User and Group Management

Linux User and Group Management

User and group management is fundamental to Linux system administration, controlling access to files, directories, and system resources. Here’s a comprehensive guide:

User Management

1. Viewing Users

cat /etc/passwd         # List all users
getent passwd           # Alternative method
id username             # Show user details
whoami                  # Show current user

2. Creating Users

useradd username        # Basic user creation
useradd -m username    # Create with home directory
useradd -G group1,group2 username  # Add to supplementary groups
useradd -s /bin/bash username  # Set default shell
useradd -u 1005 username  # Specify UID

3. Modifying Users

usermod -aG sudo username  # Add to sudo group
usermod -s /bin/zsh username  # Change shell
usermod -L username      # Lock account
usermod -U username      # Unlock account
usermod -d /new/home username  # Change home directory

4. Deleting Users

userdel username        # Delete user (keeps home dir)
userdel -r username     # Delete user and home directory

5. Password Management

passwd username         # Set/change password
passwd -l username      # Lock password
passwd -u username      # Unlock password
passwd -e username      # Expire password (force change)
chage -l username       # View password aging info

Group Management

1. Viewing Groups

cat /etc/group          # List all groups
getent group            # Alternative method
groups username         # Show user's groups

2. Creating Groups

groupadd groupname      # Create new group
groupadd -g 1005 groupname  # Create with specific GID

3. Modifying Groups

groupmod -n newname oldname  # Rename group
groupmod -g 1006 groupname  # Change GID

4. Deleting Groups

groupdel groupname      # Delete group

5. Managing Group Memberships

gpasswd -a user group   # Add user to group
gpasswd -d user group   # Remove user from group
usermod -aG group1,group2 user  # Add to multiple groups

Important Files

1. /etc/passwd - User account information

   • Format: username:x:UID:GID:description:homedir:shell

2. /etc/shadow - Encrypted passwords and aging info (root only)

3. /etc/group - Group definitions

   • Format: groupname:x:GID:members

4. /etc/gshadow - Secure group information (root only)

Special Users

1. Root (UID 0) - Superuser with full system access
2. System Users (UID 1-999) - For services/daemons (varies by distro)
3. Regular Users (UID 1000+) - Normal user accounts

Best Practices

1. Use -aG with usermod to append groups rather than replace
2. Always use visudo to edit sudoers file
3. Regularly audit users with last, who, and w commands
4. Set appropriate password policies in /etc/login.defs
5. Use adduser instead of useradd on Debian-based systems (more user-friendly)

Sudo Privileges

To grant administrative privileges:

usermod -aG sudo username  # Debian/Ubuntu
usermod -aG wheel username # RHEL/CentOS

Then edit sudoers file safely:

visudo
# Add line: username ALL=(ALL) ALL

User Environment

User-specific configuration files:

• ~/.bashrc - Shell configuration
• ~/.profile - Login configuration
• ~/.ssh/ - SSH keys and config

Practical Examples

1. Create a developer user with home directory and add to appropriate groups:

useradd -m -G developers,sudo -s /bin/bash devuser
passwd devuser

2. Find all users belonging to a specific group:

getent group developers | cut -d: -f4

3. List all users with UID ≥ 1000 (normal users):

awk -F: '$3 >= 1000 {print $1}' /etc/passwd

4. Set password expiration policy:

chage -M 90 -W 7 username  # Expire after 90 days, warn 7 days before

Proper user and group management is essential for system security and resource access control in Linux environments.