# Cheatsheet & Examples: usermod

The `usermod` command modifies a user account's properties on a Linux system. It allows administrators to change various attributes associated with a user, such as the user's login name, home directory, shell, groups, and password expiry information.

## Changing User's Login Name

Example Usage:
`usermod -l new_username old_username`

What it does:
Changes the user's login name from `old_username` to `new_username`.

Command-line Arguments Explained:

- `-l new_username`: Specifies the new login name for the user.
- `old_username`: Specifies the existing username whose login name needs to be changed.

## Modifying User's Home Directory

Example Usage:
`usermod -d /new/home/directory username`

What it does:
Changes the user's home directory to the specified path.

Command-line Arguments Explained:

- `-d /new/home/directory`: Sets the new home directory path for the user.
- `username`: Specifies the username whose home directory is being modified.

## Changing User's Primary Group

Example Usage:
`usermod -g groupname username`

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

Command-line Arguments Explained:

- `-g groupname`: Specifies the new primary group by its group name.
- `username`: Specifies the username to modify.

## Adding a User to Supplementary Groups

Example Usage:
`usermod -a -G group1,group2 username`

What it does:
Adds the user to the specified supplementary groups without removing them from their existing groups.

Command-line Arguments Explained:

- `-a`:  Appends the user to the supplementary group list. Requires the `-G` option.
- `-G group1,group2`: Specifies a comma-separated list of group names to add the user to.
- `username`: Specifies the username to modify.

## Changing User's Shell

Example Usage:
`usermod -s /bin/bash username`

What it does:
Changes the user's default shell to the specified shell.

Command-line Arguments Explained:

- `-s /bin/bash`: Sets the user's login shell (e.g., `/bin/bash`, `/bin/zsh`).
- `username`: Specifies the username to modify.

## Locking and Unlocking a User Account

Example Usage:
`usermod -L username` (Lock)
`usermod -U username` (Unlock)

What it does:
Locks or unlocks a user account, preventing or allowing login.

Command-line Arguments Explained:

- `-L`: Locks the user's password, effectively disabling login by prefixing the password hash in `/etc/shadow` with `!`.
- `-U`: Unlocks the user's password by removing the `!` from the password hash in `/etc/shadow`, enabling login.
- `username`: Specifies the username to modify.

## Setting Password Expiration Information

Example Usage:
`usermod -e YYYY-MM-DD username`

What it does:
Sets the date after which the user's password will expire.

Command-line Arguments Explained:

- `-e YYYY-MM-DD`: Sets the account expiration date.  Use YYYY-MM-DD format.
- `username`: Specifies the username to modify.

## Modifying User's UID and GID

Example Usage:
`usermod -u 1001 username`
`usermod -g 1002 username`

What it does:
Changes the user's User ID (UID) and Group ID (GID).

Command-line Arguments Explained:

- `-u 1001`: Changes the user's UID to 1001. This needs to be a unique ID.
- `-g 1002`:  Changes the user's GID to 1002. This should match an existing group ID.
- `username`: Specifies the username to modify.