User Management in Linux

User management is a crucial aspect of system administration in Linux, allowing you to control access and permissions for different users on the system. This section covers essential commands for managing users and groups.

Important Note: On many systems (including most Linux distributions), you’ll need root privileges to add, remove, or modify users and groups. This means you’ll often need to use sudo before these commands. In a Docker container, you often are the root user by default, but it’s still best practice to be mindful of privileges.

passwd

  • Description: Changes a user’s password.

  • Example Usage:

    passwd        # Change your *own* password (no sudo needed)
sudo passwd newuser  # Set or change the password for 'newuser' (requires root privileges)

    Sample Output (when changing your own password):

    Changing password for user.
(current) UNIX password:  # You'll be prompted to enter your current password
New password:             # Then, enter your new password (twice for confirmation)
Retype new password:
passwd: password updated successfully

    Sample Output (setting a new password as root):

    Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully

adduser

  • Description: Creates a new user account. This command typically does the following:

    • Adds a new entry to /etc/passwd (user information).
    • Adds a new entry to /etc/shadow (secure password information).
    • Adds a new entry to /etc/group (if a primary group is created).
    • Creates a home directory for the new user (usually in /home).
    • Copies default files (e.g., .bashrc) to the new user’s home directory.

  • Example Usage:

    sudo adduser newuser  # Creates a new user named 'newuser' (requires root privileges)

    Sample Output (truncated - it’s interactive):

      Adding user `newuser' ...
  Adding new group `newuser' (1001) ...
  Adding new user `newuser' (1001) with group `newuser' ...
  Creating home directory `/home/newuser' ...
  Copying files from `/etc/skel' ...
  New password:
  Retype new password:
  passwd: password updated successfully
  Changing the user information for newuser
  Enter the new value, or press ENTER for the default
    Full Name []: New User
    Room Number []:
    Work Phone []:
    Home Phone []:
    Other []:
  Is the information correct? [Y/n] y

    Note: adduser is generally preferred over useradd on Debian/Ubuntu systems because adduser is a higher-level command that performs additional setup steps (like creating the home directory). useradd is a lower level command.

deluser

  • Description: Deletes a user account.

  • Example Usage:

    sudo deluser newuser          # Removes the user 'newuser' (but keeps their home directory)
sudo deluser --remove-home newuser  # Removes 'newuser' *and* their home directory
sudo deluser --remove-all-files newuser # Removes all files owned by the user.

    Sample Output (for sudo deluser newuser):

    Removing user `newuser' ...
Warning: group `newuser' has no more members.
Done.

usermod

  • Description: Modifies an existing user account’s properties (login name, home directory, group memberships, etc.).

  • Example Usage:

    sudo usermod -l newlogin oldlogin   # Changes the login name from 'oldlogin' to 'newlogin'
sudo usermod -d /new/home/dir -m newuser # Changes 'newuser's home directory to /new/home/dir and *moves* the contents
sudo usermod -aG sudo newuser        # Adds 'newuser' to the 'sudo' group (giving them sudo privileges)
sudo usermod -g newgroup user      # change the user's primary group.
sudo usermod -L user     # lock the account by making the password invalid.

    Key Options for usermod:

    • -l newlogin: Changes the user’s login name.
    • -d /new/home/dir: Changes the user’s home directory.
    • -m (with -d): Moves the contents of the old home directory to the new one. Important!
    • -aG group1,group2,...: Adds the user to the specified groups. -a (append) is crucial here; without it, the user will be removed from all other groups!
    • -g: change user’s primary group.
    • -L: lock the user account.

su (Switch User)

  • Description: Switches to another user account within the current terminal session.

  • Example Usage:

    su newuser       # Switch to the 'newuser' account (you'll be prompted for 'newuser's password)
su - newuser    # Switch to 'newuser' and *also* load their environment (like their .bashrc) - highly recommended
su               # Switch to the root user (you'll be prompted for the root password)
su -             # Switch to the root user and load the root environment.
exit             # Exit the switched-to user and return to your previous user.

    Note: The - (dash) with su is extremely important. It makes the shell a login shell, which means it sources the user’s profile scripts (like .bashrc). This ensures you have the correct environment for that user. Without the -, you might have unexpected behavior.

Groups and Permissions

  • Groups: Groups are collections of users. They are used to manage permissions on files and directories. For example, you might create a group called developers and give that group read/write access to a project directory.

  • Key Files:

    • /etc/passwd: Contains user account information (username, user ID, home directory, etc.).
    • /etc/shadow: Contains secure password information (hashed passwords).
    • /etc/group: Contains group information (group name, group ID, members).

  • Commands for Viewing Group Information:

    cat /etc/group      # Display all groups on the system
groups              # Show the groups the *current* user belongs to
groups username     # Show the groups a specific user belongs to
id                  # Show the current user's ID, primary group ID, and group memberships
id username         # Show the user ID, primary group ID, and group memberships for a specific user

    Sample Output:

    uid=1000(user) gid=1000(user) groups=1000(user),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),126(sambashare)

  • Commands for Managing Group:

      sudo groupadd newgroup  # Create a new group named 'newgroup'
  sudo groupdel groupname  # Delete a group
  sudo usermod -aG newgroup username # Adds the user to the group.
  sudo gpasswd -d user groupname # remove user from the group