Users and Groups
groupadd group_name	create a new group
groupdel group_name	delete a group
groupmod -n new_group_name old_group_name	rename a group
useradd -c "Name Surname " -g admin -d /home/user1 -s /bin/bash user1	create a new user belongs "admin" group
useradd user1	create a new user
userdel -r user1	delete a user ( '-r' eliminates home directory)
usermod -c "User FTP" -g system -d /ftp/user1 -s /bin/nologin user1	change user attributes
passwd	change password
passwd user1	change a user password (only by root)
chage -E 2005-12-31 user1	set deadline for user password
pwck	check correct syntax and file format of '/etc/passwd' and users existence
grpck	check correct syntax and file format of '/etc/group' and groups existence
newgrp group_name	log in to a new group to change default group of newly created files