File permission with chmod

File permission with chmod

The chmod command is used to give and set file permissions

Syntax 

        Chmod [permissions] filenames

Permissions: Specifies what rights are you granting

Filenames: File or directory to which you are assigning the permissions.
When you start granting permissions, you might opt for any of the following options: letters or numerical (I prefer numerical).

Permissions that you can grant:

By letter		Numerical
U	User who owns the file	400	Read by owner
g	group that owns the file	040	Read by group
o	Other	004	Read by others
a	All	200	Write by owner
r	Readable file	020	Write by group
w	Writeable file	002	Write by others
x	Executable file	100	Execute by owner
		010	Execute by group
		001	Execute by others

There is something else that we need to understand before we can actually give permissions. First we have to know what type of permissions we are granting, and to whom it will be granted.

User IDS and Group IDS UID/GID

Under the UNIX operating system, every user is member of a group by default when you create a new user that user is member of its own group or member of users. The administrator can make specific users be part of any other group. Keep in mind that when the system executes or does something under a user name, that system does not actually see that user name, instead it sees just a user ID.

• user ID stands as a number
• A user name stands as a string and it is associated to the ID number

This is exactly the same for groups:

• A group ID is a number
• A group name is a string associated to the group ID number.