How to Change File Permissions on Linux

How Linux Permissions Work

Every file on Linux has an owner, a group, and three sets of permissions: one for the owner, one for the group, and one for everyone else (other). Run ls -l to see them:

-rwxr-xr-- 1 alice devs 4096 Jun 1 10:00 deploy.sh

The permission string -rwxr-xr-- breaks down as:

• - — file type (- = file, d = directory, l = symlink)
• rwx — owner can read, write, execute
• r-x — group can read and execute, not write
• r-- — others can only read

chmod — Change Permissions

You can set permissions with symbolic or numeric (octal) notation.

Symbolic notation

chmod +x script.sh            # add execute for everyone
chmod u+w file.txt            # add write for the owner
chmod go-w secret.txt         # remove write from group and others
chmod a+r public.html         # add read for all

Letters: u=owner, g=group, o=others, a=all. Operators: + add, - remove, = set exactly.

Numeric (octal) notation

Each permission is a number: read=4, write=2, execute=1. Add them for each group:

chmod 755 deploy.sh     # owner: rwx (7), group: r-x (5), others: r-x (5)
chmod 644 config.txt    # owner: rw- (6), group: r-- (4), others: r-- (4)
chmod 600 id_rsa        # owner: rw- (6), group: none (0), others: none (0)

Common patterns to memorise:

• 755 — executable script (owner full control, others read/execute)
• 644 — regular file (owner read/write, others read-only)
• 600 — private file like SSH keys (owner only)
• 777 — everyone full access (avoid in production)

Recursive permissions

chmod -R 755 /var/www/html     # set 755 on all files in directory

chown — Change Owner and Group

chown alice file.txt               # change owner to alice
chown alice:devs file.txt          # change owner and group
chown -R www-data /var/www/html    # recursive ownership change
sudo chown root /etc/sudoers       # system files need sudo

Practical Examples

# Make a script executable and run it
chmod +x ~/scripts/backup.sh
~/scripts/backup.sh

# Fix a web server's document root
sudo chown -R www-data:www-data /var/www/html
sudo chmod -R 755 /var/www/html

# Lock down an SSH private key
chmod 600 ~/.ssh/id_ed25519