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

Default:

rw –

r – –

User Group Others

2) Default Permissions for Directory File:

ls à Read

Create or Delete Files à Write

cd à Read & Execute

Default:

rwx

r – x

User Group Others

$ ls –l à To check permissions on files

chmod:It is used to change file permissions

Syntax:

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

Examples:

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 (2^nd Method)

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

UMASK:

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

0227

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

$ umask 042.