User Permission

 

chmod (change mode)

In Unix-like operating systems, the chmod command is used to change the access mode of a file.

The references are used to distinguish the users to whom the permissions apply i.e. they are list of letters that specifies whom to give permissions

Reference	Class	Description
u	owner	file's owner
g	group	Member of file's group
o	other	Members those neither owner or file group members

Operators

Operator	Description
+	owner	Add permission
-	group	To remove permission

Permission Modes

Mode	Description
r	Read Permission
w	Write Permission
x	Execute Permission

Mode Numeric values

Value	Permissions	Description
7	rwx	Read Write Execute Permission
6	rw	Read Write Permission
5	rx	Read Execute Permission
4	r	Read Write Permission
3	wx	Write Execute Permission
0	---	No permission

Login with root user and create an empty file and check its permission

sudo su
touch tmpfile.txt
ls -l tmpfile.txt

output
-rw-r--r-- 1 root root 0 Sep 25 11:36 tmpfile.txt

Change permission to this file owner can read and write but group users can read and other donot have any permission to access this file

chmod 640 tmpfile.txt
ls -l tmpfile.txt

Login with a user and check other user can read it or notcat: tmpfile.txt: Permission denied

su testuser
cat tmpfile.txt

output will be
cat: tmpfile.txt: Permission denied

Login agin with root user and provide read access to other users

exit
echo "Test file content" >> tmpfile.txt
chmod o+r tmpfile.txt

Login with a user and check other user can read it or notcat: tmpfile.txt: Permission denied

su testuser
cat tmpfile.txt

output
Test file content