systemctl Use systemd to Start a Linux Service at Boot -

What is systemd?

systemd is a Linux system tool initially developed by the Red Hat Linux team. It includes many features, including a bootstrapping system used to start and manage system processes. It is currently the default initialization system on most Linux distributions. Many commonly used software tools, such as SSH and Apache, ship with a systemd service.

It is simple to create a custom systemd service that will run any script or process you choose. Although there are several ways to run a script or start a process when your Linode boots, a custom systemd service makes it easy to start, stop, or restart your script, as well as configure it to start automatically on boot. systemd offers the advantage of using a standardized interface that is consistent across all Linux distributions that support it.

Create a Custom systemd Service

Create a script or executable that the service will manage. This guide uses a simple Bash script as an example:
File: test_service.sh
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DATE=`date '+%Y-%m-%d %H:%M:%S'` echo "Example service started at ${DATE}" | systemd-cat -p info while : do echo "Looping..."; sleep 30; done
This script will log the time at which it is initialized, then loop infinitely to keep the service running.

Copy the script to /usr/bin and make it executable:

sudo cp test_service.sh /usr/bin/test_service.sh
sudo chmod +x /usr/bin/test_service.sh

Create a Unit file to define a systemd service:
File: /lib/systemd/system/myservice.service
```
[Unit]
Description=Example systemd service.

[Service]
Type=simple
ExecStart=/bin/bash /usr/bin/test_service.sh

[Install]
WantedBy=multi-user.target
```
This defines a simple service. The critical part is the ExecStart directive, which specifies the command that will be run to start the service.
Copy the unit file to /etc/systemd/system and give it permissions:
```
sudo cp myservice.service /etc/systemd/system/myservice.service
sudo chmod 644 /etc/systemd/system/myservice.service
```
For more information about the unit file and its available configuration options, see the systemd documentation.

Start and Enable the Service

Once you have a unit file, you are ready to test the service:
```
sudo systemctl start myservice
```

Check the status of the service:

sudo systemctl status myservice

If the service is running correctly, the output should resemble the following:

  ● myservice.service - Example systemd service.
   Loaded: loaded (/lib/systemd/system/myservice.service; enabled; vendor preset: enabled)
   Active: active (running) since Tue 2018-05-01 18:17:14 UTC; 4s ago
 Main PID: 16266 (bash)
    Tasks: 2
   Memory: 748.0K
      CPU: 4ms
   CGroup: /system.slice/myservice.service
           ├─16266 /bin/bash /usr/bin/test_service.sh
           └─16270 sleep 30

May 01 18:17:14 localhost systemd[1]: Started Example systemd service..
May 01 18:17:14 localhost cat[16269]: Example service started at 2018-05-01 18:17:14
May 01 18:17:14 localhost bash[16266]: Looping...

The service can be stopped or restarted using standard systemd commands:
```
sudo systemctl stop myservice
sudo systemctl restart myservice
```

Finally, use the enable command to ensure that the service starts whenever the system boots:

sudo systemctl enable myservice

  Created symlink from /etc/systemd/system/multi-user.target.wants/myservice.service to /lib/systemd/system/myservice.service.

Reboot your Linode from the Linode Manager and check the status of the service:

sudo systemctl status myservice

You should see that the service logged its start time immediately after booting:

  ● myservice.service - Example systemd service.
   Loaded: loaded (/usr/lib/systemd/system/myservice.service; enabled; vendor preset: disabled)
   Active: active (running) since Wed 2018-05-02 15:03:07 UTC; 48s ago
 Main PID: 2973 (bash)
   CGroup: /system.slice/myservice.service
           ├─2973 /bin/bash /usr/bin/test_service.sh
           └─3371 sleep 30

May 02 15:03:07 localhost systemd[1]: Started Example systemd service..
May 02 15:03:07 localhost systemd[1]: Starting Example systemd service....
May 02 15:03:07 localhost bash[2973]: Looping...
May 02 15:03:37 localhost bash[2973]: Looping...

Troubleshooting

“Example service started at …” line does not appear in the output of the status command. The systemd-cat output is not reliable because of a race condition. As a workaround update the test_service.sh file as follows:

File: test_service.sh

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info=/tmp/myservice-systemd-cat-pipe-info
mkfifo "$info"
trap "exec 3>&-; rm $info" EXIT
systemd-cat -p info < "$info" &
exec 3>"$info"

DATE=`date '+%Y-%m-%d %H:%M:%S'`
echo "Example service started at ${DATE}" | systemd-cat -p info

while :
do
echo "Looping...";
sleep 30;
done