File permissions notation in Linux

Every file or folder in Linux has access permissions. There are three types of permissions (what allowed to do with a file):

• read access
• write access
• execute access

Permissions are defined for three types of users:

• owner of the file
• group that the owner belongs to
• other users

Numeric (octal) representation

Octal digit	Text equivalent	Binary value	Meaning
0	---	000	All types of access are denied
1	--x	001	Execute access is allowed only
2	-w-	010	Write access is allowed only
3	-wx	011	Write and execute access are allowed
4	r--	100	Read access is allowed only
5	r-x	101	Read and execute access are allowed
6	rw-	110	Read and write access are allowed
7	rwx	111	Everything is allowed

Difference in access permissions for files and folders

Access type	File	Folder
Read	can read	can listing
Write	can write to file	can create, rename, delete files in direcory
Execute	can be executed	can pass through or into

Special bit

setuid

Execute file with owner permission (example: program su)

-rws------

bit:4000

setgid

If file execute this with group permission.

If folder save every file into folder with folder group permission. All file in folder have been the same group.

-rwxrws---

bit:2000

sticky

If file already is not supported

If folder has sticky bit can delete files into folder only owner of file or superuser.

-rwxrwxrwt

bit:1000

File types

Regular file

-

Directory

d

Link

l

Special files

c

Socket

s

Named pipe

p

Block device

b

Examples

• chmod 644 myFile.txt
• chmod 4700 /bin/ls (setuid)
• chmod 2770 /bin/ls (setgid)
• chmod 1777 myFolder (sticky)
• chown tomas myFile.txt
• chgrp user myFile.txt
• chown tomas.user myFile.txt