Unix File Permissions

Unix File Permissions | What is Chmod command in unix?

Unix File Permissions :

In previous chapter we have discussed different file commands,directory commands. We had also discussed about process commands  with examples and grep command with example in previous articles. In this article i will try to explain the most important topic which is related to Unix File Permissions,directories,how to give the permissions using chmod command. File permissions are most important component of unix operating system because it secures the files by giving the specific permissions.There are following types of permissions :

Owner Permissions :

Owner Permissions are Unix File Permissions given to the specific owner to open,read,write or execute the file

Group Permissions :

Group permissions are Unix File Permissions  to specific group of users to open,read,write and execute the file.

World Permissions :

These are Unix File Permissions  to other user to perform open,read,write, execute file permission.


1) Default Permissions for Regular File:

Open File                                                         Read

Write or Modify                                               Write

Execute                                                            Read & Execute


rw – r – – r – –

User        Group   Others

2) Default Permissions for Directory File:

ls                                                                      à Read

Create or Delete Files                                     à Write

cd                                                                    à Read & Execute


rwx r – x r – x

User        Group   Others

$ ls –l                                                               à To check permissions on files

chmod:It is used to change file permissions


chmod who/[+,-,=]/Permission File_Name/Directory_Name

Who:                                                                                                                           Permissions:

User/Owner    u                              (+) Add Permission                                  Read                 r

Group              g                              (-) Deny Permission                                   Write               w

Other               o                              (=) Assign Permission                                Execute           x



1) $ chmodu+xPermission_File      

It adds Execute Permission to User Members

$ ls –l Permission_File& see the O/P

2) chmod u-wx,g-x,o-x Permission_File  

It removes Permissions of Write, Execute for User and Execute for Group & Others from Permission_File

$ ls –l Permission_File& see the O/P

3) $ chmod u=w Permission_File  

It adds only Write Permission to User members and it deletes Read and Execute Permission from User members.

$ ls –l Permission_File& see the O/P

Octal Code (2nd Method)

Read                4

Write               2

Execute           1

0          No Permissions   4              r

1          x                              5              r, x

2          w                             6             r, w

3          w, x                         7             r, w, x

1) chmod 477 Permission_File    

It gives Read Permission to User, Read/Write/Execute  Permissions to Group & Others.

$ ls –l Permission_File& see the O/P

2) chmod700Permission_File  

It gives Read/Write/Execute Permission to User and No  Permissions to Group & Others

$ ls –l Permission_File& see the O/P

3) chmod777Permission_File      

It gives Read/Write/Execute Permission to User, Group & Others

$ ls –l Permission_File& see the O/P

4) chown:To change Owner of the File/Directory

Syntax:$ chownOwner_Name File/Directory

5) chgrp: To change Group of the File/Directory

Syntax:$ chgrpGroup_Name File/Directory


Default file creation permissions will be obtained from the umask value. Mask of umask will give the actual permissions. Mask is subtract value from 7

If umask is 021, then the default file permissions are 7-0,7-2,7-1

i.e. 756 are the permissions (rwxr-xrw-)

The current umask value can be had by:

$ umask


first 0 indicate the value is octal; umask can be assigned new value by:

$ umask 042.

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Unix Tutorials :

1.What is unix?

2.Basic unix commands

3.File Commands in unix

4.Create File in Unix using multiple ways

5.Cat Command

6.Touch Command

7.Mkdir command

8.rmdir Command

9.pwd command

10.Cd Command

11.cut Command

12.paste Command

13.tr Command

14.Cp Command

15.wc command

16.cmp command

17.Rm Command

18.Grep Command

19.Egrep Command

20.FGrep Command

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