3. Access Control

This chapter introduces Unix access control: users, groups, permissions, defaults, and environment settings that shape secure command-line work.

• Identity: Connect users, groups, UIDs, and GIDs to filesystem access decisions.

• Permissions: Read symbolic and octal permission forms and change them deliberately.

• Defaults: Use umask to reason about permissions assigned to new files.

• Privilege: Recognize when elevated access is needed and when it is risky.

• Environment security: Treat PATH and environment variables as part of a system’s trust boundary.

Chapter Video

How to Modify File Permissions in Bash (9:41) by Microsoft Developer

Hosted on YouTube; property of its original creator, embedded here for educational reference only.

Learning Objectives

By the end of this chapter, you will be able to:

1. Explain how users, groups, and privileges identify access rights.

2. Read and change symbolic and octal file permissions.

3. Use umask to reason about default permissions.

Chapter Flow

1. Users, Groups & Privileges

2. Permission Symbols

3. Umask & Defaults

Glossary

#	Term	Explanation
1	User	An account identity used to own files and run processes.
2	UID	A numeric user identifier used internally by Unix-like systems.
3	Group	A collection of users that can share file access.
4	GID	A numeric group identifier used internally by the system.
5	Root	The superuser account with administrative privileges.
6	sudo	A command that runs another command with elevated privileges when authorized.
7	Owner	The user account associated with a file or directory.
8	Group owner	The group associated with a file or directory.
9	Permission bits	The read, write, and execute settings stored for a file.
10	Read permission	Permission to inspect file contents or list a directory.
11	Write permission	Permission to modify a file or change entries in a directory.
12	Execute permission	Permission to run a file as a program or traverse a directory.
13	chmod	The command used to change file permissions.
14	chown	The command used to change file ownership.
15	chgrp	The command used to change group ownership.
16	Octal notation	A numeric permission form such as 755 or 644.
17	Symbolic mode	A permission form such as u+x or go-w.
18	umask	A mask that removes permissions from newly created files and directories.
19	Special permissions	Permission bits such as setuid, setgid, and sticky bit.
20	PATH security	The risk that command lookup can run an unintended program.