File Permission in Linux

Each file and directory on your system is assigned access rights for the owner of the file, the members of a group of related users, and everybody else. Rights can be assigned to read a file, to write a file, and to execute a file (i.e., run the file as a program).

To see the permission settings for a file, we can use the ls command as follows:

ls -l

Here we can see:

The file "vmware-install.pl" is owned by user "root"
User "root" has the right to read, write and execute this file
The file is owned by the group "root"
Members of the group "root" can also read and execute this file
Everybody else can also read execute this file

Chmod:

The chmod command is used to change the permissions of a file or directory. To use it, you specify the desired permission settings and the file or files that you wish to modify

There are two ways to use this command:

1. Absolute mode

2. Symbolic mode

Absolute (Numeric) Mode:

In this mode file permissions are not represented as characters but a three digit octal number

The table below gives numbers for all for permissions types:

Value	Binary	String	Description
0 1 2 3 4 5 6 7	0 1 10 11 100 101 110 111	--- - -x -w- -wx r-- r-x rw- rwx	No Permission Execute only Write only Write & Execute Read only Read & Execute Read & Write Read, Write & Execute

Let's see the chmod command in action:

chmod 764 (permission) vmware-install.pl (filename)

In the above given image, we have changed the permissions of the file 'vmware-install.pl’ to '764'

764 absolute code says the following:

Owner can read, write and execute
User group can read and write
Other can only read

This is how you can change the permissions on a file by assigning an absolute number

Symbolic Mode

In the symbolic mode you can modify permissions of a specific owner. It makes use of mathematical symbols to modify the file permissions

Operator	Description
+	Adds a permission to a file or directory
-	Removes the permission
=	Sets the permission and overrides the permissions set earlier

The various owners are represented as :

User Denotation
u	User/Owner
g	Group
o	Other
a	All

We will be using permissions in characters like rwx:

chmod u-w,g-rw,o-w (permission) vmware-install.pl (filename)

In the above given image, we are removing write permission for user, read & write permission for group and write permission for others

Chmod u+w,g+rx,o+x (permission) vmware-install.pl (filename)

In the above image, we are adding write permission for user, read & execute permission for group and execute permission for other

Changing file ownership

You can change the owner of a file by using the chown command. Here's an example:

ls -l

Chown user1 (username) vmware-install.pl (filename)

In the first above image, we can see the file vmware-install.pl belongs to user root. Now using chown we can change the file owner to user1 as shown in above given second image