Linux User Management

This page provides a detailed guide on using the useradd and usermod commands in Linux for managing users.

User related information save in /etc/passwd

Group related information save in /etc/group

User Password related information save in /etc/shadow

Group pasword related information save in /etc/gshadow

To view the contents of these files, you can use the cat command:

cat /etc/passwd

User related information

        1. User ID (UID)
        2. Group ID (GID)
        3. Default directory (/home/username)
        4. Default shell (/bin/bash)
        5. User information save (/etc/passwd)
        6. Group information save (/etc/group)
        7. User password information save (/etc/shadow)
        8. Group password information save (/etc/gshadow)
        9. User's full name
        10. User's home directory
        11. User's login shell
        12. User's password (x means password is stored in /etc/shadow)
        13. User's group (x means group is stored in /etc/gshadow)
    

User Default authentication structure

        cat /etc/login.defs
        cat /etc/default/useradd
        The UID and GID ranges are defined in the /etc/login.defs file. The default range for UIDs is typically from 1000 to 60000, while GIDs can range from 1000 to 60000 as well.
        The /etc/default/useradd file may also contain settings for the minimum and maximum UID and GID values.
    

UID and GID range

        1. UID range: 1000-60000
        2. GID range: 1000-60000
        3. UID and GID for system users: 0-999
        4. UID and GID for regular users: 1000-60000
        5. UID and GID for special users: 60001-99999
    

Passwd file entry structure

        username:password:UID:GID:comment:home_directory:login_shell
        1. username: The name of the user.
        2. password: The password for the user (x means password is stored in /etc/shadow).
        3. UID: The user ID (a unique number assigned to the user).
        4. GID: The group ID (the primary group of the user).
        5. comment: Additional information about the user (e.g., full name).
        6. home_directory: The user's home directory.
        7. login_shell: The shell that the user will use when logging in.
    

Group file entry structure

        groupname:password:GID:member1,member2,...
        1. groupname: The name of the group.
        2. password: The password for the group (x means password is stored in /etc/gshadow).
        3. GID: The group ID (a unique number assigned to the group).
        4. member1,member2,...: The members of the group (comma-separated list).
    

1. useradd Command

The useradd command is used to create a new user in Linux.

Options

• -c: Add a comment (e.g., full name).
• -d: Specify the user's home directory.
• -e: Set the account expiration date.
• -g: Specify the primary group.
• -G: Specify additional groups.
• -m: Create the home directory if it does not exist.
• -s: Specify the login shell.
• -u: Specify the user ID (UID).

Examples

sudo useradd -m -d /home/abhishek -s /bin/bash abhishek

sudo useradd -u 1001 -g developers abhishek

2. usermod Command

The usermod command is used to modify an existing user account.

Options

• -c: Change the comment (e.g., full name).
• -d: Change the home directory.
• -e: Change the account expiration date.
• -g: Change the primary group.
• -G: Add the user to additional groups.
• -l: Change the login name.
• -s: Change the login shell.
• -u: Change the user ID (UID).
• -L: Lock the user account.
• -U: Unlock the user account.

Examples

sudo usermod -d /new/home/directory abhishek

sudo usermod -aG sudo abhishek

sudo usermod -L abhishek

3. userdel Command

The userdel command is used to delete a user account.

Options

• -r: Remove the user's home directory and mail spool.
• -f: Force the removal of the user account.
• -Z: Remove the SELinux user mapping.

Examples

sudo userdel -r abhishek

sudo userdel -f abhishek

sudo userdel -Z abhishek