Linux® Command Line and Shell Scripting Bible

Chapter 6. Understanding Linux File Permissions

IN THIS CHAPTER

• Understanding Linux security

• Decoding file permissions

• Working with Linux groups

No system is complete without some form of security. There must be a mechanism available to protect files from unauthorized viewing or modification. The Linux system follows the Unix method of file permissions, allowing individual users and groups access to files based on a set of security settings for each file and directory. This chapter discusses how to use the Linux file security system to protect data when necessary and share data when desired.

Linux Security

The core of the Linux security system is the user account. Each individual who accesses a Linux system should have a unique user account assigned. The permissions users have to objects on the system depend on the user account they log in with.

User permissions are tracked using a user ID (often called a UID), which is assigned to an account when it's created. The UID is a numerical value, unique for each user. However, you don't log in to a Linux system using your UID. Instead, you use a login name. The login name is an alphanumeric text string of eight characters or fewer that the user uses to log in to the system (along with an associated password).

The Linux system uses special files and utilities to track and manage user accounts on the system. Before we can discuss file permissions, we need to discuss how Linux handles user accounts. This section describes the files and utilities ...