Service Basics

Using Services

• systemctl enable [service] - registers the [service] to be started upon system bootup.
• systemctl start [service] - immediately start the [service].
• systemctl stop [service] - immediately stop the [service].
• systemctl restart [service] - immediately stop (if running) then start the [service].

Creating a Custom Service

Learning Outcomes

This section walks through creating a service named myservice that shall:

• have a unit file (/etc/systemd/system/myservice.service) so the system can handle the service properly.
• use a configuration file (/etc/myservice.conf) to pass information to it.
• execute a start script (/usr/bin/myservice/start.sh) upon service start up
• execute a stop script (/usr/bin/myservice/stop.sh) upon service shut down
• optionally be started by the system upon boot up.

note: all running services are stopped upon system shutdown

Tutorial

follow the steps below to create the service described earlier:

1. think of a name for your service. myservice will be used as the name of our newfangled custom service for this tutorial.

2. create the myservice.service service unit file in the /etc/systemd/system directory:

    # cat << EOF > /etc/systemd/system/myservice.service
    > [Unit]
    > Description=this service does something useful
    > After=default.target
    > ConditionPathExists=/etc/myservice.conf
    > ConditionPathExists=/usr/bin/myservice/start.sh
    > ConditionPathExists=/usr/bin/myservice/stop.sh
    > 
    > [Service]
    > Type=oneshot
    > RemainAfterExit=yes
    > EnvironmentFile=/etc/myservice.conf
    > ExecStart=/usr/bin/myservice/start.sh ${param_foo} ${param_bar}
    > ExecStop=/usr/bin/myservice/stop.sh ${param_foo} ${param_bar}
    > 
    > [Install]
    > WantedBy=default.target
    > EOF
   

   there are a couple of important things to note about the service declaration above:

   • ConditionPathExists prevents the service from executing if the path does not exist.
   • EnvironmentFile specifies the location of the configuration file for the service. this is where the values for ${param_foo} and ${param_bar} are loaded from.
   • ExecStart specifies a command to execute upon starting the service.

3. create the myservice.conf configuration file in the /etc directory:

    # cat << EOF > /etc/myservice.conf
    > param_foo=value one
    > param_bar=value2
    > EOF
   

   the configuration file created above defines two variables: param_foo with the value value one, and param_bar with the value value2.

4. create the start.sh start script that is run upon starting the service in the /usr/bin/myservice directory.

    # cat << EOF > /usr/bin/myservice/start.sh
    > #!/bin/bash
    > echo "param_foo is $1 and param_bar is $2. service has started."
    > EOF
    # chmod 755 /usr/bin/myservice/start.sh
   

   the start script logs a message involving some command-line arguments.

5. create the stop.sh stop script that is run upon stopping the service in the /usr/bin/myservice directory.

    # cat << EOF > /usr/bin/myservice/stop.sh
    > #!/bin/bash
    > echo "param_foo is $1 and param_bar is $2. service has stopped."
    > EOF
    # chmod 755 /usr/bin/myservice/stop.sh
   

   the stop script logs a message involving some command-line arguments.

6. start the service then check the status of the service to make sure that the service has started correctly.

    # systemctl start myservice
    # systemctl status myservice
   

7. stop the service then check the status of the service to make sure that the service has stopped correctly.

    # systemctl stop myservice
    # systemctl status myservice
   

8. congratulations! :) you’ve just created yourself a service. consider enabling it to have the system automatically start up the service upon booting up.

   # systemctl enable myservice