Unix File Permissions – An Introduction

Everything in Unix is considered to be a file, including physical devices such as USB devices, floppy drives, DVD-ROMs, etc.

Every file or directory is uniquely identified by its name, the directory in which it resides and a unique identifier, usually called an inode.

There are several types of files available in Unix, each file type is different from another and have different purpose to serve.

Unix File Permissions

For every unix file or directory, mostly there are three kinds of permissions

     r   –   read    => Read/view the file

     w   –   write   => create, edit or delete              

     x   –   execute => run a script/program or enter a directory

There are three levels which defines who can access the file

     u   –    user  => owner of the file

     g   –    group => a bunch of people in the same project

     o   –    other => everyone on the system

Let’s take an example of the output of “ls -lt” command to explain the file permission

 -rwxr-xr-x 1 root quest 3743 Apr 30 2012 server.sh

Setting up the file permissions

To setup the file permissions we can use “chmod” command, but first we should understand the meaning of different permissions

The number and letter represents :

Read      ==      r     ==     4

Write     ==     w    ==     2

Execute ==     x     ==     1

You can use chmod command in different ways :

Examples :

chmod 755 file_name

-rwxr-xr-x    1      root   quest   3743   Apr  30  2012      file_name

chmod –R 755 directory_name

-rwxr-xr-x    1      root   quest   3743   Apr  30  2012      directory_name

-R is used to recursively change the content of directory as well

chmod 755 *

-rwxr-xr-x    1      root   quest   3743   Apr  30  2012      file_name

-rwxr-xr-x    1      root   quest   3743   Apr  30  2012      file1_name

-rwxr-xr-x    1      root   quest   3743   Apr  30  2012      file2_name

* denotes everything in current directory

chmod 777 file_name

-rwxrwxrwx     1     root   quest   3743   Apr  30  2012      file_name

Detailed description about chmod command can be found at wiki

This article is just to explain the Unix File System, We will cover several basic/advanced Unix commands in our next article.

So, till then please be tuned and enjoy reading…