Linux file permissions – My Point of View

Linux is UNIX clone. For effective security, Linux divides authorization into 2 levels, Ownership and Permission.

Every file and directory is assigned 3 types of owner; User, Group and Other.

User is the owner of the file. By default, the person who creates a file becomes its owner. A logged-in user sometimes called an owner.
A user-group can contain multiple users. All users belonging to a group can have similar permissions. In this case the ownership belong to a group.
Other means setting permissions to the world. This person has neither create a file, nor belongs to a user group.

How Linux distinguish between these three users? This is where permissions comes in. Permission defines the user behavior.

Every file and directory has 3 permissions defined for 3 owners.

Read permission gives the authority to open and read a file. Read permission on directory gives the ability to lists its content.
Write permission gives the authority to modify the contents of a file. The write permission on a directory gives the authority to add, remove and rename files. Consider a scenario where you have to write permission on 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 to rename, move or remove the file from the directory.
Execute permission gives the authority to execute a program. 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 run it.

To summarize this, the Owner assigned permissions on every File and Directory.

Some Examples

ls -l

The highlighted “-rw-rw-r–“ code is the one that tells us the permission given to owner, group or the world.

The first ‘–‘ tells us that we have selected a file.

If this were a directory, d would be shown here;

The characters are pretty easy to remember.

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

Let’s read and understand our sample file;

The first part of the code is ‘rw-‘. This is for the user. This tells us that the user can:

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

The second part is ‘rw-‘. This is for the user group for example ‘Home’. 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 permission using ‘chmod’ command

To restrict user access (owner, group and the world) to files and folders we will use permissions (read, write, execute) using chmod (change mode) command.

chmod permissions filename

We can use Absolute (Numeric) mode or Symbolic mode to do this.

In Absolute mode, file permissions are not represented as characters but three-digit octal number. Here is a table that shows all possible permission 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

Some Example

Checking current file permission;

chmod 764 and checking permission again;

chmod 764 MySmpleFile.txt

764 absolute code means;

Owner can read, write and execute (7). Usergroup can read and write (6). World can only read (4). This is shown as ‘-rwxrw-r–. In this mode we change permission for all 3 owners.

In Symbolic mode we change permissions of specific owner. It uses mathematical symbols.