Systemd

# Start / stop / restart a service
sudo systemctl start servicename
sudo systemctl stop servicename
sudo systemctl restart servicename

# Reload config without full restart (if supported)
sudo systemctl reload servicename

# Enable at boot / disable
sudo systemctl enable servicename
sudo systemctl disable servicename

# Enable and start in one command
sudo systemctl enable --now servicename

# Check status
systemctl status servicename

# Is it running?
systemctl is-active servicename

# Is it enabled at boot?
systemctl is-enabled servicename

# List all running services
systemctl list-units --type=service --state=running

# List all failed units
systemctl --failed

# List all units (running, stopped, failed)
systemctl list-units --type=service

# Reboot / shutdown / halt
sudo systemctl reboot
sudo systemctl poweroff
sudo systemctl halt

# View all logs (oldest first)
journalctl

# Follow live (like tail -f)
journalctl -f

# Logs for a specific service
journalctl -u servicename

# Follow logs for a service live
journalctl -fu servicename

# Last 50 lines for a service
journalctl -u servicename -n 50

# Since last boot
journalctl -b

# Logs from a previous boot (-1 = one boot ago)
journalctl -b -1

# Logs since a specific time
journalctl --since "2024-01-01 12:00:00"
journalctl --since "1 hour ago"

# Filter by priority (0=emerg, 3=err, 4=warning, 6=info, 7=debug)
journalctl -p err

# Show kernel messages
journalctl -k

# Disk space used by journal
journalctl --disk-usage

# Vacuum journal to limit size
sudo journalctl --vacuum-size=500M
sudo journalctl --vacuum-time=7d

# /etc/systemd/system/myapp.service

[Unit]
Description=My Application
Documentation=https://example.com/docs
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
User=myappuser
Group=myappuser
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/bin/myapp --config /etc/myapp/config.yaml
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure
RestartSec=5
StandardOutput=journal
StandardError=journal

# Environment variables
Environment=NODE_ENV=production
# Or load from file
EnvironmentFile=/etc/myapp/.env

[Install]
WantedBy=multi-user.target

# After creating the file, reload systemd and enable
sudo systemctl daemon-reload
sudo systemctl enable --now myapp

Restart=no           # Never restart (default)
Restart=on-failure   # Restart only on non-zero exit
Restart=always       # Always restart
Restart=on-abnormal  # Restart on signal, timeout, watchdog

# /etc/systemd/system/backup.service
[Unit]
Description=Run Backup Script

[Service]
Type=oneshot
User=youruser
ExecStart=/usr/local/bin/backup.sh

# /etc/systemd/system/backup.timer
[Unit]
Description=Daily Backup Timer
Requires=backup.service

[Timer]
OnCalendar=daily
OnCalendar=*-*-* 02:00:00   # Every day at 2am
Persistent=true              # Run if missed (e.g. system was off)

[Install]
WantedBy=timers.target

sudo systemctl daemon-reload
sudo systemctl enable --now backup.timer

# Check timer status and next run time
systemctl list-timers

# View current target
systemctl get-default

# Set default target
sudo systemctl set-default multi-user.target   # no GUI
sudo systemctl set-default graphical.target    # with GUI

# Common targets
# multi-user.target  → like runlevel 3 (multi-user, no GUI)
# graphical.target   → like runlevel 5 (multi-user + GUI)
# rescue.target      → single user / rescue mode
# emergency.target   → minimal emergency shell

# Service won't start — see why
sudo systemctl status servicename
journalctl -u servicename -n 50 --no-pager

# Find unit file location
systemctl cat servicename

# Show all properties of a unit
systemctl show servicename

# Check for dependency issues
systemctl list-dependencies servicename