Show contents of an archive

tar -tf archive.tar

Allows write opening of a file only append mode

chattr +a file

check correct syntax and file format of ‘/etc/passwd’ and users existence

pwck

Check correct syntax and file format of ‘/etc/group’ and groups existence

grpck

Delete a user ( ‘-r’ eliminates home directory)

userdel -r user1

Set deadline for user password

chage -E 2008-12-31 user_name

Change a user’s attributes

usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1

Create a new user belongs “admin” group called ‘user1′

useradd -c "Name Surname " -g admin -d /home/user1 -s /bin/bash user1

There is a simpler command too…

useradd user1

Show space used by deb packages installed sorted by size (ubuntu, debian and like)

dpkg-query -W -f='${Installed-Size;10}t${Package}n' | sort -k1,1n

Rename a group

groupmod -n new_group_name old_group_name