Category Archives: Linux

1.  ls   list all the contents of a directory  (Display One File Per Line Using ls -1 | Display File Size in Human Readable Format Using ls -lh | Display Hidden Files Using ls -a (or) ls -A | )

2. ll list all the contents of a directory with its permissions, size etc

3. cd / cd .. move in an out of directories

4. mv / cp:         Copy file1 to file2 preserving the mode, ownership and timestamp.

$ cp -p file1 file2

Copy file1 to file2. if file2 exists prompt for confirmation before overwritting it.

$ cp -i file1 file2

mv command examples

Rename file1 to file2. if file2 exists prompt for confirmation before overwritting it.

$ mv -i file1 file2

Note: mv -f is just the opposite, which will overwrite file2 without prompting.

mv -v will print what is happening during file rename, which is useful in some cases.

$ mv -v file1 file2

5. man to find documentation about a command

6. mkdir: to make a directory

mkdir command examples Following example creates a directory called temp under your home directory.

$ mkdir ~/temp

Create nested directories using one mkdir command. If any of these directories exist already, it will not display any error. If any of these directories doesn’t exist, it will create them.

$ mkdir -p dir1/dir2/dir3/dir4/

7. rmdir  to delete a directory

8. touch to create an empty file

9.rm  to remove a file

10. clear (same as cls in winows) clear the clutter from terminal window

11. rm -rf / << the command that we should never use. it will clean the whole system. 

12.TAR: Create a new tar archive.

$ tar cvf archive_name.tar dirname/

Extract from an existing tar archive.

$ tar xvf archive_name.tar

View an existing tar archive.

$ tar tvf archive_name.tar

13. grep : to extract data. check example.

nesi@Alpha:~$ ip route
default via 192.168.1.1 dev enp7s0  proto static  metric 100
default via 192.168.75.3 dev wlp6s0  proto static  metric 600
192.168.1.0/24 dev enp7s0  proto kernel  scope link  src 192.168.1.95  metric 100
192.168.75.0/24 dev wlp6s0  proto kernel  scope link  src 192.168.75.94  metric 600
192.168.250.0/24 dev anbox0  proto kernel  scope link  src 192.168.250.1
nesi@Alpha:~$ ip route | grep default 
default via 192.168.1.1 dev enp7s0  proto static  metric 100
default via 192.168.75.3 dev wlp6s0  proto static  metric 600

14. sudo / su:
su command examples : Switch to a different user account using su command. Super user can switch to any other user without entering their password. some linux distributions nowadays only allow sudo.

$ su - USERNAME

Execute a single command from a different account name. In the following example, john can execute the ls command as raj username. Once the command is executed, it will come back to john’s account.

[john@dev-server]$ su - raj -c 'ls'

[john@dev-server]$

Sudo: is same like “run as administrator” in windows

15. pwd (print working directory) show us the current directory in which the terminal is running on.

16. ps show currently running processes

17. passwd command  : Change your password from command line using passwd. This will prompt for the old password followed by the new password.

$ passwd

Super user can use passwd command to reset others password. This will not prompt for current password of the user.

# passwd USERNAME

Remove password for a specific user. Root user can disable password for a specific user. Once the password is disabled, the user can login without entering the password.

# passwd -d USERNAME

18. ifconfig command :  Use ifconfig command to view or configure a network interface on the Linux system.View all the interfaces along with status.

$ ifconfig -a

Start or stop a specific interface using up and down command as shown below.

$ ifconfig eth0 up

$ ifconfig eth0 down

set IP and SM :

$ ifconfig eth0 192.168.50.5 netmask 255.255.255.0

linux-network-configuration-and-troubleshooting-commands

19. uname command : Uname command displays important information about the system such as — Kernel name, Host name, Kernel release number,
Processor type, etc.,Sample uname output from a Ubuntu laptop is shown below.

$ uname -a
Linux Alpha 4.10.0-35-generic #39~16.04.1-Ubuntu SMP Wed Sep 13 09:02:42 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

20. whatis command : Whatis command displays a single line description about a command.

$ whatis ls
ls		(1)  - list directory contents

$ whatis ifconfig
ifconfig (8)         - configure a network interface

21. whereis command : When you want to find out where a specific Unix command exists (for example, where does ls command exists?), you can execute the following command.

