Managing Users and Permissions on Linux<

This guide shows you how to create and manage users on your Linux server and assign the appropriate permissions. Clean user management is essential for your server's security.

Note: Detailed subpages on security and advanced topics can be found in the left navigation.

Why Multiple Users?

• Security: Don't do everything as root – otherwise the entire system is affected by mistakes
• Access control: Each user only sees their own files
• Traceability: Who made which changes?

Creating and Managing Users

Create a New User

# Create user with home directory
adduser username

# Or with useradd (more options)
useradd -m -s /bin/bash username

# Set password
passwd username

Add Users to Groups

# Add to sudo group (admin rights)
usermod -aG sudo username

# Add to a specific group
usermod -aG groupname username

# Show groups of a user
groups username

List and Delete Users

# Show all users
cat /etc/passwd

# Only "real" users (with shell)
grep -v /nologin /etc/passwd

# Delete user (with home directory)
userdel -r username

Understanding File Permissions

-rwxr-xr-- 1 user group 1024 May 28 12:00 file.txt
 ─┬─ ─┬─ ─┬─
  │   │   └── Permissions for others (Others)
  │   └────── Permissions for the group (Group)
  └────────── Permissions for the owner (Owner)

• r = Read – value: 4
• w = Write – value: 2
• x = Execute – value: 1

Change Permissions with chmod

# File readable and writable for owner, readable for group and others
chmod 644 file.txt

# Folder: owner has all rights, group and others can read and enter
chmod 755 folder

# Make script executable
chmod +x script.sh

# Recursively for all files in a folder
chmod -R 755 /var/www

Change Owner with chown

# Change owner and group
chown user:group file.txt

# Recursively for an entire folder
chown -R www-data:www-data /var/www

Important System Users

Common Problems

"Permission denied" despite correct permissions:

• Check if the parent folder is readable – the path must be accessible from top to bottom
• Check permissions of all parent folders with ls -la /path/to/folder

sudo not working:

• Is the user in the sudo group? groups username
• After adding to the group: The user must log out and log back in

Detailed guides on security and advanced topics can be found in the articles in the left navigation.