Master Linux User & Group Management: Commands, Files, and Best Practices

Overview

Linux is a multi‑user, multitasking operating system where each user must have an account. Accounts allow administrators to track usage, control resource access, and provide security.

User Management Files

/etc/passwd

This file stores basic user attributes; it is world‑readable. Each line contains seven colon‑separated fields: username, password placeholder, UID, GID, comment, home directory, and login shell.

/etc/shadow

Contains encrypted passwords; only root can read it. Fields include login name, encrypted password, last change date, minimum and maximum age, warning period, inactivity period, and expiration date.

/etc/group

Defines groups; format similar to /etc/passwd with fields: group name, password placeholder, GID, and member list.

User Management Commands

1. Add User (useradd)

Creates a new user. Common options: -c (comment), -d (home directory), -m (create home), -e (expire date), -g (primary group), -G (supplementary groups), -s (login shell), -u (UID), -p (encrypted password).

2. Modify User (usermod)

Changes user attributes such as login name (-l), home directory, group membership, or locks the account (-L). Unlock with -U.

3. Delete User (userdel)

Removes a user; -r also removes the home directory.

4. Set Password (passwd)

Root can set or change any account password; users can change their own.

5. Lock/Unlock Password

Use passwd -l to lock and passwd -u to unlock; check status with passwd -S.

6. Group Management

Create groups with groupadd, modify with groupmod (rename with -n, change GID with -g), and delete with groupdel. Use usermod or gpasswd to add or remove users from groups, and assign group administrators.

Additional utilities such as id, whoami, and groups display user and group information.