Create New Service Units in SystemD Using Shell Script on CENTOS 7

Source : http://www.tecmint.com/create-new-service-units-in-systemd/

In a simple way target is a collection of service units. If you want to have a look at service units running in yourgraphical.target level, type:

# systemctl --type=service

List All Service Units in CentOS 7

As you can see some services are active and “running” all the time, while others run one-time and terminate (exited). If you want to check the status of a service, type:

# systemctl status firewalld.service

Check Status of Service in CentOS 7

As you can see I checked the status of firewalld.service (tip: you can use the auto-complete for the name of the service). It informs me that firewalld service is running all the time and it is enabled.

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Enabled and disabled means the service will be permanently loaded or not, during the next boot respectively. On the other hand to start and stop a service has the limitation of the present session and it’s not permanent.

For example, if you type:

# systemctl stop firewalld.service
# systemctl status firewalld.service

Manage Services in CentOS 7

You can see that the firewalld.service is inactive (dead) but it is still enabled, which means that during next boot it will be loaded. So if we want a service to be loaded during boot time in the future we must enabled it. What a great conclusion! Lets create one, it’s easy.

If you go to the folder:

# cd /etc/systemd/system
# ls -l

SystemD System Files

You can see some link files of unit services and some directories of the “wants” of a target. For example: what the multi-user target wants to be loaded when the boot procedure reaches its level, is listed in the directory with name /etc/systemd/system/multi-user.target.wants/.

# ls multi-user.target.wants/

Multi User Targets Services

As you can see it doesn’t contain only services but also other targets which are also collections of services.

Let’s make a service unit with the name connection.service.

# vim connection.service

and type the following (hit “i” for insert mode), save it and exit (with “esc” and “:wq!” ) :

[Unit]
Description = making network connection up
After = network.target

[Service]
ExecStart = /root/scripts/conup.sh

[Install]
WantedBy = multi-user.target

Create New Service Units in CentOS 7

To explain the above: we have created a unit of service type (you can also create units of target type), we have set it to be loaded after the network.target (you can understand that the booting procedure reaches the targets with a defined order) and we want every-time the service starts to execute a bash script with the name conup.shwhich we are going to create.

The fun starts with the last part [install]. It tells that it will be wanted by “multi-user.target”. So if we enable our service a symbolic link to that service will be created inside the multi-user.target.wants folder! Got it? And if we disable it that link will be deleted. So simple.

Just enable it and check:

# systemctl enable connection.service

it informs us that the symbolic link in the multi-user.target.wants folder has been created. Check it:

# ls multi-user.target.wants/

Enable Service in CentOS 7

As you can see “connection.service” is ready for next booting, but we must create the script file first.

# cd /root
# mkdir scripts
# cd scripts
# vim conup.sh

Add the following line inside vim and save it:

#!/bin/bash
nmcli connection up enp0s3

Of course if you want your script to execute something else, you could type whatever you want instead of the second line.

For example,

#!/bin/bash
touch /tmp/testbootfile

that would create a file inside /tmp folder (just to check that your service is working).

We must also make the script executable:

# chmod +x conup.sh

Now we are ready. If you don’t want to wait until next boot (it’s already enabled) we can start the service for the current session typing:

# systemctl start connection.service

Voila! My connection is up and running!

If you ‘ve chosen to write the command “touch /tmp/testbootfile” inside the script, just to check its functionality, you will see this file created inside /tmp folder.