Understanding Users and Groups

A user is an individual entity that uses the system. Each user is assigned a unique User ID (UID), where UID 0 generally represents the root user, while normal users are assigned UIDs greater than 1000. Each user has a home directory where they can store personal files and configurations, and they can set their preferred shell, environment variables, and access permissions individually.

In contrast, a group is a collection of users that can be managed as a unit, allowing permissions to be assigned to files or directories at the group level. Users can belong to multiple groups simultaneously, typically having a primary group assigned during account creation and additional supplementary groups added as needed. Groups also have a unique Group ID (GID) like users.

Significance of File Ownership

In Linux, every file and directory has both an owner and an owning group. This forms the basis for determining access permissions to that file. For example, when running a web server, a dedicated user account like www-data typically runs the web server processes, and developers belonging to the webdev group can be granted the ability to modify files in the web server.

File ownership can be viewed using the ls -l command:

-rw-r--r-- 1 john developers 123 Feb 17 15:50 project.txt

Here, john represents the owner of the file, while developers represent the owning group.

Role of the chown Command

The command for changing file ownership is chown, short for change owner. It allows you to modify the owner or owning group of a file or directory. It’s important to note that only the root user can change ownership.

The basic usage is as follows:

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# Change owner only
chown user file

# Change both owner and group
chown user:group file

# Change group only
chown :group file

Practical Use Cases

One of the most common use cases is managing files for a web server. Since web servers like nginx or Apache run as the www-data user, files that the web server needs to access should have their ownership set appropriately:

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# Set ownership of web root
chown -R www-data:www-data /var/www/html/

# Grant write permission to developer group
chown :developers /var/www/html/dev/
chmod g+w /var/www/html/dev/

Database servers can be managed in a similar manner:

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# Set ownership of DB data directory
chown -R mysql:mysql /var/lib/mysql/

# Set permissions for backup directory
chown :dbadmin /backup/mysql/
chmod 775 /backup/mysql/

Caveats

Ownership of system files should be treated with special care, as incorrect ownership can affect the entire system:

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# Correct ownership of system files
chown root:root /etc/passwd
chown root:shadow /etc/shadow

Proper ownership of user home directories is also important:

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# Set ownership of home directory
chown -R user:user /home/user/

# SSH configuration files are particularly sensitive
chown -R user:user ~/.ssh/
chmod 700 ~/.ssh/

Best Practices

The key to effective ownership management is to make only minimal changes when necessary. In particular, it’s recommended to:

  1. Leave system file ownership alone
  2. Utilize group permissions for shared directories
  3. Regularly review ownership of critical files
  4. Document any changes you make