User and Group Management

• useradd: Creates new users or updates default user information

  • Can specify home directory, default groups, expiration dates

  • Example: useradd clark (creates user with defaults)

  • -M: Don't create home directory

  • -g: Specify primary group

• groupadd: Creates new groups

  • Can specify group ID instead of using system default

  • -g: Specify custom group ID

  • Example: groupadd dev or groupadd -g 1200 ops

• usermod: Modifies existing user accounts

  • Change group membership, home directory, user ID

  • -G: Add user to supplemental groups

  • -u: Change user ID

  • -d: Change home directory

  • Example: usermod -G dev,ops -u 1300 clark

• groupmod: Modifies group definitions

  • Change group ID or group name

  • -n: Change group name

  • Example: groupmod -n dev2 dev

• userdel: Removes user accounts

  • -r: Also removes home directory and files

  • Example: userdel clark

• groupdel: Removes groups

  • Important: Cannot delete a group if it's the primary group of an existing user

  • Must delete the user first

  • Example: groupdel dev2

• passwd: Changes user passwords

  • Regular users can change their own password: passwd

  • Root user can change any user's password: passwd username

  • Example: passwd barry

• /etc/group: Contains all group information

• /etc/passwd: Contains all user information (UIDs, GIDs, home directories, login shells)

• Primary Group: The main group a user belongs to

• Supplemental Groups: Additional groups a user can belong to

• UID: User ID number

• GID: Group ID number

• Root user has privileges to modify any user/group

useradd [username]

• Creates a new user account

• Can be used with options like -M (no home directory), -g (specify primary group)

• Example: useradd -g ops -M barry

groupadd [groupname]

• Creates a new group

• Use -g option to specify custom group ID

• Example: groupadd -g 1200 ops (creates group with GID 1200)

The -M option tells useradd to NOT create a home directory for the new user, even though a default home directory path may still be listed in /etc/passwd.

• -g: Sets the PRIMARY group (only one)

• -G: Sets SUPPLEMENTAL/additional groups (can be multiple, comma-separated)

• Example: usermod -G dev,ops clark (adds clark to dev and ops groups)

usermod [options] [username]

• Common options:

  • -G: Add to supplemental groups

  • -u: Change user ID

  • -d: Change home directory

• Example: usermod -u 1300 -G dev,ops clark

groupmod -n [new_name] [old_name]

• Example: groupmod -n dev2 dev

• The -n option specifies the new name

• Group ID remains the same

userdel [username]

• Use -r option to also delete the user's home directory and files

• Example: userdel -r clark (removes user AND home directory)

The -r option removes the user's home directory and all files within it.

• Without -r: User account deleted but home directory remains

• With -r: Both user account and home directory are removed

• Example: userdel -r username

NO. You cannot delete a group if it's the primary group of an existing user. You must first delete the user, then you can delete the group.

• Supplemental groups can be deleted without removing users

• Command: groupdel [groupname]

/etc/passwd

• Contains: username, UID, GID, home directory, default shell

• View with: cat /etc/passwd

• Format: username:x:UID:GID:comment:home_directory:shell

/etc/group

• Contains: group name, GID, and group members

• View with: cat /etc/group

• Format: groupname:x:GID:member1,member2

passwd [username]

• Root can change ANY user's password

• Regular users can only change their own: passwd (no username)

• Example: passwd barry (root changing barry's password)

id [username]

• Shows: UID (user ID), GID (primary group ID), and all groups the user belongs to

• Example output: uid=1004(clark) gid=1004(clark) groups=1004(clark),1005(dev),1200(ops)

• Primary Group: The main group assigned to a user (only ONE), set with -g

• Supplemental Groups: Additional groups a user belongs to (can be MULTIPLE), set with -G

• Every user must have exactly one primary group but can have zero or more supplemental groups

useradd -M -g [groupname] [username]

• -M: Don't create home directory

• -g: Specify primary group

• Example: useradd -M -g ops barry

1. Create the directory: mkdir /home/flash

2. Change ownership: chown user:group /home/flash

3. Update user's home directory: usermod -d /home/flash username

1. Create: useradd / groupadd

2. Modify: usermod / groupmod

3. Delete: userdel / groupdel

• Plus: passwd for password management

The root user (superuser) can change passwords for any user on the system, including their own.

• Regular users can only change their own password

• Root uses: passwd [username]

usermod -G group1,group2,group3 username

• Use -G flag followed by comma-separated group names (no spaces)

• Example: usermod -G dev,ops,admin clark

• This sets the supplemental groups

[command] [options] [name]

• Command: useradd, usermod, userdel, groupadd, groupmod, groupdel

• Options: flags like -g, -G, -M, -u, -d, -r, -n

• Name: username or groupname

• Example: usermod -G dev,ops clark