FILE PERMISSION IN LINUX/UNIX

Permissions

Every file and directory in your UNIX/Linux system has following 3 permissions defined for all the 3 owners discussed above

• Read : This permission give you the authority to open and read a file. Read permission on a directory gives you the ability to lists it's content.
• Write: The right permission gives you the authority to modify the contents of a file. The write permission on a directory gives you the authority to add , remove and rename files stored in the directory.  Consider a scenario where you have write permission on the file but do not have write permission on the directory where the file is stored. You will be able to modify the file contents. But you will not be able rename, move or remove the file from the directory.
• Execute : In windows an executable program usually has an extension ".exe" and which you can easily run. In Unix/Linux , you cannot run a program unless the execute permission is set. If the execute permission is not set, you might still be able to see/modify the program code(provided read & write permissions are set), but not actually run it.

Lets see this in action

ls - l on terminal gives

Here, we have highlighted '-rw-rw-r--'and this weird looking code is the one that tells us about the permissions given to the owner, user group and the world.

Here, the first '-' implies that we have selected a file. 

Else, if it were a directory , d would have been shown.

The characters are pretty easy to remember.

r = read permission
w = write permission
x = execute permission
- = no permission

Let us look at it this way.

The first part of the code is 'rw-'. This suggests that the owner 'Home' can:

Read the file Write or edit the file He cannot execute the file since , the execute bit is set to '-'.

By design many Linux distributions like Fedora , CentOS , Ubuntu etc , will add users to a group of the same group name as the user name. Thus, a user 'tom' is added to a group named 'tom'.

The second part is 'rw-'. It for the user group 'Home' and group-members can:

• Read the file
• Write or edit the file

The third part is for the world which means any user . It says 'r--'. This means the user can only:

• Read the file

Changing file/directory permissions with 'chmod' command

Say you do not want your colleague to see your personal images.  This can be achieved by changing file permissions.

We can use the 'chmod' command which stands for 'change mode'. Using the command, we can set permissions (read, write, execute) on a file/directory for the owner, group and the world.

Syntax:

chmod  permissions filename

There are 2 ways to use the 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.

                

Number	Permission Type	Symbol
0	No Permission	---
1	Execute	--x
2	Write	-w-
3	Execute + Write	-wx
4	Read	r--
5	Read + Execute	r-x
6	Read +Write	rw-
7	Read + Write +Execute	rwx

Let's see the chmod command in action.

In the above given terminal window, we have changed the permissions of the file 'sample to '764'. 

'764' absolute code says the following: Owner can read, write and execute Usergroup can read and write World can only read This is shown as '-rwxrw-r-

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

Symbolic Mode

In the Absolute mode you change permissions for all  3 owners. 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 Denotations
u	user/owner
g	group
o	other
a	all

We will not be using permissions in numbers like 755 but characters like rwx. Let's look into an example

Changing Ownership and Group

For changing the ownership of a file/directory , you can use the following command:

chown user

In case you want to change the user as well as group for a file or directory use the command

chown user:group

Let's see this in action 

In case , you want to change  group-owner only , use the command

chgrp  

'chgrp' stands for change group. 

Tip

• The file /etc/group contains all the groups defined in the system
• You can use the command "groups" to find all the groups you are a member of 
• 
• You can use the command newgrp to work as a member a group other than your default group
• .
• You cannot have 2 groups owning the same file. 
• You do not have nested groups in Linux. One group can not be sub-group of other
• x- eXecuting a directory means  Being allowed to "enter" a dir and gain possible access to sub-dirs
• There are other permissions that you can set on Files and Directories which will be covered in a later advanced tutorial