File & Directory Permission in Linux

We can change permissions of file/directory using `chmod` command.

Basic

Permission	Number	Character
Read	4	r
Write	2	w
Execute	1	x

We can use these value combination for changing permission. For example,

Permission	Number	Character
Read + Write + Execute	4+2+1=7	rwx
Read + Write	4+2=6	rw
Read	4	r

Cheat Sheet

Read/Write/Execute

Target	Command
User	chmod 700 sample.txt
	chmod u+rwx sample.txt
Group	chmod 070 sample.txt
	chmod g+rwx sample.txt
Others	chmod 007 sample.txt
	chmod o+rwx sample.txt
All	chmod 777 sample.txt
	chmod a+rwx sample.txt

Read/Write

Target	Command
User	chmod 600 sample.txt
	chmod u+rw sample.txt
Group	chmod 060 sample.txt
	chmod g+rw sample.txt
Others	chmod 006 sample.txt
	chmod o+rw sample.txt
All	chmod 666 sample.txt
	chmod a+rw sample.txt

Read/Execute

Target	Command
User	chmod 500 sample.txt
	chmod u+rx sample.txt
Group	chmod 050 sample.txt
	chmod g+rx sample.txt
Others	chmod 005 sample.txt
	chmod o+rx sample.txt
All	chmod 555 sample.txt
	chmod a+rx sample.txt

Write/Execute

Target	Command
User	chmod 300 sample.txt
	chmod u+wx sample.txt
Group	chmod 030 sample.txt
	chmod g+wx sample.txt
Others	chmod 003 sample.txt
	chmod o+wx sample.txt
All	chmod 333 sample.txt
	chmod a+wx sample.txt

Set SUID/GUID

By adding SUID or GUID, we can execute the file as the file owner/group. This may cause local privilege escalation.

# SUID for user owner
chmod u+s /opt/example.sh

# GUID for group owner
chmod g+s /opt/example.sh