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How to create a daemon service

Page IDdaemon-service-linuxUpdated

Overview

A daemon is a background process that starts automatically at boot and runs without user interaction. Modern Linux distributions use systemd to manage daemons. This guide shows how to create a systemd service and, optionally, a legacy SysV init script.

Prerequisites

  • A Linux distribution with systemd (e.g., Ubuntu 20.04+, Debian 10+, CentOS 7+, Fedora)

  • Root or sudo privileges

  • An executable script or binary you want to run as a daemon

1. Prepare the Executable

Create a simple script to test the service. Save it as /usr/local/bin/hello.sh and make it executable:

#!/bin/bash
while true; do
    echo "$(date) - Hello from daemon" >> /var/log/hello-daemon.log
    sleep 60
done

sudo chmod +x /usr/local/bin/hello.sh

2. Write a systemd Service Unit

Create a file /etc/systemd/system/hello.service with the following content:

[Unit]
Description=Hello Daemon Service
After=network.target

[Service]
Type=simple
ExecStart=/usr/local/bin/hello.sh
Restart=on-failure
User=root
StandardOutput=append:/var/log/hello-daemon.log
StandardError=append:/var/log/hello-daemon.log

[Install]
WantedBy=multi-user.target

Tip: Use Type=simple for scripts that run in the foreground. If your program daemonizes itself, use Type=forking.

3. Enable and Start the Service

  • Reload systemd to recognize the new unit

  • Enable the service to start at boot

  • Start the service now

sudo systemctl daemon-reload
sudo systemctl enable hello.service
sudo systemctl start hello.service

4. Manage the Service

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5. Verify Automatic Startup

Reboot the machine and confirm the service is active:

sudo reboot
# After reboot
sudo systemctl status hello.service

Optional: Legacy SysV Init Script

For distributions without systemd, create an init script in /etc/init.d/hello:

#!/bin/sh
### BEGIN INIT INFO
# Provides:          hello
# Required-Start:    $remote_fs $syslog
# Required-Stop:     $remote_fs $syslog
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Hello daemon
### END INIT INFO

DAEMON=/usr/local/bin/hello.sh
PIDFILE=/var/run/hello.pid

case "$1" in
    start)
        echo "Starting hello..."
        start-stop-daemon --start --background --make-pidfile --pidfile $PIDFILE --exec $DAEMON
        ;;
    stop)
        echo "Stopping hello..."
        start-stop-daemon --stop --pidfile $PIDFILE
        ;;
    restart)
        $0 stop
        $0 start
        ;;
    status)
        status_of_proc -p $PIDFILE $DAEMON hello && exit 0 || exit $?
        ;;
    *)
        echo "Usage: $0 {start|stop|restart|status}"
        exit 1
        ;;
esac
exit 0

sudo chmod +x /etc/init.d/hello
sudo update-rc.d hello defaults
sudo service hello start

Common Pitfalls

  • Forgetting to chmod +x the script makes the service fail to start.

  • Using the wrong Type in the unit file can cause systemd to think the service has exited.

  • Not reloading systemd after editing unit files (daemon-reload).

You now have a functional daemon managed by systemd (or SysV init). Adjust paths, users, and restart policies to fit your application.