Unix File Permissions (chmod) Explained with Octal

What is chmod?

chmod changes file permissions on Unix/Linux/macOS. Modes can be symbolic (u+x) or octal (755). Octal is compact: three digits encode owner, group, and others.

Permission Bits

Each category has three bits:

Bit	Value	Meaning
r (read)	4	View file / list directory
w (write)	2	Modify file / create in directory
x (execute)	1	Run script / enter directory

Sum bits for the digit: rwx = 4+2+1 = 7, rw- = 6, r-x = 5, r— = 4.

Common Modes

Octal	Symbolic	Typical Use
755	rwxr-xr-x	Executable scripts, directories
644	rw-r—r—	Regular files
600	rw-------	Private files (SSH keys)
777	rwxrwxrwx	Open (avoid in production)

Example: chmod 755 script.sh

Owner: 7 = rwx (full). Group: 5 = r-x (read+execute). Others: 5 = r-x.

Octal ↔ Binary

Each digit = 3 bits. 7 = 111, 5 = 101. See Octal Conversion Guide.

Tools

Convert permission digits with Number System Converter — enter octal base 8 to see binary breakdown.

Frequently Asked Questions

What is umask?

Default permission mask subtracted from new files — often 022 giving 644 for files.

Can directories be executable?

Yes — x on directories means “search/enter” permission.

Why not use decimal for chmod?

Historical Unix convention — octal maps cleanly to 3-bit groups.

What is sticky bit?

Special fourth digit (e.g. 1777) — advanced topic beyond basic chmod.

Where learn more number systems?

Number System Guide on Numverto.