Linux File Permissions and Ownership (chmod, chown, groups)

Understanding Linux File Permissions

Run ls -l to see file permissions:

-rw-r--r--  1 alex  users   1234 Aug 16 10:00 notes.txt

• – → file type (- file, d directory, l link)
• rw- → owner permissions (read + write)
• r– → group permissions (read only)
• r– → others permissions (read only)

Linux File Permissions Types

• r = read (4)
• w = write (2)
• x = execute (1)

Numbers combine these values (e.g., 7 = rwx, 6 = rw-, 5 = r-x).

Changing Permissions with chmod

# symbolic method
chmod u+x script.sh     # give execute to user
chmod g-w report.txt    # remove write from group
chmod o-r secret.txt    # remove read from others

# numeric method
chmod 755 script.sh     # rwx for owner, r-x for group & others
chmod 644 notes.txt     # rw- for owner, r-- for group & others

Changing Ownership with chown

# change file owner
sudo chown alex notes.txt

# change owner and group
sudo chown alex:staff notes.txt

# recursively change a directory
sudo chown -R alex:staff /var/www/html

Groups in Linux

Groups allow multiple users to share file access.

# add user to group
sudo usermod -aG developers alex

# list groups for user
groups alex

# create a new group
sudo groupadd editors

Special Permissions

• Setuid (s): run file with owner’s privileges.
• Setgid (s): files created in directory inherit group.
• Sticky bit (t): in shared dirs, only owner can delete their files (common in /tmp).

# example: sticky bit on /shared
sudo chmod +t /shared