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Some Linux Commands


This isn't majorly a Mechanics post but I've noticed that a lot of people have started using HPC's (which use Linux OS almost all the time), and Linux OS based desktops. It's slightly difficult for people to begin using linux but once you do there's no turning back. Here I aim to create a Linux Command cheat sheet that one can refer to. Or just have it printed and kept in the lab. 

Most of these were borrowed from but I've added in some more explanations and commands.


$ Instructions on how to enter commands

$ command is case sensitive, the flags are used after command. For ex:

$ ls -l  = here ls is the command and -l is the flag

$ multiple flags can be written in minute form by putting them together

$ ls -l -a -R can be written as ls -laR

$ Press up to see command history

$ Press tab to autofill commands

$ Use wildcards to work with specific items:

$ example:  ls *.jpg will list only jpg items

$ ls test.* will show only filenames with *

$ ls **or** will list files with or in their filenames

$ some of these commands might only work on redhat and not on other distros

$ like ubuntu, for example the yum search and yum install commands

$ are redhat commands and for ubuntu have to be replaced with

$ apt-get



1.uptime Command

In Linux uptime command shows how long your system has been running and the number of users currently logged in and also displays load average for 1,5 and 15 minutes intervals.


2. w Command

It will displays users currently logged in and their running process. Also shows the login name, tty name, remote host, login time, idle time, JCPU, PCPU, command and processes.

-h : displays no header entries.

-s : without JCPU and PCPU.

-f : Removes from field.

-V : (upper letter) – Shows versions.


3. users Command

Users command displays currently logged in users. 


4. who Command

who command simply returns user name, date, time and host information. who command is similar to w command. Unlike w command who doesn’t print what users are doing. 

-b : Displays last system reboot date and time.

-r : Shows current runlet.

-a, –all : Displays all information in cumulatively.


5. whoami Command

whoami command prints the name of current user. You can also use “who am i” command to display the current user. If you are logged in as a root using sudo command.


6. ls Command 

ls command displays list of files in human readable format.


Sort file as per last modified time.

-l shows file or directory, size, modified date and time, file or folder name and owner of file and it’s permission.

ls -a

List all files including hidden file starting with ‘.‘.


shows sizes in human readable format.


add the ‘/’ Character at the end each directory.


display files and directories in reverse order.


will list very long listing directory trees.


displays file size in order, will display big in size first.


check ls version (yes, its a double dash)

-l <direcctory name>

list files under directory

-ld <directory name>

displays information

-d */

display only directories


7.  halt

If you are logged in as root, issuing the halt command will cease all CPU function on the system. On most systems, this will drop you into single-user mode and then power off the machine. (Probably a red hat only command)


8. less Command

allows quickly to view file. You can page up and down. Press ‘q‘ to quit from less window.


9. more Command

more command allows to quickly view file and shows details in percentage. You can page up and down. Press ‘q‘ to quit out from more window.

If some text quickly removes itself from the terminal window then you can use more as an addendum to force text output to be page by page controlled by the user like this:

ls -l |more


10. cp Command

Copy file from source to destination.

example: cp fileA fileB  

example cp /tmp/fileA.jpg  /usr/AnotherFolder/fileB.jpg


You will be prompted before overwrite to file.


11. mv Command

Rename fileA to fileB or move a file to a different directory

example of rename: mv fileA fileArenamed

example of move: mv fileA /tmp/directory/subdirectory/


You will be prompted before overwrite to file.


12. cat Command

cat command is used to view multiple file at the same time.

example: cat fileA fileB    or     cat fileA;cat fileB; cat FileC;

cat can  show contents of files

example: cat /usr/directory1/file.txt

creates a file, Awaits input from user, type desired text and press CTRL+D (hold down Ctrl Key and type ‘d‘) to exit. The text will be written in file. You can see contains of file with following cat command.

example: cat >test

redirect text of a file(s) into a new file or else existing file with ‘>‘ (greater than) symbol. Careful, existing contains of test3 will be overwritten by contains of test, test1 and test2 file.

example: cat test test1 test2 > test3

Appends in existing file with ‘>>‘ (double greater than) symbol. Here, contains of test file will be appended at the end of test1 file.

example: cat test >> test1


see the line numbers of all lines of a file


see the line numbers of non empty lines only 


13. Cd command (change directory)

example: cd /directory1/directory2/


14. pwd command (print working directory)

pwd command returns with present working directory. Great for bash scripting


15. Sort command

Sorting lines of text files in ascending order.


descending order


16. vim Command

Vim is a popular text editor available on most of the UNIX-like OS.

example: vim test.txt

will create test.txt or open it if it exists.

Press :q to exit. Press :q! to force exit. Press :w to save. Press :wq to save and quit.


17. ssh Command (Secure Shell)

SSH command is used to login into remote host. Great for working remotely. Use ifconfig to find out  inet address and then ssh into that computer using its inet address.

example:   ssh  



Switch on X11 forwarding


switch on trusted X11 Forwarding

ANY linux system (even your personal computer) can be remoted into if it has the open-ssh libraries installed. Install open ssh libraries in ubuntu with sudo get-apt open-ssh

All you need to know about the computer you want to remote into is the ipaddress which you can find using command 36: ifconfig


18. ftp or sftp Command

ftp or sftp command is used to connect to remote ftp host. ftp is (file transfer protocol) and sftp is (secure file transfer protocol).

Upload multiple files in remote host with mput

ftp > mput test1.txt

mget to download multiple files from remote host.

ftp > mget test2.txt


19. service Command

Service command call script located at /etc/init.d/ directory and execute the script. There are two ways to start the any service. For example we start the service called httpd with service command.

example: service <service name> start


20. free command

Free command shows free, total and swap memory information in bytes.


shows total memory used and available to use in bytes.


