systemd

# Start / stop / restart a service
systemctl start nginx
systemctl stop nginx
systemctl restart nginx

# Reload config without restart
systemctl reload nginx

# Enable / disable at boot
systemctl enable nginx
systemctl disable nginx

# Enable and start in one step
systemctl enable --now nginx

# Check service status
systemctl status nginx

# Check if active / enabled
systemctl is-active nginx
systemctl is-enabled nginx

# List all failed units
systemctl --failed

# All logs for a service
journalctl -u nginx

# Follow live logs
journalctl -u nginx -f

# Last 100 lines
journalctl -u nginx -n 100

# Logs since last boot
journalctl -u nginx -b

# Logs in time range
journalctl -u nginx \
  --since "2026-05-01 00:00:00" \
  --until "2026-05-01 12:00:00"

# Kernel messages
journalctl -k

# Priority filter (err, warning, info, debug)
journalctl -u nginx -p err

# Output as JSON
journalctl -u nginx -o json | jq '.'

# Disk usage of journal
journalctl --disk-usage

# /etc/systemd/system/my-app.service

[Unit]
Description=My Application
After=network.target
Wants=network-online.target

[Service]
Type=simple
User=myapp
WorkingDirectory=/opt/my-app
ExecStart=/opt/my-app/bin/server --port 8080
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure
RestartSec=5s
StandardOutput=journal
StandardError=journal
Environment=NODE_ENV=production
EnvironmentFile=/etc/my-app/env

[Install]
WantedBy=multi-user.target

# /etc/systemd/system/cleanup.timer

[Unit]
Description=Run cleanup daily

[Timer]
OnCalendar=daily
RandomizedDelaySec=1h
Persistent=true

[Install]
WantedBy=timers.target

---
# /etc/systemd/system/cleanup.service

[Unit]
Description=Cleanup job

[Service]
Type=oneshot
ExecStart=/usr/local/bin/cleanup.sh

---
# Enable and start timer
systemctl enable --now cleanup.timer

# List all timers
systemctl list-timers

# List all active targets
systemctl list-units --type=target

# Get default boot target
systemctl get-default

# Set default boot target
systemctl set-default multi-user.target

# Common targets:
# multi-user.target  — CLI, no GUI (default for servers)
# graphical.target   — multi-user + GUI
# rescue.target      — single user, minimal services
# network.target     — networking is up

# Show dependency tree for a unit
systemctl list-dependencies nginx

# Show reverse dependencies (what depends on it)
systemctl list-dependencies --reverse nginx

# Reload systemd after editing unit files
systemctl daemon-reload

# Edit a unit file (creates override)
systemctl edit nginx

# Edit the full unit file
systemctl edit --full nginx

# Show effective unit (with overrides applied)
systemctl cat nginx

# Revert override
systemctl revert nginx

# Mask a unit (prevent start)
systemctl mask nginx

# Unmask
systemctl unmask nginx