Working with Files and Text

File Permissions

File permissions are the core security mechanism that controls who can do what with a file or directory.

They answer three questions:

1. Who can access this?
2. What can they do with it?
3. How do we change it?

Permission Categories

File permissions in Linux and Unix-like systems are fundamental to controlling who can access files and directories, and what actions they can perform. They are a core component of the traditional Unix access control model.

Purpose and Overview

Every file and directory in Linux possesses a set of permissions that dictate who can access them and how. These permissions limit access in three main ways:

1. Restrict access to the owner alone.
2. Allow users in a predesignated group to have access.
3. Permit anyone on the system to have access (referred to as "others" or "the world").

User Categories

Permissions are defined for three distinct categories of users:

1. Owner (u): The user who created the file or directory.
2. Group (g): A collection of users assigned to a group.
3. Others (o): All other users on the system.

The symbol a represents all categories (owner, group, others).

Class	Symbol	Description
User	u	The owner of the file or directory.
Group	g	Users who belong to the file's group.
Other	o	Everyone else on the system.

Types of Permissions

Each category can have three types of permissions:

• Read (r)
• Write (w)
• Execute (x)

Permissions for Files

• Read (r): Allows the file to be opened or viewed.
• Write (w): Allows modification of file contents.
• Execute (x): Allows the file to be executed as a program.

Permissions for Directories

• Read (r): Allows listing of directory contents.
• Write (w): Allows creation, deletion, and renaming of files inside the directory.
• Execute (x): Allows entering the directory using commands like cd.

Permission	Symbol	On a File	On a Directory
Read	r	View file contents	List directory contents
Write	w	Modify file	Create, delete, rename files
Execute	x	Run file as program	Enter the directory

Displaying Permissions

The ls -l command displays detailed file information including permissions.

The first 10 characters represent file attributes.

First Character (File Type)

• - Regular file
• d Directory
• l Symbolic link
• c Character device
• b Block device
• s Local socket
• p Named pipe

Next Nine Characters

These represent permissions for owner, group, and others.

• r Read permission
• w Write permission
• x Execute permission
• - Permission not granted

Example

-rw-r--r-- 1 chris weather 207 Feb 20 11:55 mydata

• - Regular file
• rw- Owner has read & write
• r-- Group has read
• r-- Others have read

Changing Permissions

The chmod command is used to change file permissions. Only the file owner or root can change permissions.

1. Symbolic Method

Uses symbols:

• u owner
• g group
• o others
• a all users

Operators:

• + add permission
• - remove permission
• = set exact permissions

Examples:

chmod u+x lsc
chmod g+rw mydata
chmod o+r-wx mydata
chmod a-x file
chmod u+x,go=rx file

2. Absolute (Octal) Method

Uses a three-digit number representing permissions.

• 4 = Read
• 2 = Write
• 1 = Execute

Number	Permission
0	---
1	--x
2	-w-
3	-wx
4	r--
5	r-x
6	rw-
7	rwx

Examples:

chmod 600 foo.txt
chmod 700 myprog
chmod 755 myprog

Ownership and Group Ownership

Files and directories are owned by a user and a group.

chown (Change Owner)

chown robert mydata
chown tony:tony myfile.txt

chgrp (Change Group)

chgrp forecast today

Default Permissions

The umask command controls the default permissions when files are created.

Examples:

umask
umask 0002
umask -S u=rwx,g=rx,o=rx