Permissions in Linux

The Linux filesystem gives us three types of permissions. Here is a simplified review:

Owner permissions − The owner’s permissions specifies what actions the owner of the file can perform on the file.

Group permissions − The group’s permissions specifies what actions a user, who is a member of the group that a file belongs to, can perform on the file.

Other (world) permissions − The permissions for others specifies what action all other users can perform on the file.

User (or user owner)
Group (or owner group)
Other (everyone else)

File Access Modes –

Access modes defines the level of permissions a user/group or others have for a file or directory :

Read – Grants the capability to read, i.e., view the contents of the file.

Write – Grants the capability to modify, or remove the content of the file.

Execute – User with execute permissions can run a file as a program.

Read
Write
eXecute

The permissions can be assigned/removed in form of numbers using +, – or = operators :-

Number	Permission Representation	Ref
0	No permission	—
1	Execute permission	–x
2	Write permission	-w-
3	Execute and write permission: 1 (execute) + 2 (write) = 3	-wx
4	Read permission	r–
5	Read and execute permission: 4 (read) + 1 (execute) = 5	r-x
6	Read and write permission: 4 (read) + 2 (write) = 6	rw-
7	All permissions: 4 (read) + 2 (write) + 1 (execute) = 7	rwx

Number	Chmod operator	Description
1	+	Adds the permission
2	–	Removes the permission
3	=	Sets the permission

chmod :-

chmod is used to manipulate the file/directory’s permissions.

Example :

Original permission –

# ls -l test
-rw-r--r-- 1 root root 18 Jun 29 21:56 test

Added execute permission for the file’s “owner”, “group” as well as “others”

# chmod +x test

# ls -l test
-rwxr-xr-x 1 root root 18 Jun 29 21:56 test

Added write and execute permission to the “owner” and “group”(hence 7(4+2+1),7(4+2+1)) ; read and execute permission to “others” (5 (4+1)) hence

# chmod 775 test

# ls -l test
-rwxrwxr-x 1 root root 18 Jun 29 21:56 test
#

chown :-

chown is used to change the ownership of a file or directory.

# ls -l test
-rwxrwxr-x 1 root root 18 Jun 29 21:56 test



# chown jay test
# ls -l test
-rwxrwxr-x 1 jay root 18 Jun 29 21:56 test
#

owner and group both can be changed with a single chown command –

# chown jit:root test

# ls -l test
-rwxrwxr-x 1 jit root 18 Jun 29 21:56 test
#

chgrp :-

This is used to change the group for the file or directory.

# ls -l test
-rwxrwxr-x 1 jay root 18 Jun 29 21:56 test


# chgrp sys test
# ls -l test
-rwxrwxr-x 1 jay sys 18 Jun 29 21:56 test

#