File ownership is an important component of UNIX that provides a secure

method for storing files. Every file in UNIX has the following attributes −

 Owner permissions − The owner's permissions determine what actions the owner of the file can perform on the
file.

 Group permissions − The group's permissions determine what actions a user, who is a member of the group
that a file belongs to, can perform on the file.

 Other (world) permissions − The permissions for others indicate what action all other users can perform on
the file.

The Permission Indicators

While using ls -l command it displays various information related to file
permission as follows −

$ls -l /home/amrood

-rwxr-xr-- 1 amrood users 1024 Nov 2 00:10 myfile

drwxr-xr--- 1 amrood users 1024 Nov 2 00:10 mydir

Here first column represents different access mode ie. permission

associated with a file or directory.

The permissions are broken into groups of threes, and each position in the
group denotes a specific permission, in this order: read (r), write (w),
execute (x) −

 The first three characters (2-4) represent the permissions for the file's owner.
For example -rwxr-xr-- represents that onwer has read (r), write (w) and
execute (x) permission.

 The second group of three characters (5-7) consists of the permissions for
the group to which the file belongs. For example -rwxr-xr-- represents that
group has read (r) and execute (x) permission but no write permission.

 The last group of three characters (8-10) represents the permissions for
everyone else. For example -rwxr-xr-- represents that other world has read
(r) only permission.

File Access Modes

The permissions of a file are the first line of defense in the security of a Unix
system. The basic building blocks of Unix permissions are the read, write,
and execute permissions, which are described below −

1. Read
Grants the capability to read ie. view the contents of the file.
2. Write
Grants the capability to modify, or remove the content of the file.

3. Execute
User with execute permissions can run a file as a program.

Directory Access Modes

Directory access modes are listed and organized in the same manner as any
other file. There are a few differences that need to be mentioned:

1. Read
Access to a directory means that the user can read the contents. The user
can look at the filenames inside the directory.

2. Write
Access means that the user can add or delete files to the contents of the
directory.

3. Execute
Executing a directory doesn't really make a lot of sense so think of this as a
traverse permission.

A user must have execute access to the bin directory in order to execute ls
or cd command.

Changing Permissions
To change file or directory permissions, you use the chmod (change mode)
command. There are two ways to use chmod: symbolic mode and absolute
mode.

Using chmod in Symbolic Mode

The easiest way for a beginner to modify file or directory permissions is to
use the symbolic mode. With symbolic permissions you can add, delete, or
specify the permission set you want by using the operators in the following
table.

Chmod operator Description

+ Adds the designated permission(s) to a file or directory.

 - Removes the designated permission(s) from a file or directory.

= Sets the designated permission(s).

Here's an example using testfile. Running ls -1 on testfile shows that the

file's permissions are as follows −

$ls -l testfile

-rwxrwxr-- 1 amrood users 1024 Nov 2 00:10 testfile

Then each example chmod command from the preceding table is run on
testfile, followed by ls -l so you can see the permission changes −

$chmod o+wx testfile

$ls -l testfile

-rwxrwxrwx 1 amrood users 1024 Nov 2 00:10 testfile

$chmod u-x testfile

$ls -l testfile

-rw-rwxrwx 1 amrood users 1024 Nov 2 00:10 testfile

$chmod g=rx testfile

$ls -l testfile

-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile

Here's how you could combine these commands on a single line:

$chmod o+wx,u-x,g=rx testfile

$ls -l testfile

-rw-r-xrwx 1 amrood users 1024 Nov 2 00:10 testfile

Using chmod with Absolute Permissions

The second way to modify permissions with the chmod command is to use a
number to specify each set of permissions for the file.

Each permission is assigned a value, as the following table shows, and the
total of each set of permissions provides a number for that set.

Number Octal Permission Representation Ref

0 No permission ---

1 Execute permission --x

2 Write permission -w-

3 Execute and write permission: 1 (execute) + 2 (write) = 3 -wx

4 Read permission r--

 5 Read and execute permission: 4 (read) + 1 (execute) = 5 r-x

6 Read and write permission: 4 (read) + 2 (write) = 6 rw-

7 All permissions: 4 (read) + 2 (write) + 1 (execute) = 7 rwx

Here's an example using testfile. Running ls -1 on testfile shows that the

file's permissions are as follows −

$ls -l testfile

-rwxrwxr-- 1 amrood users 1024 Nov 2 00:10 testfile

Then each example chmod command from the preceding table is run on
testfile, followed by ls -l so you can see the permission changes −

$ chmod 755 testfile

$ls -l testfile

-rwxr-xr-x 1 amrood users 1024 Nov 2 00:10 testfile

$chmod 743 testfile

$ls -l testfile

-rwxr---wx 1 amrood users 1024 Nov 2 00:10 testfile

$chmod 043 testfile

$ls -l testfile

----r---wx 1 amrood users 1024 Nov 2 00:10 testfile

Changing Owners and Groups

While creating an account on Unix, it assigns a owner ID and a group ID to
each user. All the permissions mentioned above are also assigned based on
Owner and Groups.

Two commands are available to change the owner and the group of files −

 chown − The chown command stands for "change owner" and is used to change the owner of a file.

 chgrp − The chgrp command stands for "change group" and is used to change the group of a file.

Changing Ownership
The chown command changes the ownership of a file. The basic syntax is as
follows −

$ chown user filelist

The value of user can be either the name of a user on the system or the
user id (uid) of a user on the system.
Following example −

$ chown amrood testfile

Changes the owner of the given file to the user amrood.

NOTE: The super user, root, has the unrestricted capability to change the
ownership of a any file but normal users can change only the owner of files
they own.

Changing Group Ownership

The chrgp command changes the group ownership of a file. The basic
syntax is as follows −

$ chgrp group filelist

The value of group can be the name of a group on the system or the group
ID (GID) of a group on the system.

Following example −

$ chgrp special testfile

Changes the group of the given file to special group.