Create new user
sudo adduser newuser
NOTE: Creating a new user also creates a group by the same name known as the primary group
Change user password
sudo passwd newuser
Grant user admin privileges
visudo
with unlimited root access
newuser ALL=(ALL) all
with restricted access
Cmnd_Alias ADMIN1PRIVILEDGES = /usr/bin/adduser, /usr/bin/usermod, /usr/bin/addgroup
newuser ALL=(root) ADMIN1PRIVILEGES
Change user home directory
usermod --home /home/newuser newuser
Change user shell
usermod --shell /bin/sh
Add descriptive comment to user
usermod --comment "Here is a new demo user" newuser
Add an account expiry date
usermod --expiredate 2022-12-31 newuser
Lock account
usermod --lock newuser
Unlock account
usermod --unlock newuser
Add a password change policy of 60 days
change --maxdays 60 newuser
Delete user account
deluser newuser
Delete user with all files
deluser -r newuser
Create a group
addgrop newgroup
Delete a group
delgroup newgroup
Add a user to a group
usermod -aG newgroup newuser
Remove a user from a group
deluser newuser newgroup
Change file owner to a user
chown newuser file1.txt
Change file group to a group
chgrp newgroup file1.txt
Change file permissions
chmod a+rwx file1.txt //give read+write+execute to all
chmod u+rwx file1.txt //give read+write+execute to owner
chmod o-w file1.txt //remove write access from others different from file owner and group
Or, in the octal form:
chmod 755 file1.txt //equivalent to u+rwx, g+rx, o+rx
permission bits | binary | octal
--x 001 1
-wx 011 3
rwx 111 7
r-x 101 5
rw- 110 6
r-- 100 4
-w- 010 2
To prevent a user from deleting files owned by other users, set the sticky bit on the directory
chmod o+t directory1
To enable others to access the file with the same permission as the owner
chmod u+s file1.txt //apply the setuid bit
To enable others to access the file with the same permission as the group
chmod g+s file1.txt //apply the setgid bit
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