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Linux Permissions & Sudo

1 Global Definitions

File Permissions: Rules that define who can read, write, or execute a file in Linux.
Ownership: Each file is owned by a user and a group, which determines access rights.
Sudo (Superuser Do): A command that allows authorized users to execute commands with root privileges.

1.1 Permission Basics

Permission Types

Linux permissions are represented as three categories: user, group, and others.

r → read
w → write
x → execute

Viewing Permissions

Use the ls -l command to view file permissions.

Example: -rwxr-xr--
First character: file type (-=file, d=directory)
Next nine characters = user/group/others permissions

Changing Permissions

Modify permissions using the chmod command.

chmod 755 script.sh
Symbolic mode: chmod u+x file.sh

1.2 File Ownership

Changing Ownership

Use chown and chgrp to change file ownership.

chown user file.txt → change file owner
chgrp group file.txt → change group ownership

Default Permissions (umask)

Defines the default permission settings for new files and directories.

umask → shows current mask
umask 022 → sets default permissions

1.3 Special Permissions

SetUID

Executes a file with the permissions of the file’s owner.

Example: passwd command
Represented as s in user permissions

SetGID

Executes a file with the permissions of the file’s group.

Applied to directories → new files inherit group ownership
Represented as s in group permissions

Sticky Bit

Restricts file deletion in shared directories.

Only file owner or root can delete files
Common in /tmp directory
Represented as t in others’ execute field

1.4 Using Sudo

Basic Usage

Run commands with elevated privileges.

sudo command
sudo apt update → update system packages

Sudoers File

Defines which users can run commands as root.

Located at /etc/sudoers
Edit with visudo for safety

Security Considerations

Sudo prevents unnecessary use of root account.

Logs all privileged commands
Allows fine-grained control (e.g., specific commands)

1.5 Why Permissions & Sudo Matter

Linux permissions form the first line of defense in system security by controlling access to files and resources. Sudo provides a safe way to perform administrative tasks without exposing the root account. Together, they reduce the risk of accidental damage and unauthorized privilege escalation.