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Linux File Permissions

Linux is a multi-user OS. To protect user data, it employs a strict permissions system. Every file and directory is owned by a specific user and a specific group, and has access rights defined for three categories of users.

Permission Categories​

  • User (u): The owner of the file.
  • Group (g): Other users who are members of the file's group.
  • Others (o): Everyone else.

Permission Types​

  • Read (r): View file contents or list directory contents. Value = 4.
  • Write (w): Modify file contents or create/delete files within a directory. Value = 2.
  • Execute (x): Run a file as a program or enter a directory. Value = 1.

Practical Examples​

Viewing Permissions​

Run ls -l to view permissions.

-rwxr-xr-- 1 user group 1024 Jan 1 12:00 script.sh

This means:

  • User has read, write, execute (rwx)
  • Group has read, execute (r-x)
  • Others have read only (r--)

Changing Permissions (chmod)​

You can use symbolic or numeric (octal) modes.

# Give owner execute permission
chmod u+x script.sh

# Set permissions to rwxr-xr-x (755)
chmod 755 script.sh

Changing Ownership (chown)​

Only root can change file ownership.

# Change owner to 'alice' and group to 'devs'
chown alice:devs project.txt