User Management in Linux

Linux is a multi-user operating system, meaning multiple users can operate on a system simultaneously. Proper user management ensures security, controlled access, and system integrity.

Types of Users in Linux

• Root User / Superuser: Has full control over the system. Can perform any task such as installing software, changing system settings, and managing other users.

• Regular User / Standard User: Has limited access. Can read, write, and execute files, but restricted from critical system areas.

• System User: Non-human users that run background services and processes. These accounts are intangible to regular users.

Key Files Involved in User Management

• /etc/passwd – Stores user account details.
• /etc/shadow – Stores encrypted user passwords.
• /etc/group – Stores group information.
• /etc/gshadow – Stores secure group details.

Commands for Creating Users and Groups

• Create a new user:

  useradd username

• Create a user with a specific UID:

  useradd -u UID username

• Create a new group:

  groupadd groupname

• Create a group with a specific GID:

  groupadd -g GID groupname

Managing User Passwords

• To set or change a user’s password:

passwd username

Enforcing Password Policies

• Password expiration: Set password expiry days

chage -M 90 username

• Lock a user account

passwd -l username

• Unlock a user account

passwd -u username

Modifying Users and Groups

• Change an existing username:

  usermod -l new_username old_username

• Change an existing group name:

  groupmod -n new_groupname old_groupname

• Check groups a user belongs to:

  groups username

• Create a user with a specific UID and primary group (group must exist):

  useradd -u UID -g groupname username

or

  useradd -u UID -g GID username

• Append a user to another group (without removing them from existing groups):

  usermod -aG groupname username

• Change a user’s UID:

  usermod -u UID username

• Change a user’s primary GID:

  usermod -g GID username

Deleting Users

• To remove a user but keep their home directory:

userdel username

• To remove a user and their home directory:

userdel -r username