Managing Linux Users Cheat Sheet

Adding and Removing Users

Command	Description
useradd <username>	Adds a new user to the system. Replace <username> with the actual username you want to create.
useradd -m <username>	Adds a new user and creates a home directory for them at /home/<username>. Without the -m flag, no home directory will be created.
useradd -G <groupname> <username>	Adds a user to a specific additional group. By default, a user is added to their own group, but you can assign them to other groups using this command.
useradd -s <shell> <username>	Specifies which shell the user will use. The default is usually /bin/bash, but you can set others like /bin/sh.
userdel <username>	Deletes a user from the system, but does not remove their home directory.
userdel -r <username>	Deletes the user and their home directory, along with any files they own in /home/<username>. Use this with caution.

Modifying Users

Command	Description
usermod -l <newusername> <oldusername>	Changes the login name of an existing user. Useful when renaming a user.
usermod -aG <groupname> <username>	Adds a user to an additional group, without removing them from their current groups. The -a stands for "append".
usermod -L <username>	Locks a user's account, preventing them from logging in. The user's files remain intact.
usermod -U <username>	Unlocks a previously locked user account.

Adding Groups

Groups are collections of users. They help simplify permissions management. For example, adding multiple users to the "developers" group makes it easier to assign the same access to all developers.

Command	Description
groupadd <groupname>	Creates a new group. Replace <groupname> with the name of the group you want to create.
groupdel <groupname>	Deletes a group. Be cautious — deleting a group may affect users associated with it.

Modifying Groups

Just like users, groups can be renamed using groupmod.

Command	Description
groupmod -n <newgroupname> <oldgroupname>	Changes the name of an existing group.

Checking Users and Groups

• The /etc/passwd file contains essential information about user accounts, such as their username, user ID, and home directory.
• The /etc/shadow file stores encrypted password data and expiration dates.
• The /etc/group file lists all the system groups and the users assigned to them.

Command	Description
cat /etc/passwd	Displays a list of all the user accounts on the system.
cat /etc/shadow	Shows password and account expiration information. This file is only viewable by the root user.
cat /etc/group	Displays a list of all the groups on the system and their members.

Password Management

Command	Description
passwd <username>	Changes the password for a specific user.
passwd -l <username>	Locks a user's password, preventing them from logging in.
passwd -u <username>	Unlocks a locked password, allowing the user to log in again.