21. top Command

top command displays processor activity of your system and also displays tasks managed by kernel in real-time. It’ll show processor and memory are being used. Press ‘O‘ (uppercase letter) to sort as per desired by you. Press ‘q‘ to quit from top screen.

-u <user_name>

shows username specific details


22. Tar Command

tar command is used to compress files and folders in Linux. For example the below command will create a archive for /home directory with file name as archive-name.tar.

tar -cvf archive-name.tar /home

To extract tar archive file use the option as follows.

tar -xvf archive-name.tar


Create a new archive


extract files from archive


verbosely list files processed


use archive file


list the contents of an archive


append files to end of archive


23. Grep Command

grep search for a given string in a file. 


ignore case sensitivity


24. find Command

Find command used to search files, strings and directories.

example: find /usr/ -name test

finds a file called test in directory and subdirectories of usr

-name <text>

search for files with name 'text'


ignores case sensitivity

-type d -name

finds directories only

-type f -name

finds extensions

! -name <text>

Finds files without the text , 'text'

-type f -empty

finds empty files

-type f -name ".*"

finds  all hidden files

/ -mtime 50

finds files modified in the last 50 days

/ -atime 50

finds files accessed in the last 50 days

/ -cmin -30

finds files changed in the last 30 mins

/ -mmin -30

finds files modified in the last 30 mins

/ -amin -30

find files accessed in last 30 mins

/ -size +50M -size -100M

finds files greater than 50 mb less than 100 mb


Combine these flags together to use modified options such as 


find / -size +50M -size -100M -type f ! -name "*.mp3"

Will find any files greater than 50 mb less than 100 mb with an extension that is NOT mp3 


25. lsof Command

List of all open files. 

-u <username>

Opens list of files opened by particular user 


26. last command

With last command we can watch user’s activity in the system. It will display complete user’s info like terminal, time, date, system reboot or boot and kernel version. Useful command to troubleshoot.

example: last      or last <username>


27. ps command

ps command displays about processes running in the system. 

example: ps -ef | grep init


28. kill command

Use kill command to terminate process. First find process id with ps command as shown below and kill process with kill -9 command.

example: 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

kill- 9 7508


29. rm command

rm command used to remove or delete a file without prompting for confirmation.

example: rm test.txt


get confirmation before removing


force delete

WARNING: Permanent deletion occurs of all files when rm is used

to delete contents of the entire directory use:

rm <directoryname>/*


30. mkdir command and rmdir command

mkdir command is used to create directories. rmdir command is used to delete directories


31. alias

make shortcuts to large commands that are used daily

example: alias trialcommand="find / -size +50M -size -100M -type f ! -name "*.mp3""

Now using the command trialcommand will find any files greater than 50 mb less than 100 mb with an extension that is NOT mp3 

example: trialcommand

WARNING: aliases are forgotten everytime you logout

Shows date and time. Can be saved in a variable and used for timed bash scripting


33. chmod

Changes permissions from read only, to read and access only or read and write etc and vice versa. Very important command if you want to execute a file and getting a permission error

example: chmod <filename> number



1st digit owner permission

2nd digit group permission

3rd digit others permission


7read, write and execute

6read and write

5read and execute

4read only

3write and execute

2write only

1execute only



example chmod 764 test.for

test.for will have following permissions:

user can read, write and execute

group can read and write but not execute 

others can only read but not write and execute


recursive, i.e. include objects in subdirectories


force, forge ahead with all objects even if errors occur


verbose, show objects processed


34. man or help

The manual command or help command can be used to pull up help files for all commands

example: man ls  


35. matlab -nodesktop

Opens matlab at the terminal, useful when you do not require the GUI and want to write quick codes or run matlab quickly


35. abaqus commands

abaqus licensing ru

shows licensing details of abaqus, will show how many licenses are being used


abaqus cae -mesa

starts abaqus CAE with mesa drivers, works better if you're on an older compuer


abaquse viewer odb=FILENAME

opens the abaqus viewer with the odb file opened


abaqus cae noGUI

Run the python script


abaqus job=JOBNAME inp=INPUTFILENAME cpus=NO.OF.CPUS interactive

Run an interactive job 


abaqus help |more

Shows the abaqus command help page


abaqus doc

opens the HTML Documentation


35. scp

secure copy can be used to copy files while remoting into a computer from the remote computer to user computer and vice versa


36. ifconfig

Outputs the computer's inet address which you can use to ssh into that computer.


37. kinit

Obtain and cache kerberos ticket granting ticket


38. aklog

obtain tokens for authentication to AFS


39. sudo su -

become root


40. yum search app_name

search for app in RHEL


41 sudo yum install app_name -y

install app


42. echo "3.4562+5.6543" |bc -l

does simple arithematic, use s(), c() for sin cos, a() for arctan, l() natural log,e() exp

example: echo '2*sqrt(25)+s(3.1415/2)' | bc -l  

output: 10.999991282731238


****The below 3 commands are for Red Hat Linux

43. system-config-printer

configure printer


44. system-config-authentication

configure authentication


47. system-config-date

configure date and time setting



48. variable name=value;  echo "$variable_name"

Will output value. This is how you can use variables in the bash shell


49. time <other_command>

Tells you the time it took to run the command other_command

example: time echo 'scale=5000;4*a(1)' |bc -l 

this will calculate pi to 5000 digits and give you the time to calculate it 


50. cat /proc/cpuinfo

Shows cpuinfo and computer info


51. umount /media/<USB Name>

Unmount USB drive


52. poweroff

Shutdown computer


53. reboot

Restart computer





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