Linux Administration — Learner Cheat Sheet

Topic Summary

Linux is a secure, multi-user operating system that uses permissions, systemd services, and process isolation to manage server resources. This cheat sheet covers the essential commands to manage users, file permissions, running processes, system services, networking, packages, and bash automation.

Key Concepts

User & Group IDs (UID/GID): Linux tracks users by ID numbers. root is UID 0 (master administrator). Standard users are UID 1000+.
sudo (Superuser Do): Allows standard users to temporarily elevate their privileges to execute administrative commands securely and auditably.
Process (PID): A running instance of a program, tracked by a Process ID. systemd is always PID 1.
Systemd (Daemon Manager): The master init system that launches and manages background services (daemons).
Package Manager (apt/dnf): Automates the installation, upgrading, and dependency resolution of software from centralized repositories.
Shebang (#!/bin/bash): The first line of a bash script that tells the kernel which interpreter to use.

Important Files & Directories

/etc/passwd: Master list of all user accounts on the server.
/etc/shadow: Encrypted password hash file (requires sudo).
/etc/sudoers: Policy file defining which users/groups can use sudo (edit ONLY with sudo visudo).
/usr/lib/systemd/system/ (or /etc/systemd/system/): Directory containing systemd unit (.service) configuration files.

User & Group Administration

Task	Command	Memory Aid / Note
View active identity	`id [username]`	Prints your active UID, GID, and secondary groups.
Create user	`sudo useradd -m -s /bin/bash username`	`-m` makes the home dir; `-s` sets default shell.
Add user to group	`sudo usermod -aG groupname username`	`-aG` = append to Group.
Delete user	`sudo userdel -r username`	`-r` completely removes their home directory.

Related Lesson: MOD-LINUX-ADM-01

File Permissions & Ownership

Task	Command	Memory Aid / Note
Add execute permission	`chmod u+x script.sh`	user + x (execute).
Set standard directory	`chmod 755 folder/`	`rwxr-xr-x` (User:7, Group:5, Others:5).
Set secure secrets file	`chmod 600 key.pem`	`rw-------` (User:6). Mandatory for SSH keys!
Change owner & group	`sudo chown user:group file`	Changes both owner and group simultaneously.

[!CAUTION] Never use chmod 777 to fix permission issues! It grants read/write/execute to everyone in the world and destroys system security.

Related Lesson: MOD-LINUX-ADM-02

Process & Job Management

Task	Command	Memory Aid / Note
Find running process	`ps aux \| grep process_name`	Takes a static snapshot of all processes.
View live resources	`top` or `htop`	Real-time CPU/Memory monitor. (Press `q` to quit).
Run in background	`[command] &`	Adds job to background, returning your prompt.
Polite shutdown	`kill [PID]`	Sends `SIGTERM` to safely shut down a process.

[!CAUTION] Avoid using kill -9 [PID] (SIGKILL) unless a process is completely frozen. It forces instant termination without saving data or closing database connections cleanly.

Related Lesson: MOD-LINUX-ADM-03

Service Management (systemctl)

Task	Command	Memory Aid / Note
Check service health	`systemctl status [service]`	Displays active status and recent logs.
Start / Stop service	`sudo systemctl start\|stop [service]`	Powers the daemon on or off.
Reload config safely	`sudo systemctl reload [service]`	Re-reads configuration without dropping active users.
Enable on boot	`sudo systemctl enable [service]`	Service will start automatically when server turns on.
View live logs	`sudo journalctl -u [service] -f`	`-f` (follow) streams live log output to screen.

Related Lesson: MOD-LINUX-ADM-04

Networking Basics

Task	Command	Memory Aid / Note
View IP addresses	`ip addr show`	Replaces the deprecated `ifconfig`.
Test connectivity	`ping -c 4 [hostname]`	`-c 4` sends exactly 4 packets (prevents infinite loop).
View listening ports	`sudo ss -tulpn`	tcp, udp, listening, process, numeric.
Fetch HTTP headers	`curl -I http://example.com`	`-I` fetches headers only (e.g., `HTTP 200 OK`).

Related Lesson: MOD-LINUX-ADM-05

Package Management (APT)

Task	Command	Memory Aid / Note
Update catalog cache	`sudo apt update`	Do this before installing anything!
Install non-interactive	`sudo apt install -y [package]`	`-y` auto-answers “yes”, crucial for scripts.
Upgrade installed apps	`sudo apt upgrade -y`	Upgrades software based on updated catalog.

Related Lesson: MOD-LINUX-ADM-06

Bash Scripting Automation

#!/bin/bash
set -euo pipefail # Strict safety: Abort on errors or undefined variables
 
BACKUP_DIR="/tmp/backup" # No spaces around equals sign!
 
if [ ! -d "$BACKUP_DIR" ]; then # Mandatory spaces inside brackets [ ]
    mkdir -p "$BACKUP_DIR"
fi
 
for SERVER in "app-01" "app-02"; do
    echo "Processing $SERVER..." # Double quotes interpolate variables
done
 
echo "Finished with exit code: $?" # $? holds the exit code of last command

Don’t forget to run chmod +x script.sh to make it executable!

Related Lesson: MOD-LINUX-ADM-07 Related Lab: LAB-MOD-LINUX-ADM-01

Common Beginner Mistakes

Using chmod 777: Destroys file security. Use chmod u+x for scripts, 600 for keys, 755 for directories.
Forgetting -y in apt install: Causes automated scripts (like Dockerfiles) to freeze indefinitely.
Spaces in Bash Variables: VAR = "hello" fails. Must be VAR="hello".
Missing sudo with ss -tulpn: Running ss -tulpn without sudo will hide the process names/PIDs owning the ports.
Confusing apt update with apt upgrade: update only refreshes the list of available software; upgrade actually updates the installed programs.

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