chmod & File Permissions Cheatsheet

A quick reference for Unix file permissions — numeric (octal) and symbolic modes, common permission patterns, and special bits.

Updated Apr 11, 2026

Sections

Bit	Octal	Symbolic	Meaning
`Read`	4	r	View file contents / list directory
`Write`	2	w	Modify file / create & delete in directory
`Execute`	1	x	Run as program / enter directory
`None`	0	-	No permission

Three octal digits represent permissions for owner (u), group (g), and others (o). Each digit is the sum of read (4), write (2), execute (1).

Octal	Symbolic	Typical Use
`755`	rwxr-xr-x	Directories, executables — owner can write, others can read & execute
`644`	rw-r--r--	Regular files — owner can write, others read-only
`600`	rw-------	Private files — SSH private keys, .env files
`700`	rwx------	Private directories or executables
`777`	rwxrwxrwx	Fully open — avoid in production; only for temp dirs
`666`	rw-rw-rw-	Writable by everyone — avoid
`444`	r--r--r--	Read-only for everyone
`400`	r--------	Owner read-only — common for SSH private keys
`775`	rwxrwxr-x	Shared project dirs — owner & group can write
`664`	rw-rw-r--	Shared files — owner & group can write

Syntax	Example	Description
`u+x`	chmod u+x file	Add execute for owner
`g-w`	chmod g-w file	Remove write from group
`o=r`	chmod o=r file	Set others to read-only (exactly)
`a+x`	chmod a+x file	Add execute for all (owner, group, others)
`u=rwx,go=rx`	chmod u=rwx,go=rx file	Owner full, group and others read+execute
`+x`	chmod +x file	Add execute for all (shorthand for a+x)
`-R`	chmod -R 755 dir/	Apply recursively to directory and contents

Bit	Octal	Symbolic	Effect
`Setuid (SUID)`	4xxx	u+s	Execute as the file owner — e.g. chmod 4755 (rwsr-xr-x)
`Setgid (SGID)`	2xxx	g+s	Execute as the group owner; new files in dir inherit group
`Sticky bit`	1xxx	+t	Only owner can delete files in directory — e.g. /tmp (rwxrwxrwt)

Command	Description
`ls -l`	List files with permission strings (e.g. -rw-r--r--)
`stat file`	Show detailed file info including octal permissions
`chmod 644 file`	Set permissions using octal
`chmod u+x file`	Set permissions using symbolic mode
`chmod -R 755 dir/`	Recursively set permissions on a directory
`chown user:group file`	Change file owner and group
`chown -R user:group dir/`	Recursively change owner and group
`umask`	Show default permission mask (subtracted from 666/777 on new files/dirs)
`umask 022`	Set umask — new files get 644, new dirs get 755