Linux for Developers 2026: Essential Commands, SSH and Systemd

# Key directories every developer should know
/           # root
/home/user/ # user home directory (~)
/etc/       # system configuration files
/var/log/   # log files
/var/lib/   # application data (databases, docker)
/opt/       # optional/third-party software
/usr/local/ # user-installed software
/tmp/       # temporary files (cleared on reboot)
/proc/      # virtual filesystem — kernel and process info
/dev/       # device files

# Navigation
cd ~                  # go home
cd -                  # go back to previous directory
ls -la                # list all with permissions
pwd                   # print working directory
find . -name "*.log"  # find files
locate filename       # fast find (uses index)

# View processes
ps aux                    # all processes
ps aux | grep nginx       # find nginx
top                       # interactive, real-time
htop                      # better interactive
pgrep -f "python"         # find by name

# Kill processes
kill 1234                 # graceful (SIGTERM)
kill -9 1234              # force kill (SIGKILL)
killall nginx             # kill by name
pkill -f "python app.py"  # kill by pattern

# Background jobs
./long_task.sh &          # run in background
nohup ./task.sh &         # persist after logout
jobs                      # list background jobs
fg %1                     # bring job 1 to foreground
bg %1                     # send to background

# System resources
free -h                   # memory usage
df -h                     # disk space
du -sh *                  # size of items in current dir
iostat -x 1               # I/O statistics

# Check connectivity
ping google.com
traceroute google.com
dig google.com            # DNS lookup
nslookup google.com

# Ports and services
ss -tulnp                 # list open ports + process
netstat -tulnp            # same (older)
lsof -i :8000             # what's using port 8000
curl -I https://example.com  # HTTP headers only

# SSH
ssh user@server.com
ssh -i ~/.ssh/key.pem ubuntu@ec2-host.compute.amazonaws.com
scp file.txt user@server:/path/to/dest/
rsync -avz ./src/ user@server:/app/src/

# SSH config (~/.ssh/config)
Host myserver
    HostName 203.0.113.1
    User ubuntu
    IdentityFile ~/.ssh/mykey.pem
# Now: ssh myserver

# Managing services
sudo systemctl start nginx
sudo systemctl stop nginx
sudo systemctl restart nginx
sudo systemctl reload nginx  # reload config without restart
sudo systemctl enable nginx  # start on boot
sudo systemctl disable nginx
sudo systemctl status nginx

# View logs
journalctl -u nginx          # nginx logs
journalctl -u nginx -f       # follow nginx logs
journalctl -n 100            # last 100 system log entries
journalctl --since "1 hour ago"

# Create your own service
# /etc/systemd/system/myapp.service
[Unit]
Description=My App
After=network.target

[Service]
Type=simple
User=deploy
WorkingDirectory=/app
ExecStart=/usr/bin/python3 -m uvicorn main:app --host 0.0.0.0 --port 8000
Restart=always
RestartSec=5

[Install]
WantedBy=multi-user.target

# Edit crontab
crontab -e
crontab -l  # list current crons

# Cron syntax:
# MIN HOUR DAY MONTH WEEKDAY command
# 0   2    *   *     *       /usr/bin/backup.sh

# Examples:
0 * * * *      /app/hourly_task.sh     # every hour
0 2 * * *      /app/daily_backup.sh    # 2am daily
*/15 * * * *   /app/health_check.sh   # every 15 min
0 9 * * 1-5    /app/weekday_report.sh  # 9am Mon-Fri
0 0 1 * *      /app/monthly_report.sh  # 1st of month

# Logging cron output
0 2 * * * /app/backup.sh >> /var/log/backup.log 2>&1

# Docker on Linux
sudo apt install docker.io
sudo usermod -aG docker $USER  # run docker without sudo
newgrp docker

# Docker-compose
sudo apt install docker-compose-plugin
docker compose up -d

# Cloud instances: most common tasks
# Connect
ssh -i key.pem ubuntu@your-server-ip

# Update and install
sudo apt update && sudo apt upgrade -y
sudo apt install -y nginx python3-pip git

# Firewall
sudo ufw enable
sudo ufw allow ssh
sudo ufw allow 80
sudo ufw allow 443
sudo ufw status