systemd Units & Targets

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

# Key targets
# multi-user.target  — Multi-user mode (no GUI)
# graphical.target   — Graphical mode (with GUI)
# rescue.target      — Single-user mode
# emergency.target   — Emergency mode (minimal)
# default.target     — Default boot target (symlink to graphical or multi-user)

# Check default target
systemctl get-default
# multi-user.target

# Change default target
sudo systemctl set-default multi-user.target

# Switch targets at runtime
sudo systemctl isolate rescue.target
sudo systemctl isolate multi-user.target

# Check target dependencies
systemctl list-dependencies graphical.target

# Types of dependencies
# Requires — Strong dependency (fail if dependency fails)
# Wants   — Weak dependency (try to start, but don't fail)
# After   — Ordering (start after this unit)
# Before  — Ordering (start before this unit)
# BindsTo — Stronger than Requires (stop if dependency stops)
# PartOf  — Restart/stop when dependency restarts/stops
# Conflicts — Cannot run simultaneously
# WantedBy — reverse dependency (used in [Install] section)

# View dependencies of a service
systemctl list-dependencies sshd.service
# sshd.service
# ├─sshd-keygen.service
# ├─network.target
# └─sysinit.target

systemctl list-dependencies --after sshd.service
systemctl list-dependencies --before sshd.service

# Service management
sudo systemctl start nginx        # Start a service
sudo systemctl stop nginx         # Stop a service
sudo systemctl restart nginx      # Restart a service
sudo systemctl reload nginx       # Reload configuration (if supported)
sudo systemctl enable nginx       # Enable at boot
sudo systemctl disable nginx      # Disable at boot
sudo systemctl status nginx       # Check status
sudo systemctl is-active nginx    # Check if active
sudo systemctl is-enabled nginx   # Check if enabled

# Mask/unmask (prevent manual start)
sudo mask nginx                   # Prevent starting
sudo unmask nginx                 # Allow starting

# List all services
systemctl list-units --type=service
systemctl list-unit-files --type=service

# /etc/systemd/system/myapp.service
[Unit]
Description=My Application
After=network.target postgresql.service
Requires=postgresql.service
Wants=redis.service

[Service]
Type=simple
User=appuser
Group=appgroup
WorkingDirectory=/opt/myapp
ExecStart=/opt/myapp/bin/myapp --config /opt/myapp/config.yaml
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure
RestartSec=5
Environment=NODE_ENV=production

[Install]
WantedBy=multi-user.target

# Key fields:
# Type=simple — Process stays in foreground (no daemonize)
# Type=forking — Process forks and exits parent
# Type=oneshot — Runs once, then exits
# Restart=on-failure — Restart only on non-zero exit

# Show full dependency tree
systemctl list-dependencies --all nginx.service

# Show only required dependencies
systemctl list-dependencies --require nginx.service

# Analyze boot time
systemd-analyze
# Startup finished in 5.234s (kernel) + 12.345s (initrd) + 45.678s (userspace)

# Show time per unit
systemd-analyze blame | head -10
# 15.234s network-manager.service
# 12.345s postgresql.service
# 10.123s docker.service

# Critical chain (longest dependency path)
systemd-analyze critical-chain
# The time after the product of the longest chain.

# Verify unit file syntax
systemd-analyze verify /etc/systemd/system/myapp.service