Linux user management controls who can log in, what they can access, and how processes run. Here’s a deeper guide to accounts, groups, commands, and safety tips.

Accounts, groups, and shells

• User (UID): Each account has a UID, name, primary GID, home dir, and shell.
• Primary group (GID): The default group for file ownership when a user creates files.
• Secondary groups: Extra access (e.g., sudo, docker, www-data); a user can belong to many.
• Shell: The program launched on login (/bin/bash, /bin/sh, /usr/sbin/nologin for service accounts).

Key files (for reference)

• /etc/passwd: User records (name, UID, GID, home, shell).
• /etc/shadow: Password hashes and policies (root-readable only).
• /etc/group: Group definitions and memberships.

Create users and groups

• Create a group: sudo groupadd developers
• Create a user (low-level): sudo useradd -m -s /bin/bash -G developers alice
-m make home, -s shell, -G secondary groups. Set a password: sudo passwd alice.
• Create a user (interactive helper, Debian/Ubuntu): sudo adduser bob
  Then add to a group: sudo usermod -aG developers bob.

Modify users and groups

• Add secondary groups: sudo usermod -aG docker alice
• Change shell: sudo chsh -s /bin/bash alice
• Lock/unlock account: sudo usermod -L alice (lock) · sudo usermod -U alice (unlock)
• Change password: sudo passwd alice
• Change primary group: sudo usermod -g developers alice

Remove users

• Delete user, keep home: sudo userdel alice
• Delete user and home: sudo userdel -r alice

Check info

• Show a user: id alice
• List users: getent passwd
• List groups: getent group
• Switch user: su - alice (needs password) or sudo -u alice -i

File permissions refresh

• Ownership: owner:group. Change with chown user:group file.
• Permissions: read/write/execute for owner, group, others. Change with chmod (e.g., chmod 640 file).
• Use groups to grant shared access instead of loosening “others.”

Good practices

• Create one account per person/service; avoid shared logins.
• Use groups for access control; don’t sprinkle permissive chmods.
• Use /usr/sbin/nologin for service accounts that should not get shells.
• Lock or remove unused accounts promptly.
• Protect /etc/shadow; keep sudo restricted and logged.

With these commands and habits, you can add, modify, audit, and remove users safely while keeping permissions tidy.