Some linux commands that I’ll probably need in the future

User management

Add user to a group

sudo usermod -aG group user


sudo adduser <user> <group>

Delete user

userdel user

Delete the user’s home directory and mail spool:

userdel -r user

Remove user from a group

sudo gpasswd -d user group

list all users:

$ getent passwd

list all groups:

$ getent group

list all groups of the current user:

$ groups

list all groups of a user:

$ groups <user>

Change shell for user user to bash

chsh -s /bin/bash user

Disable user from login including ssh

sudo usermod --expiredate 1 user1

Reenable the user:

sudo usermod --expiredate "" user1


Kill processes occupying a certain port:

fuser -k 8080/tcp

Detach process

Sometimes I need to detach from a process running on a remote machine so that it continues running after I logout.

Using the Job Control of bash to send the process into the background:

  • Ctrl+Z to stop (pause) the program and get back to the shell.
  • bg to run it in the background.
  • disown -h [job-spec] where [job-spec] is the job number (like %1 for the first running job; find about your number with the jobs command) so that the job isn’t killed when the terminal closes.


Unrfortunately disown is specific to bash and not available in all shells.

Certain flavours of Unix (e.g. AIX and Solaris) have an option on the nohup command itself which can be applied to a running process:

nohup -p pid

The first of the commands below starts the program abcd in the background in such a way that the subsequent logout does not stop it.

$ nohup abcd &
$ exit



Scan IP range

Generally, nmap is useful to quickly scan networks.

To install nmap, enter

sudo apt-get install nmap

Once it is installed, enter

nmap -sn

This will show you which hosts responded to ping requests on the network between and



Replace text in a file using command line

replace regex in files using command line and sed:

sed -i -E 's/source/destination/g' ./file.txt

all occurrences of source will be replaced with destination and the substitution will be done in-place. No backup!

More complex example:

sed -i.back -E 's/^(\S+) 123 (\S+)$/\1 456 \2/g' ./file.txt

Here I replace 123 to 456 when it is between two other words in a string. For example

123some_word_without_spaces 123 must_match
aaa 123 multiple words - not matched

will become

123some_word_without_spaces 456 must_match
aaa 123 multiple words - not matched

Also there will be a backup file with .back extension.

If your text contain slashes, you can use another delimiter:

xargs sed -i.original "s|text/to/find/|text/to/put|g" ./file.txt

Text replacement recursively in many files

replacing all occurences of source with destination in files *.txt, inplace, with a backup.

find . -name "*.txt" -exec sed -i.back -E 's/source/destination/g' {} \;


Encode/decode binary file to ascii using command line


Restart now:

shutdown -r 0

Disk space

df - check free disk space

baobab - free disk space

See also