User & Group Management in Unix

Unix is a multi-user system, meaning multiple users can log in and work simultaneously. Each user has:

• A username (e.g., alice)

• A UID (user ID, a unique number)

• A primary group and possibly supplementary groups

• A home directory and shell

File access is controlled through users, groups, and permissions.

1. useradd – Add a New User

Creates a new user account.

sudo useradd alice              # Add user "alice"
sudo useradd -m bob             # Add "bob" and create home directory
sudo useradd -s /bin/bash charlie  # Set shell for "charlie"

Check user details:

id alice

2. passwd – Set or Change Password

Assigns or changes a user’s password.

sudo passwd alice

It will prompt for a new password.

Users can change their own password with just:

passwd

3. groups – Show Group Membership

Lists groups a user belongs to.

groups          # Show current user’s groups
groups alice    # Show groups of "alice"

To add a user to a group:

sudo usermod -aG developers alice

4. chmod – Change File Permissions

Every file/directory has permissions for:

• User (owner)

• Group

• Others

Permissions:

• r = read

• w = write

• x = execute

Numeric Mode

• r=4, w=2, x=1

• Add them up for each level.

chmod 644 file.txt   # rw- r-- r--  (owner can read/write, others can only read)
chmod 755 script.sh  # rwx r-x r-x  (owner can do everything, others can read/execute)

Symbolic Mode

chmod u+x script.sh   # Give execute to user (owner)
chmod g-w file.txt    # Remove write for group
chmod o+r file.txt    # Give read to others

5. chown – Change Ownership

Used to change file/directory owner and/or group.

sudo chown alice file.txt          # Make alice the owner
sudo chown alice:developers file.txt  # Owner = alice, Group = developers

6. Putting It Together

Example workflow:

# Create a new user
sudo useradd -m devuser

# Set password
sudo passwd devuser

# Add to group
sudo usermod -aG sudo devuser

# Give permissions
chmod 750 project.sh        # Owner: rwx, Group: r-x, Others: ---
chown devuser:developers project.sh

Summary

• useradd → create users

• passwd → set/change passwords

• groups → check group membership

• chmod → change file permissions

• chown → change file/directory ownership