chmod Command Explained: Unix File Permissions Made Simple

What Are File Permissions?

Every file and directory in Unix/Linux has three sets of permissions that control who can read, write, and execute it:

• Owner (u) — The user who owns the file
• Group (g) — Users in the file's group
• Others (o) — Everyone else

Each set has three permission types:

Permission	Symbol	Numeric	Meaning for Files	Meaning for Directories
Read	r	4	View file contents	List directory contents
Write	w	2	Modify file	Create/delete files in directory
Execute	x	1	Run as program	Enter (cd into) directory

Reading Permission Strings

When you run ls -la, you see permissions like:

-rwxr-xr-x  1 alice  staff  4096 Jan 15 10:30 script.sh
drwxr-xr-x  5 alice  staff   160 Jan 15 10:30 src/

Breaking down -rwxr-xr-x:

-    rwx    r-x    r-x
│    │      │      │
│    │      │      └── Others: read + execute
│    │      └── Group: read + execute
│    └── Owner: read + write + execute
└── File type (- = file, d = directory, l = symlink)

Numeric (Octal) Notation

Each permission has a numeric value. Add them up for each set:

r = 4
w = 2
x = 1

rwx = 4 + 2 + 1 = 7
r-x = 4 + 0 + 1 = 5
r-- = 4 + 0 + 0 = 4
--- = 0 + 0 + 0 = 0

So rwxr-xr-x = 755

Common Permission Numbers

Number	Permissions	Use Case
755	rwxr-xr-x	Executable scripts, directories
644	rw-r--r--	Regular files (HTML, CSS, images)
600	rw-------	Private files (SSH keys, .env)
700	rwx------	Private directories, scripts
777	rwxrwxrwx	Full access (avoid in production!)
444	r--r--r--	Read-only files
750	rwxr-x---	Group-accessible executables
640	rw-r-----	Group-readable files

The chmod Command

Numeric Mode

chmod 755 script.sh
chmod 644 index.html
chmod 600 .env
chmod -R 755 public/    # Recursive

Symbolic Mode

chmod u+x script.sh      # Add execute for owner
chmod g+w file.txt        # Add write for group
chmod o-r secret.txt      # Remove read for others
chmod a+r public.html     # Add read for all
chmod u=rwx,g=rx,o=rx script.sh  # Set exact permissions

Symbolic Operators

Operator	Meaning
+	Add permission
-	Remove permission
=	Set exact permission

Symbolic Targets

Target	Meaning
u	Owner (user)
g	Group
o	Others
a	All (owner + group + others)

Security Best Practices

1. Never use 777 — It gives everyone full access. If something "only works with 777", the real problem is elsewhere.

2. SSH keys must be 600 — SSH refuses to use keys with loose permissions:

chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub
chmod 700 ~/.ssh/

3. Web server files: 644, directories: 755 — The web server user needs read access, not write.

4. Environment files: 600 — .env files contain secrets and should only be readable by the owner.

5. Use groups for shared access — Instead of making files world-readable, add users to a group.

Calculate permissions visually with our Chmod Calculator — toggle permissions and get the numeric and symbolic notation instantly.

Related Tools & Articles

• Chmod Calculator — Visual permission calculator
• Linux Commands Cheat Sheet — Essential Linux commands
• Docker Commands Cheat Sheet — Docker reference
• Git Commands Cheat Sheet — Git reference