Following our previous guide on Linux file permissions, understanding user and group management is crucial for effective system administration. This comprehensive guide will walk you through the essential concepts and commands for managing users and groups in Linux.

User Management Basics

Viewing User Information

The system stores user information in several key files:

# View user information
$ cat /etc/passwd
john:x:1000:1000:John Doe:/home/john:/bin/bash

# View encrypted passwords
$ sudo cat /etc/shadow
john:$6$xyz...:18962:0:99999:7:::

Creating Users

# Create a new user
$ sudo useradd -m -s /bin/bash newuser

# Create user with additional information
$ sudo useradd -m -s /bin/bash -c "Full Name" -G sudo,developers newuser

# Set/change password
$ sudo passwd newuser

Common useradd options:

• -m: Create home directory
• -s: Specify login shell
• -c: Add comment/full name
• -G: Add to supplementary groups
• -e: Set account expiry date

Modifying Users

# Change username
$ sudo usermod -l newname oldname

# Change home directory
$ sudo usermod -d /newhome/user user

# Add to additional groups
$ sudo usermod -aG docker,developers user

# Lock/unlock account
$ sudo passwd -l user    # Lock
$ sudo passwd -u user    # Unlock

Deleting Users

# Remove user
$ sudo userdel username

# Remove user and home directory
$ sudo userdel -r username

# Backup home directory before removal
$ sudo tar -czf username-backup.tar.gz /home/username
$ sudo userdel -r username

Group Management

Viewing Groups

# List all groups
$ cat /etc/group

# View user's groups
$ groups username

# View current user's groups
$ groups

Creating and Modifying Groups

# Create new group
$ sudo groupadd developers

# Add user to group
$ sudo gpasswd -a username developers

# Remove user from group
$ sudo gpasswd -d username developers

# Change group ownership
$ sudo chgrp developers /path/to/directory

Primary vs Supplementary Groups

# Change primary group
$ sudo usermod -g newgroup username

# Add supplementary groups
$ sudo usermod -aG group1,group2 username

Advanced User Management

User Limits and Restrictions

Using /etc/security/limits.conf:

# Set maximum processes
username hard nproc 100

# Set maximum file size
@developers soft fsize 1000000

Setting Up User Templates

The /etc/skel directory contains template files:

# Add default files for new users
$ sudo cp .bashrc /etc/skel/
$ sudo cp .vimrc /etc/skel/

Best Practices

1. User Creation

   • Use strong passwords
   • Set appropriate shell and home directory
   • Add users only to necessary groups

2. Security

   • Regularly audit user accounts
   • Remove unused accounts
   • Lock system accounts

3. Group Management

   • Use descriptive group names
   • Maintain clear group purposes
   • Regular group membership audits

Practical Examples

Setting Up Development Team

# Create development group
$ sudo groupadd developers

# Create user accounts
$ sudo useradd -m -s /bin/bash -G developers dev1
$ sudo useradd -m -s /bin/bash -G developers dev2

# Set up project directory
$ sudo mkdir /opt/project
$ sudo chown :developers /opt/project
$ sudo chmod 2775 /opt/project

Temporary Access Management

# Create temporary user
$ sudo useradd -m -e $(date -d "+30 days" +%Y-%m-%d) tempuser

# Add to specific group
$ sudo usermod -aG project_team tempuser

# Set password expiry
$ sudo chage -M 30 tempuser

Troubleshooting

1. Access Denied Issues

   # Check user groups
   $ groups username
   
   # Verify file ownership
   $ ls -l /path/to/file
   
   # Check effective permissions
   $ sudo -u username ls -l /path/to/file
   

2. Group Permission Issues

   # Fix group ownership
   $ sudo chgrp -R groupname /path/to/directory
   
   # Set SGID for group inheritance
   $ sudo chmod g+s /path/to/directory
   

Conclusion

Effective user and group management is fundamental to Linux system administration and security. Combined with proper file permissions (as discussed in our previous article), these skills form the backbone of system access control.

In our next guide, we’ll explore Linux file system security, building upon the concepts we’ve covered in both file permissions and user management.