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User Modify (usermod)

The usermod command in Linux is used to modify existing user accounts. It allows system administrators to change user attributes such as username, home directory, groups, shell, and more.

Basic Syntax

sudo usermod [options] username

  • Requires sudo or root privileges.
  • Replace [options] with the appropriate flags to modify user attributes.

Useful usermod Switches

1. Change Username (-l)

What it does: Renames an existing user account.
Syntax:

usermod -l new_username old_username

**Example**:  
```bash
usermod -l user1_new user1_old
  • Changes the username from user1_old to user1_new.

2. Change Home Directory (-d)

What it does: Changes the user's home directory.
Syntax:

usermod -d /new/home/dir -m username

Example:

usermod -d /home/user1_new -m user1
  • -d: Specifies the new home directory.
  • -m: Moves the contents of the old home directory to the new one.

3. Change User's Shell (-s)

What it does: Changes the user's login shell.
Syntax:

usermod -s /path/to/shell username

Example:

usermod -s /usr/bin/zsh user1
  • Sets the user's login shell to /usr/bin/zsh.

4. Add User to Supplementary Groups (-aG)

What it does: Adds the user to additional groups without removing existing group memberships.
Syntax:

usermod -aG group1,group2 username

Example:

usermod -aG sudo,developers user1
  • -a: Append to existing groups (prevents overwriting).
  • -G: Specifies the groups to add the user to.

5. Lock/Unlock User Account (-L, -U)

What it does: Locks or unlocks a user account.
Syntax:

  • Lock: 
    usermod -L username
  • Unlock: 
    usermod -U username

6. Change User ID (-u)

What it does: Changes the user's UID (User ID).
Syntax:

usermod -u new_uid username

Example:

usermod -u 1100 user1
  • Updates the user's UID to 1100.

7. Change Group ID (-g)

What it does: Changes the user's primary group.
Syntax:

usermod -g new_gid username

Example:

usermod -g 1050 user1
  • Updates the user's primary group to 1050.

8. Set Account Expiry Date (-e)

What it does: Sets an expiry date for the user account.
Syntax:

usermod -e YYYY-MM-DD username

Example:

usermod -e 2025-12-31 user1
  • The account will expire on December 31, 2025.

9. Change User's Full Name (-c)

What it does: Updates the user's comment field (often used for the full name).
Syntax:

usermod -c "Full Name" username

Example:

usermod -c "John Doe" user1
  • Updates the user's comment field to "John Doe".

10. Move User's Home Directory (-m)

What it does: Moves the contents of the user's home directory to a new location.
Syntax:

usermod -d /new/home/dir -m username

Example:

usermod -d /home/user1_new -m user1
  • Moves the contents of /home/user1 to /home/user1_new.

11. Force Password Change (-f)

What it does: Sets the number of days after which the password must be changed.
Syntax:

usermod -f days username

Example:

usermod -f 8 user1
  • The user must change their password after 8 days.

12. Allow Non-Unique UID (-o)

What it does: Allows duplicate UIDs (not recommended for security reasons).
Syntax:

usermod -o -u 0 username

Example:

usermod -o -u 0 user1
  • Sets the UID to 0 (root), even if another user has the same UID.

13. Set Encrypted Password (-p)

What it does: Sets an encrypted password for the user.
Syntax:

usermod -p 'encrypted_password' username

Example:

Generate Encrypted Password :

openssl passwd 123
$1$dr5MGUev$uZ8QHz4ilePmWugMkj7yA.

Sets the encrypted password :

usermod -p '$1$dr5MGUev$uZ8QHz4ilePmWugMkj7yA.' user1

User Deletion (userdel)

The userdel command in Linux is used to delete user accounts from the system. It removes the user's entry from the /etc/passwd, /etc/shadow, and /etc/group files, effectively removing the user from the system.

Syntax:

userdel [options] username

Examples:

  • Delete a user: 
    userdel user1
  • Delete a user and their home directory: 
    userdel -r user1
  • Forcefully delete a user and their home directory: 
    userdel -f -r user1

Password Management (passwd)

The passwd command in Linux is used to manage user passwords. It allows users to change their own passwords and enables administrators to manage passwords for other users. The command also provides options to lock, unlock, and check the status of passwords.

Syntax:

passwd [options] username

Examples:

  • Change a user's password: 
    passwd user1
  • Delete a user's password: 
    passwd -d user1
  • Lock a user's password: 
    passwd -l user1
  • Unlock a user's password: 
    passwd -u user1
  • Check password status: 
    passwd -S user1
  • Forces user1 to change their password on their next login.: 
    passwd -e user1
  • Forces user1 to change their password on their next login.: 
    passwd -e user1

Summary Table

Command Description Example
usermod -l Change username usermod -l user1_new user1_old
usermod -d Change home directory usermod -d /home/user1_new -m user1
usermod -s Change login shell usermod -s /usr/bin/zsh user1
usermod -aG Add user to groups usermod -aG sudo,dev user1
usermod -L/U Lock/unlock account usermod -L user1
usermod -u Change UID usermod -u 1100 user1
usermod -g Change primary GID usermod -g 1050 user1
usermod -e Set account expiry date usermod -e 2025-12-31 user1
usermod -c Update comment (e.g., full name) usermod -c "John Doe" user1
usermod -m Move home directory contents usermod -d /home/user1_new -m user1
usermod -f Force password change usermod -f 8 user1
usermod -o Allow non-unique UID usermod -o -u 0 user1
usermod -p Set encrypted password usermod -p $(openssl passwd -1 '123')
userdel Delete user userdel -r user1
passwd Manage passwords passwd -S user1