The usermod command in Linux is used to modify or update a user’s account settings. With usermod, you can change various attributes of an existing user, such as their username, home directory, group memberships, login shell, and more.

1. Basic Syntax of usermod

The syntax for the command is:

sudo usermod [options] <username>

This command requires administrative privileges (sudo) and is followed by various options to specify the modifications.

2. Examples of usermod with Expected Output

Example 1: Changing a User’s Primary Group

To change a user’s primary group, use the -g option.

Command:

sudo usermod -g developers alice

Expected Output:

(no output)

There is no output on success. This command changes alice's primary group to developers. You can verify the change by checking the /etc/passwd file or using the id command:

id alice

Expected Output:

uid=1001(alice) gid=1002(developers) groups=1002(developers)

Example 2: Adding a User to a Secondary Group

To add a user to additional groups (secondary groups), use the -aG option. The -a (append) flag is important; without it, -G would replace the user's current groups instead of adding new ones.

Command:

sudo usermod -aG sudo,developers alice

Expected Output:

(no output)

This command adds alice to the sudo and developers groups without affecting her other group memberships. To confirm, use:

id alice

Expected Output:

uid=1001(alice) gid=1002(developers) groups=1002(developers),27(sudo)

Example 3: Changing a User’s Username

To change a user’s username, use the -l option.

Command:

sudo usermod -l alicia alice

Expected Output:

(no output)

This command changes the username from alice to alicia. However, the home directory remains the same unless specified with the -d option. Verify by checking the /etc/passwd file.

Example 4: Changing a User’s Home Directory

To change the home directory, use the -d option and -m to move existing files to the new directory.

Command:

sudo usermod -d /home/newalice -m alicia

Expected Output:

(no output)

This command changes the home directory for alicia to /home/newalice and moves all files from the old home directory to the new one. Check the new home directory by listing it:

ls /home/newalice

Example 5: Locking and Unlocking a User Account

To lock a user’s account, preventing login, use the -L option.

Command:

sudo usermod -L alicia

Expected Output:

(no output)

This locks alicia's account by placing an exclamation mark (!) before the encrypted password in /etc/shadow. To unlock the account, use:

sudo usermod -U alicia

3. Common Options for usermod

• -g <group>: Change the primary group.
• -aG <group1,group2,...>: Add to secondary groups.
• -d <directory>: Specify a new home directory.
• -m: Move the contents of the current home directory to the new directory.
• -l <newname>: Change the username.
• -s <shell>: Change the default shell (e.g., /bin/bash).
• -L / -U: Lock or unlock a user account.

4. Important Considerations

• Username Change: Changing usernames can affect ownership of files and permissions, so it should be done cautiously.
• Locking Accounts: Use account locks for temporary suspensions; this does not delete the user or their files.

The usermod command is versatile and powerful, but each change must be carefully implemented to ensure the account functions as intended.