Cheat Sheet

Chmod

Unix file permissions reference: octal values, symbolic notation, common chmod values, special bits (SUID, SGID, sticky), and useful commands.

Unix file permissions control who can read (r), write (w), and execute (x) a file or directory. Permissions are set for three groups: Owner, Group, and Others.

Permission Bits

Symbol	Octal	Permission	On file	On directory
r	4	Read	View file contents	List directory contents
w	2	Write	Modify or delete file	Create, delete, rename files inside
x	1	Execute	Run as a program	Enter (cd) the directory
-	0	None	No permission	No permission

Common Octal Values

Octal	Symbolic	Meaning	Typical use
777	rwxrwxrwx	Everyone has full access	Avoid — dangerous
755	rwxr-xr-x	Owner full, group/others read+execute	Executables, public dirs
750	rwxr-x---	Owner full, group read+execute	Restricted executables
700	rwx------	Owner full, others none	Private scripts
644	rw-r--r--	Owner read+write, others read-only	Regular files, configs
640	rw-r-----	Owner read+write, group read-only	Sensitive configs
600	rw-------	Owner read+write only	SSH keys, secrets
400	r--------	Owner read-only	Read-only credentials

Symbolic Notation (chmod)

chmod +x fileAdd execute for all

chmod -x fileRemove execute for all

chmod u+x fileAdd execute for owner only

chmod g-w fileRemove write from group

chmod o=r fileSet others to read-only exactly

chmod a=rx dirSet everyone to read + execute

chmod -R 755 dir/Recursive — apply to all files in dir

Who

u user (owner)

g group

o others

a all (u+g+o)

Operator

+ add permission

- remove permission

= set exactly

Permission

r read

w write

x execute

Reading `ls -l` Output

-rwxr-xr-x  1  alice  staff  4096  Jan 10  file.sh
drwxr-x---  2  alice  staff  128   Jan 10  mydir/

-rwxr-xr-xFile type (1 char) + permissions (9 chars: owner / group / others)

1Hard link count

aliceOwner user

staffOwner group

4096File size in bytes

Jan 10Last modification date

First character: - file · d directory · l symlink · b block device

Special Bits

4xxxSetUID (SUID)chmod u+s file

When set on an executable, it runs as the file owner, not the user running it. Shown as "s" in owner execute position. E.g. /usr/bin/passwd.

2xxxSetGID (SGID)chmod g+s dir

On a file: runs as the file group. On a directory: new files inherit the directory's group instead of the creator's.

1xxxSticky Bitchmod +t dir

On a directory, only the file owner (or root) can delete or rename files inside. Shown as "t". Used on /tmp.

Useful Commands

chmod 755 fileSet permissions by octal

chmod -R 644 dir/Recursive chmod

chown alice fileChange owner

chown alice:staff fileChange owner and group

chown -R alice:staff dir/Recursive chown

umask 022Set default permission mask (files get 644, dirs 755)

umaskShow current umask

stat fileShow full metadata including permissions in octal

find . -perm 777Find files with specific permissions

find . -perm /o+wFind world-writable files

Calculate permissions visually with our Chmod Calculator →

Click checkboxes and get the numeric and symbolic notation instantly.