Systemd Services: Create, Enable & Manage Linux Services

Understanding Systemd

Systemd is the init system and service manager for most modern Linux distributions. It uses "units" to manage system resources:

• Services — Execute and manage programs/daemons
• Sockets — Listen on ports and start services on-demand
• Targets — Group units together (similar to runlevels)
• Timers — Schedule tasks (like cron, but with systemd)

Basic systemctl Commands

# Start a service
sudo systemctl start nginx

# Stop a service
sudo systemctl stop nginx

# Restart a service
sudo systemctl restart nginx

# Reload configuration (without restart)
sudo systemctl reload nginx

# Check service status
systemctl status nginx

# Enable service to start on boot
sudo systemctl enable nginx

# Disable service from starting on boot
sudo systemctl disable nginx

# Check if service is enabled
systemctl is-enabled nginx

# Check if service is active (running)
systemctl is-active nginx

Targets and Runlevels

# List available runlevel targets
ls -al /lib/systemd/system/runlevel*

# Get current default target (runlevel)
systemctl get-default

# Set default target
sudo systemctl set-default multi-user.target  # No GUI
sudo systemctl set-default graphical.target   # With GUI

# Switch target immediately
sudo systemctl isolate multi-user.target

Creating a Custom Service

Step 1: Create the Service File

# Service files go in /lib/systemd/system/ (or /etc/systemd/system/)
sudo nano /lib/systemd/system/myapp.service

Step 2: Write the Service Configuration

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

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

[Install]
WantedBy=multi-user.target

Step 3: Reload and Test

# Reload systemd to recognize new service
sudo systemctl daemon-reload

# Start the service
sudo systemctl start myapp

# Check status and logs
sudo systemctl status myapp
sudo journalctl -xeu myapp

Step 4: Enable on Boot

# Enable the service (creates symlink in multi-user.target.wants/)
sudo systemctl enable myapp

# Verify it's enabled
systemctl is-enabled myapp

Service Types Explained

# Most common types:
Type=simple      # Default. Process started by ExecStart is the main process
Type=forking     # Service forks and parent exits (traditional daemons)
Type=oneshot     # Process exits after doing its work (scripts)
Type=notify      # Like simple, but service sends notification when ready
Type=idle        # Similar to simple, but waits until all jobs finish

Example: Forking Service

[Service]
Type=forking
PIDFile=/run/myapp/myapp.pid
ExecStart=/opt/myapp/bin/myapp --daemon
ExecStop=/bin/kill -TERM $MAINPID

Example: Oneshot Service (Script)

[Unit]
Description=Initialize Application Data

[Service]
Type=oneshot
ExecStart=/opt/myapp/init-data.sh
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Environment Variables

[Service]
# Single variable
Environment="NODE_ENV=production"

# Multiple variables
Environment="NODE_ENV=production" "PORT=3000" "DB_HOST=localhost"

# From a file
EnvironmentFile=/etc/myapp/environment
EnvironmentFile=-/etc/myapp/local.env  # '-' means optional

Example environment file (/etc/myapp/environment):

NODE_ENV=production
PORT=3000
DATABASE_URL=postgres://localhost/myapp
SECRET_KEY=your-secret-key

Restart Policies

[Service]
# Restart options
Restart=no           # Don't restart (default)
Restart=on-success   # Restart only if exited cleanly
Restart=on-failure   # Restart on non-zero exit, signal, timeout
Restart=on-abnormal  # Restart on signal, timeout
Restart=on-abort     # Restart on signal only
Restart=on-watchdog  # Restart on watchdog timeout
Restart=always       # Always restart

# Delay before restart
RestartSec=5

# Maximum restart attempts (10 restarts within 60 seconds)
StartLimitBurst=10
StartLimitIntervalSec=60

Service Dependencies

[Unit]
# Start after these units
After=network.target postgresql.service

# Require these units (fail if they fail)
Requires=postgresql.service

# Want these units (don't fail if they fail)
Wants=redis.service

# Don't start if these are starting
Before=nginx.service

# Conflict with these units
Conflicts=other-webserver.service

Viewing Logs with journalctl

# View logs for a specific service
journalctl -u myapp

# Follow logs in real-time (like tail -f)
journalctl -fu myapp

# View logs since last boot
journalctl -u myapp -b

# View logs from specific time
journalctl -u myapp --since "2025-01-10 10:00:00"
journalctl -u myapp --since "1 hour ago"

# Show only errors
journalctl -u myapp -p err

# Extended output with explanations
journalctl -xeu myapp

# Show kernel messages
journalctl -k

# Disk usage of journal
journalctl --disk-usage

# Clean old logs
sudo journalctl --vacuum-time=7d
sudo journalctl --vacuum-size=500M

Complete Example: Node.js Application

Here's a production-ready service file for a Node.js application:

# /lib/systemd/system/nodeapp.service
[Unit]
Description=Node.js Application
Documentation=https://github.com/yourcompany/nodeapp
After=network.target

[Service]
Type=simple
User=node
Group=node
WorkingDirectory=/var/www/nodeapp
Environment=NODE_ENV=production
Environment=PORT=3000
ExecStart=/usr/bin/node /var/www/nodeapp/server.js
ExecReload=/bin/kill -HUP $MAINPID
KillMode=mixed
KillSignal=SIGTERM
TimeoutStopSec=30
Restart=on-failure
RestartSec=10
StandardOutput=journal
StandardError=journal
SyslogIdentifier=nodeapp

# Security hardening
NoNewPrivileges=yes
ProtectSystem=strict
ProtectHome=yes
PrivateTmp=yes
ReadWritePaths=/var/www/nodeapp/uploads

[Install]
WantedBy=multi-user.target

# Deploy the service
sudo systemctl daemon-reload
sudo systemctl enable --now nodeapp
sudo systemctl status nodeapp

Troubleshooting

# Analyze boot time
systemd-analyze

# Check which services take longest to start
systemd-analyze blame

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

# Show service dependencies
systemctl list-dependencies myapp

# List all failed units
systemctl --failed

# Reset failed state
sudo systemctl reset-failed myapp

Resources

• Systemd Documentation
• Arch Wiki - Systemd

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