License: Public domain

                  Introduction to Service Management on Systemd
                                - taiprogramer -

                                                      Date created: Oct 13 2021
                                                     Last modified: Oct 21 2021

Section 1: Basic terminology

What is daemon?
- daemon is a process running in the background of the system.

What is init?
- init is a first process (pid=1) run when the system start and continue
  running until the computer shutdown, it's a parent of all other processes.

What is systemd?
- systemd is a init replacement that starts other processes in parallel.

Section 2: Practice

Basic service management

- Some things you can do with service
systemctl {enable, status, start, stop, restart} service-name.service

For example, to get current status of OpenSSH Server.

systemctl status sshd.service

- See all log messages of service
journalctl --unit service-name.service

- See log messages for current boot only
journalctl --unit service-name.service --boot

Create new service

- Create .sh script file act as service body

Location: ~/showlove.sh

------------- showlove.sh -------------
#!/bin/sh

i=0
while true; do
	echo "$i. I love you."
	i=`expr $i + 1`
	sleep 1
done
------------- showlove.sh -------------

- Create showlove.service file to register our service

Location: /etc/systemd/system/showlove.service

------------- showlove.service -------------
[Unit]
Description=Show love service

[Service]
ExecStart=/home/YOUR_USERNAME/showlove.sh
------------- showlove.service -------------

- Start the service

sudo systemctl start showlove.service

That's it. Now, you can see the output in a log file located in /var/log/syslog.

sudo tail /var/log/syslog

- Watch output of service in realtime

sudo journalctl -u showlove.service -f

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