$ whereis ls
ls: /bin/ls /usr/share/man/man1/ls.1.gz

22. locate command : Using locate command you can quickly search for the location of a specific file (or group of files). Locate command uses the database created by updatedb. The example below shows all files in the system that contains the word crontab in it.

$ locate crontab
/etc/anacrontab
/etc/crontab
/usr/bin/crontab
/usr/share/doc/cron/examples/crontab2english.pl.gz
/usr/share/man/man1/crontab.1.gz

23. tail command : read a text file on terminal.
usage examples: Print the last 10 lines of a file by default.

$ tail filename.txt

Print N number of lines from the file named filename.txt N refers to the number of lines you want to print

$ tail -n N filename.txt

View the content of the file in real time using tail -f. This is useful to view the log files, that keeps growing. The command can be terminated using CTRL-C.

$ tail -f log-file

24. less command : less is very efficient while viewing huge log files, as it doesn’t need to load the full file while opening.

$ less huge-log-file.log

One you open a file using less command, following two keys are very helpful.

CTRL+F – forward one window
CTRL+B – backward one window

25. Kill Command – Kill the process by specifying its PID:  to kill conventions by sending the TERM signal to the specified process. For the signals, either the signal name or signal number can be used. You need to lookup the pid (process ID) for the process and give it as an argument to kill. Use PS or Top to find pid 
exmple:

nesi@Alpha:~$ ps
  PID TTY          TIME CMD
11671 pts/1    00:00:00 bash
12194 pts/1    00:00:00 ps
nesi@Alpha:~$ top
9658 nesi      20   0 1790380 367880  32344 S  12.5  9.3   1:00.76 chromium-b+ 
 3604 nesi       9 -11  592688   6124   4160 S   6.2  0.2   3:39.36 pulseaudio  
12212 nesi      20   0   59776   3524   2948 R   6.2  0.1   0:00.01 top         
    1 root      20   0  185312   2104    840 S   0.0  0.1   0:01.52 systemd     
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.01 kthreadd    
    4 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/0:+ 
    6 root      20   0       0      0      0 S   0.0  0.0   0:00.15 ksoftirqd/0 
nesi@Alpha:~$ kill 9658

in this case the kill command will kill the chromium app

26. Xkill Command – kill a client by X resource:  xkill is the simplest way to kill a malfunctioning program. When you want to kill a process, initiate xkill which will offer an cross-hair cursor. Click on the window with left cursor which will kill that process. command: xkill

27. top shows information about the top processes in the system (sorted by CPU usage by default): top may not be installed by default. Htop is another app that gives a more detailed approach to this purpose.

28. alias : to assign an alias for a command. for example, to update our package repositories we need to run “ sudo apt-get update” which is a long command. using alias we can make it shorter like may be “update” see the another example below:
we often ping to google to check if our internet is working.

nesi@Alpha:~$ ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=58 time=87.8 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=58 time=86.4 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=58 time=86.6 ms
64 bytes from 8.8.8.8: icmp_seq=4 ttl=58 time=87.2 ms
nesi@Alpha:~$ alias g='ping 8.8.8.8'
nesi@Alpha:~$ g
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=58 time=87.5 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=58 time=86.5 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=58 time=86.7 ms

29. apt / apt-get | install, update, dist-upgrade, remove etc : APT-GET How to

30. hostname : display the host name of the system (in case if you want to change the hostname of your laptop, you have to do it inside /etc/hosts file. open it up in gedit or any other text editor and change it. Ex: “sudo gedit /etc/hosts” )

31. uptime – Reports how long the system has been running since last boot. Extremely useful for servers.

32. lsb_release -a or –all: Displays some relevan all of the above information. For instance, if you are running Arch Linux, this will display
LSB Version: 1.4
Distributor ID: Arch
Description: Arch Linux
Release: rolling
Codename: n/a   (lsb_release is part of a software package called the LSB core, which is not necessarily installed on your system by default. to install on ubuntu: sudo apt-get update && sudo apt-get install lsb-core | on CentOS :sudo yum update && sudo yum install redhat-lsb-core)

33. who : shows the list of currently logged in users.

34. shutdown: shuts down your computer. You can use shutdown -r to restart your computer. (you may have to run it as root, sudo /su)