How to create a systemd service unit file in Linux

systemd is a system and service manager for Linux operating systems. When you install any application from a repository, it will drop a service unit file into the systemd directory and you should not modify these files directly.

The systemd unit files will be found in the following three directories:

• /usr/lib/systemd/system/ – systemd unit files dropped when the package has installed.
• /run/systemd/system/ – systemd unit files created at run time.
• /etc/systemd/system/ – systemd unit files created by ‘systemctl enable’ command as well as unit files added for extending a service.

Sometime you may need to create a service unit file for a custom application or daemon or script. There are many parameters can be added, but we will only add a few values to make the unit file much simpler for better understanding:

Refer the following articles to know more about systemd & systemctl:

• How to manage systemd units with systemctl
• SysVinit Vs systemd Cheatsheet

For instance: To run a custom script on boot in systemd system, you need to create a custom service unit file. The following procedure describes the general process of creating a custom service unit:

Creating a custom script

The following shell script will write the welcome message in the file as follows:

$ sudo vi /usr/sbin/welcome.sh

#!/bin/bash
echo "Welcome to Linux WORLD..!!!" >> /tmp/welcome.txt

Creating systemd unit file

You will need to create a custom service unit file under the ‘/etc/systemd/system/’ directory because this is reserved for custom scripts. Any unit file in ‘/etc/systemd/system’ will override the corresponding file in ‘/lib/systemd/system’.

Syntax: The systemd unit file consists of three sections:

Section-1 [Unit]
Parameter 1
.
.
Parameter N

Section-2 [Service]
Parameter 1
.
.
Parameter N

Section-3 [Install]
Parameter 1

To demonstrate this, we will be creating a systemd service unit file named ‘custom.service’.

$ sudo vi /etc/systemd/system/custom.service

[Unit]
Description=example systemd custom service unit file
After=network.target

[Service]
Type=notify
ExecStart=/bin/bash /usr/sbin/welcome.sh

[Install]
WantedBy=multi-user.target

Section-1:

• Unit : This section provides basic information about the service
• Description : Short description of the service unit. The description appears next to the service unit name when you execute ‘systemctl status UNIT.service’ command.
• After : Defines the order in which units are started. The ‘custom.service’ unit will be started only after the ‘network.target’ unit is started.

Section-2:

• Service : The ‘Service’ section provides instructions on how to control the service.
• Type : Defines the type of systemd service. It’s identical to ‘Type=simple’, but at the same time the daemon expected to send a signal to systemd when it is ready.
• ExecStart : It used to start the service, which includes the full path to the actual service executable.

Section-3:

• Install : The ‘Install’ section provides instructions on how to install the systemd service.
• WantedBy : The ‘WantedBy’ setting indicates under which target a given service unit should be launched. In this example, custom.service uses multi-user.target, so systemd starts custom.service when loading multi-user.target on startup.

Set the executable permission to the ‘custom.service’

$ sudo chmod a+x /etc/systemd/system/custom.service

To add a new service to systemd, run:

$ sudo systemctl daemon-reload

To start the custom.service, run:

$ sudo systemctl start custom.service

To enable the custom.service on boot, run:

$ sudo systemctl enable custom.service

Finally reboot the system to check if the custom.service ran the script on boot as expected by verifying the output file.

$ sudo reboot

Yes, it worked well.

$ cat /tmp/welcome.txt
Welcome to Linux WORLD..!!!

Ref: systemd the unit file structure

Over to You

In this guide, we have explained how to create a custom systemd service unit file in Linux.

If you have any questions or feedback, feel free to comment below.