Table of contents

• Introduction
• File Permissions
• File Type
• File Mode
• Change File Mode
  • Octal Mode
  • Symbolic Mode

Introduction

The multi-user capability of Unix-like systems is a feature that is deeply ingrained into the design of the operating system.

File Permissions

On a Linux system, each file and directory is assigned access rights for the owner of the file.

You can check the permission settings by using ls -l.

$ ls -l drwxr-xr-x. 13 root  root  1027 Jan  3 12:32 bin/cat

Let's explore what drwxr-xr-x. 13 root root 1077 Jan 3 12:32 bin/cat means one by one.

File Type

File Mode

The r letter means the user has permission to read the file/directory. The w letter the user has permission to write the file/directory. And the e letter means the user has permission to execute the file/directory.

Let's take a look at the 9 letters in the command.
The first 3 letters show the permissions for the file owner, the second 3 letters show the permissions for the group owner and the last 3 letters show the permissions for other users.

rwx/ r-x/ r-x/Owner: rwxGroup: r-xUsers: r-x

Change File Modes

The chmod command is used to change the permissions of a file or directory.

Octal Mode

Each permission may be specified with an octal number: read = 4; write = 2; execute = 1; no permission = 0.

Example

The command below means giving permissions to read(4), write(2) and execute(1) to the owner and permissions to read(4) and execute(1) to the group user and permissions to read(4) to other users.

chmod 754 myfile

Symbolic Mode

The blow is the basic syntax of chmod.

% chmod who operator permission filename

You can use the following commands to change modes.

Example

In the following example, read permission are taken away from others.

% chmod o-r filea

In the following example, read and execute permissions are added for user, group, and others.

$ chmod a+rx fileb

In the following example, read, write, and execute permissions are assigned to the group.

$ chmod g=rwx filec

References

• Learning the shell - Lesson 9: Permissions