User and Group Management in Linux

A guide to Linux user and group management covering /etc/passwd and /etc/group file structures, and commands like useradd, usermod, groupadd, and passwd.

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In Linux systems, the concepts of users and groups are used to manage access permissions to files and processes.

User Account Information

User account information is primarily stored in the /etc/passwd file. This file contains basic information for all user accounts on the system, with each line corresponding to one user.

Each line in /etc/passwd consists of 7 fields separated by colons :.

[Username]:[Password]:[User ID (UID)]:[Group ID (GID)]:[Comment]:[Home Directory]:[Login Shell]
  • Password: For security reasons, the actual password hash is not stored here directly. Instead, x is displayed, and the password hash is stored in the /etc/shadow file. The /etc/shadow file is accessible only by the root user in read-only mode.

User Information Editing Commands

  • useradd: Adds a new user account.
    sudo useradd -m -s /bin/bash newuser # Create home directory and set bash as shell
sudo passwd newuser # Set password
  • usermod: Modifies an existing user account.
    sudo usermod -aG sudo newuser # Add newuser to the sudo group
sudo usermod -l new_name old_name # Change username
  • userdel: Deletes a user account.
    sudo userdel -r olduser # Delete including home directory
  • passwd: Changes a user’s password.
    passwd # Change your own password
sudo passwd username # Change another user's password (requires root privileges)

Group Account Information

Group account information is primarily stored in the /etc/group file. This file contains basic information for all groups on the system, with each line corresponding to one group.

Each line in /etc/group consists of 4 fields separated by colons :.

[Group Name]:[Password]:[Group ID (GID)]:[Members]
  • Password: Group passwords are rarely used. Typically, x is displayed.
  • Members: A comma-separated list of usernames belonging to the group.

Group Information Editing Commands

  • groupadd: Adds a new group.
    sudo groupadd newgroup
  • groupmod: Modifies an existing group.
    sudo groupmod -n new_group_name old_group_name # Change group name
  • groupdel: Deletes a group.
    sudo groupdel oldgroup
  • id: Checks the groups that the current user belongs to.
    id
# Example: uid=1000(username) gid=1000(username) groups=1000(username),4(adm),27(sudo)

Understanding and properly using these commands enables effective user and group management in Linux systems.

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