Linux - File Permission / Access
Modes
• 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, the first column represents different access modes, i.e., the permission
associated with a file or a 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 the owner 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 the
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 there is 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 have been described below −
Read
Grants the capability to read, i.e., view the contents of the file.
Write
Grants the capability to modify, or remove the content of the
file.
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 −
Read
Access to a directory means that the user can read the contents. The user
can look at the filenames inside the directory.
Write
Access means that the user can add or delete files from the directory.
Execute
Executing a directory doesn't really make sense, so think of this as a
traverse permission.
A user must have execute access to the bin directory in order to execute
the ls or the cd command.
Changing Permissions
• To change the file or the directory permissions, you
use the chmod (change mode) command. There are
two ways to use chmod — the symbolic mode and the
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.
Sr.No. Chmod operator & Description
1 +
Adds the designated permission(s) to a file or
directory.
2 -
Removes the designated permission(s) from
a file or directory.
3 =
Sets the designated permission(s).
using testfile. Running ls -1 on the 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 the 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 can 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 -wx
(write) = 3
4 Read permission r--
5 Read and execute permission: 4 (read) + 1 r-x
(execute) = 5
6 Read and write permission: 4 (read) + 2 (write) = rw-
6
7 All permissions: 4 (read) + 2 (write) + 1 (execute) rwx
=7
• Here's an example using the testfile. Running ls -1 on the
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 the 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
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 the Owner and the 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 the user can be either the name of a user on
the system or the user id (uid) of a user on the system.
• The following example will help you understand the concept
−
• $ 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 any file but normal users can
change the ownership of only those files that they own.
Changing Group Ownership
• The chgrp 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 helps you understand the
concept −
• $ chgrp special testfile
• $
Resetting the Root Password
In order to reset the root password of a Linux server, follow
these steps below.
1. Log in to the server with the root user using your existing
password
2. Now, to change the password for the root user, enter the
command:
passwd root
