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File Ownership: Who Owns What

Every file has an owner and a group. That's how Linux controls access.

Here's the thing: Ownership matters. It controls who can do what. Learn it.

The Basic Concept​

Every file has:

  • Owner - The user who created it
  • Group - The group it belongs to
  • Others - Everyone else

Think of it like a house: You own it. Your family (group) shares it. Guests (others) can visit. Different access levels.

Users and Groups​

Users​

whoami                            # Current user
id                                # Your IDs and groups
id username                       # Another user's IDs

User file: /etc/passwd

cat /etc/passwd
# Format: username:x:UID:GID:comment:home:shell
# Example: ubuntu:x:1000:1000:Ubuntu:/home/ubuntu:/bin/bash

What matters:

  • username = Login name
  • UID = User ID (0 = root, 1000+ = regular users)
  • GID = Primary group ID
  • home = Home directory
  • shell = Default shell

My take: UID 0 is root. UID 1000+ are regular users. That's what you need to know.

Groups​

groups                            # Your groups
groups username                    # Another user's groups
getent group groupname            # Group details

Group file: /etc/group

cat /etc/group
# Format: groupname:x:GID:members
# Example: sudo:x:27:ubuntu,john

My take: Groups are collections of users. Share permissions. Organize access.

chown: Change Owner​

sudo chown user:group file.txt
sudo chown user file.txt           # Just owner
sudo chown :group file.txt          # Just group
sudo chown -R user:group dir/       # Recursive

Real examples:

sudo chown www-data:www-data /var/www/html
sudo chown ubuntu:ubuntu script.sh
sudo chown -R ubuntu:ubuntu /home/ubuntu/projects

My take: Use chown to change ownership. Usually for web servers. Files need to be owned by the right user.

chgrp: Change Group​

sudo chgrp group file.txt
sudo chgrp -R group directory/     # Recursive

My take: chown can do this. chgrp is just a shortcut. Use whichever you prefer.

Common Patterns​

Web Server Files​

sudo chown -R www-data:www-data /var/www/html
sudo chmod -R 755 /var/www/html

My take: Web server needs to read files. Owner needs to write. This is the pattern.

User Files​

sudo chown -R user:user /home/user

My take: User should own their home directory. Simple.

Shared Directories​

sudo chown -R :sharedgroup /shared
sudo chmod -R 775 /shared

My take: Group ownership for shared directories. Multiple users can access.

Understanding IDs​

id
# uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),27(sudo)

Breaking it down:

  • uid = User ID
  • gid = Primary group ID
  • groups = All groups (primary + secondary)

My take: IDs are numbers. Names are labels. System uses IDs. You use names.

Common Mistakes (I've Made These)​

  1. Forgetting sudo: Changing ownership needs root. Use sudo.

  2. Wrong user/group: Make sure user and group exist. id username to check.

  3. Not using -R for directories: -R is recursive. Needed for directories.

  4. Changing system files: Don't change ownership of system files. Bad things happen.

  5. Not checking current ownership: ls -l shows ownership. Check before changing.

Real-World Examples​

Fix Web Server Permissions​

sudo chown -R www-data:www-data /var/www/html
sudo find /var/www/html -type f -exec chmod 644 {} \;
sudo find /var/www/html -type d -exec chmod 755 {} \;

Transfer Ownership​

sudo chown -R newuser:newuser /home/olduser

Shared Project Directory​

sudo chgrp -R developers /projects
sudo chmod -R 775 /projects

What's Next?​

Now that you understand ownership, you can manage file access. Or learn about Links to create file shortcuts.

Personal note: When I started, I'd use sudo chown for everything. Then I learned about proper ownership. Now I set it correctly. Ownership is Linux's way of organizing access. Learn it. Use it.