 up 34min, 1 user, load average: 0.00, 0.00, 0.08

Available options:

-h: omit the header.

-s: omit JCPU and PCPU.

-f: omit the field information.

-V: display version.

3. users command

The users command lists the usernames of users currently logged in. It has no options besides --help and --version.

# users
Tecmint

4. who command

The who command shows each logged‑in user’s name, date, time, and host information. Unlike w, it does not display what the user is doing.

# who
tecmint pts/0 2012-09-18 07:59 (192.168.50.1)

Options:

-b: show the last system boot time.

-r: show the current runlevel.

-a, --all: display all available information.

5. whoami command

The whoami command prints the effective username of the current user. The equivalent who am i also works.

# whoami
tecmint

6. ls command

The ls command lists files in a directory. Common options shown:

# ls -l
total 114
dr-xr-xr-x. 2 root root 4096 Sep 18 08:46 bin
...

To sort by modification time:

# ls -ltr
total 40
-rw-r--r--. 1 root root 6546 Sep 17 18:42 install.log.syslog
...

7. crontab command

The crontab utility manages user‑specific scheduled tasks. -l lists tasks, -e edits them (typically with vi).

# crontab -l
00 10 * * * /bin/ls >/ls.txt

# crontab -e

8. less command

The less pager allows forward and backward navigation through a file; press q to quit. # less install.log Sample output excerpt:

Installing setup-2.8.14-10.el6.noarch
warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature,keyID c105b9de: NOKEY
Installing filesystem-2.4.30-2.1.el6.i686
...

9. more command

The more pager also displays file contents page by page, showing a percentage progress indicator. # more install.log Sample output is similar to less.

10. cp command

The cp command copies files. -p preserves mode, ownership, and timestamps; -i prompts before overwriting.

# cp -p fileA fileB

# cp -i fileA fileB

11. mv command

The mv command renames or moves files. The -i option prompts before overwriting.

# mv -i fileA fileB

12. cat command

The cat command concatenates and displays file contents. It can be combined with less or more for paging.

# cat fileA fileB

# cat install.log | less

# cat install.log | more

13. cd command

The cd command changes the current working directory.

# cd /fileA

14. pwd command

The pwd command prints the absolute pathname of the current working directory.

# pwd
/root

15. sort command

The sort command orders lines of text alphabetically; -r sorts in reverse order.

# sort fileA.txt

# sort -r fileA.txt

16. vi command

The vi editor is a widely used text editor on Unix‑like systems. The -R option opens a file in read‑only mode.

# vi -R /etc/shadows

17. ssh command

The ssh command initiates a secure remote login. Use -V to display the client version.

# ssh [email protected]

# ssh -V
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010

18. ftp / sftp command

The ftp and sftp commands connect to remote FTP servers; sftp provides encrypted transfers. Commands mput and mget upload and download multiple files.

# ftp 192.168.50.2

# sftp 192.168.50.2

# ftp > mput *.txt

# ftp > mget *.txt

19. service command

The service command runs init scripts located in /etc/init.d/. Example to start the HTTP daemon: # service httpd start or directly:

# /etc/init.d/httpd start

20. free command

The free command reports memory usage, showing total, used, free, shared, buffers, and cached memory. The -t option adds a total line.

# free
total used free shared buffers cached
Mem: 1030800 735944 294856 0 51648 547696
-/+ buffers/cache: 136600 894200
Swap: 2064376 0 2064376

# free -t
...

21. top command

The top utility provides a real‑time view of system processes, CPU usage, and memory consumption. Use -u username to filter by user, O to change sorting, and q to quit.

# top -u tecmint

top – 11:13:11 up 3:19, 2 users, load average: 0.00, 0.00, 0.00
Tasks: 116 total, 1 running, 115 sleeping, 0 stopped, 0 zombie
Cpu(s): 0.0%us, 0.3%sy, 0.0%ni, 99.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 1030800k total, 736188k used, 294612k free, 51760k buffers
Swap: 2064376k total, 0k used, 2064376k free, 547704k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
1889 tecmint 20 0 11468 1648 920 S 0.0 0.2 0:00.59 sshd
...

22. tar command

The tar utility creates and extracts archive files. Example to create an archive of /home: # tar -cvf archive-name.tar /home To extract:

# tar -xvf archive-name.tar

23. grep command

The grep command searches for patterns in files. Use -i for case‑insensitive matching.

# grep tecmint /etc/passwd
tecmint:x:500:500::/home/tecmint:/bin/bash

24. find command

The find command locates files and directories based on criteria. Example searching for the string "tecmint" from the root filesystem:

# find / -name tecmint
/var/spool/mail/tecmint
/home/tecmint
/root/home/tecmint

25. lsof command

The lsof utility lists open files. Example showing files opened by user tecmint:

# lsof -u tecmint
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sshd 1889 tecmint cwd DIR 253,0 4096 2 /
sshd 1889 tecmint txt REG 253,0 532336 298069 /usr/sbin/sshd
...

26. last command

The last command displays a list of recent logins, reboots, and shutdowns, useful for auditing user activity.

# last
tecmint pts/1 192.168.50.1 Tue Sep 18 08:50 still logged in
...

Specify a username to filter:

# last tecmint

27. ps command

The ps command reports a snapshot of current processes. Example showing the init process:

# ps -ef | grep init
root 1 0 0 07:53 ? 00:00:04 /sbin/init
root 7508 6825 0 11:48 pts/1 00:00:00 grep init

28. kill command

The kill command sends signals to processes, typically to terminate them. Find the PID with ps and then use kill -9 PID to force termination.

# kill -9 7508

29. rm command

The rm command removes files. Use -i for interactive confirmation, -r for recursive deletion, and -f to force removal without prompts.

# rm filename

# rm -i test.txt

30. mkdir command

The mkdir command creates new directories. # mkdir directoryname This collection covers the most frequently used Linux commands for everyday system administration.