Create and enable a simple systemd service

This blog is a short reminder, a quick how to create a simple systemd service.
I will use two mock-up files, /etc/init.d/firewall.sh service script and the firewall.service systemd unit file.

1. Create /etc/init.d/firewall.sh service script and make it executable.

#!/bin/sh

### BEGIN INIT INFO
# Provides: Firewall
# Required-Start:
# Required-Stop:
# Default-Start:
# Default-Stop:
# Short-Description: Starts firewall rules
# Description: Starts firewall rules
### END INIT INFO

# Exit immediately if a command exits with a non-zero status.
set -e

case $1 in
        start)
                iptables -F
                iptables -A INPUT -s 192.168.200.0/24 -j ACCEPT
                iptables -A INPUT -p tcp --dport 22 -j DROP
                ;;
        stop)
                iptables -F
                ;;
        *)
        echo "Usage: /etc/init.d/firewall.sh {start|stop}"
        exit 1
        ;;
esac

2. Create the systemd service unit file /lib/systemd/system/firewall.service.

[Unit]
Description=Firewall
Requires=network-online.target
After=network-online.target

[Service]
User=root
Type=oneshot
RemainAfterExit=yes
ExecStart=/etc/init.d/firewall.sh start
ExecStop=/etc/init.d/firewall.sh stop

[Install]
WantedBy=multi-user.target

3. Enable at startup the firewall.service.

Created symlink /etc/systemd/system/multi-user.target.wants/firewall.service → /lib/systemd/system/firewall.service.

That’s it, now that we created and enabled the service, let’s see other useful commands.

Start and stop the service.

Disable the service.

Removed /etc/systemd/system/multi-user.target.wants/firewall.service.

List available systemd targets.