chmod Explained: Linux File Permissions, Octals, and rwx Notation

Every file and directory on a Unix system has a permission set that controls who can read it, write to it, or execute it. When a deployment fails because your web server cannot read a config file, or your CI script is not executable, the fix is usually one chmod command — once you understand what the numbers mean.

The three permission classes

Unix permissions apply to three categories of user:

The three permission bits

Symbolic notation: rwxr-xr--

Run ls -l and you will see something like -rwxr-xr--. Reading left to right:

• Position 1: file type (- regular file, d directory, l symlink)
• Positions 2–4: owner permissions (rwx = read, write, execute)
• Positions 5–7: group permissions (r-x = read, no write, execute)
• Positions 8–10: others permissions (r-- = read only)

A dash - in any position means that permission is not granted.

Octal notation

Each permission triplet maps to a single octal digit (0–7) because there are exactly three bits:

So chmod 755 means: owner=7(rwx), group=5(r-x), others=5(r-x).

Common permission patterns

Using chmod

# Set permissions using octal
chmod 755 deploy.sh
chmod 644 config.yaml
chmod 600 ~/.ssh/id_rsa

# Set permissions using symbolic mode
chmod u+x deploy.sh        # add execute for owner
chmod go-w sensitive.txt   # remove write from group and others
chmod a+r public.html      # add read for all (a = all)

# Recursive — apply to directory and all contents
chmod -R 755 /var/www/html

Setuid, setgid, and sticky bit

A fourth octal digit controls three special bits:

• Setuid (4xxx) — executable runs as the file's owner, not the caller. Used by sudo and passwd. Example: chmod 4755 binary
• Setgid (2xxx) — on files: runs as the file's group. On directories: new files inherit the directory's group, not the creator's. Example: chmod 2755 shared-dir/
• Sticky bit (1xxx) — on directories: only the owner can delete their own files, even if others have write permission. Used on /tmp. Example: chmod 1777 /tmp

Checking current permissions

ls -la            # show permissions for all files in directory
stat file.txt     # detailed permissions, owner, size, timestamps
find . -perm 777  # find world-writable files (security audit)