Linux & DevOps Cheatsheet

Beginner-friendly Linux notes, commands, troubleshooting tips, and practical DevOps context.

Why Linux?

Most servers, cloud virtual machines, containers, and DevOps tools run on Linux. Learning the command line makes it easier to manage systems, investigate problems, and automate work.

Navigation and Files

Command	What it does
pwd	Shows your current location.
ls -la	Lists files and permissions.
cd /path/to/folder	Moves to a folder.
mkdir project	Creates a folder.
touch notes.txt	Creates an empty file.
cp file.txt copy.txt	Copies a file.
mv old.txt new.txt	Moves or renames a file.
find . -name "*.log"	Finds log files.
rm file.txt	Deletes a file. Use carefully.

Viewing and Editing Files

cat file.txt             # print a small file
less /var/log/syslog     # view a large file; press q to exit
head -n 10 file.txt      # first 10 lines
tail -n 20 file.txt      # last 20 lines
tail -f app.log          # follow a log as it changes
nano file.txt            # simple terminal editor
grep -n "error" app.log  # find error lines

Permissions and Ownership

Linux controls who can read, write, or run files.

ls -l file.txt           # view permissions and owner
chmod +x script.sh       # allow a script to run
chmod 644 file.txt       # owner can write; others can read
sudo chown user:group file.txt  # change owner and group

sudo runs a command with administrator privileges. Use it only when needed and double-check commands before pressing Enter.

Processes and Services

ps aux                   # list running processes
top                      # live view of CPU and memory use
kill PID                 # ask a process to stop
systemctl status nginx   # check a service
sudo systemctl start nginx
sudo systemctl restart nginx
sudo systemctl enable nginx  # start after reboot

Disk, Memory, and Network Checks

df -h                    # free disk space
du -sh *                 # folder sizes in current directory
free -h                  # memory usage
uptime                   # uptime and load average
ip addr                  # network addresses
ping -c 4 example.com    # basic connectivity test
curl -I https://example.com  # check an HTTP response
ss -tulpn                # listening ports

Package Management

On Ubuntu and Debian systems:

sudo apt update
sudo apt install nginx
sudo apt remove nginx

On Red Hat, Fedora, and Amazon Linux systems, use dnf (or sometimes yum) instead of apt.

Logs and Troubleshooting

When something fails, start with the error message and recent logs.

sudo journalctl -u nginx --since "30 minutes ago"
sudo journalctl -xe
tail -n 50 /var/log/syslog

Simple troubleshooting flow:

1. Check the service: systemctl status service-name
2. Read its recent logs: journalctl -u service-name
3. Check disk and memory: df -h and free -h
4. Check ports and network access: ss -tulpn and curl
5. Make one change at a time, then test again.

Practical Examples

Find large files

find /var/log -type f -size +100M 2>/dev/null

Check a website from a server

curl -s -o /dev/null -w "%{http_code}\n" https://example.com

Securely copy a file to a server

scp backup.tar.gz user@server.example:/home/user/backups/

Good Habits

• Run pwd and ls before changing or deleting files.
• Prefer rm -i file.txt when learning; it asks for confirmation.
• Keep system updates current, especially on test machines.
• Avoid logging in as root for daily work; use a normal user with sudo.
• Read logs before restarting services, so useful error information is not lost.
• Store configuration in version control, but never commit passwords or private keys.
• Document changes made to servers and deployment environments.

DevOps Connection

Linux skills are used every day for cloud servers, Docker containers, CI/CD runners, monitoring, incident response, and application deployments.

The goal is not to memorise every command—learn how to inspect a system, understand what you see, and make safe changes.