The systemd facility replaces the older System-V initialization scripts from earlier releases. The systemd is an event-driven facility which allows non-dependent subsystems to be started, controlled, or stopped in parallel. Here we explain how to add a custom script to the systemd facility.
1. Write And Debug The Custom Script
Typically a systemd script is written as a shell script. Begin by writing your custom script using the normal conventions. We will call our script my-custom-script.sh and is straightforward:
#!/bin/sh echo I am a custom script
2. The script must be executable
Lets make the script executable:
# chmod 0755 my-custom-script.sh
3. Describe The Custom Script To systemd
With the script written and tested manually, the script is ready to be described to the systemd system. To do this, a [name].service file is needed. The syntax uses the INI format commonly used for configuration files. Continuing our example, we need a my-custom-script.service file. The executable will run exactly once for each time the service is started. The service will not be started until the networking layer is up and stable:
# This is my-custom-script.service, which describes the my-custom-script.sh file
[Unit]
Description=This is executed on shutdown or reboot
DefaultDependencies=no
Wants=network-pre.target # (if network is required before running the script)
Before=network-pre.target shutdown.target reboot.target halt.target # Defines the order in which units are stoped. #(REQUIRED)
[Service]
Type=oneshot # enables specifying multiple custom commands that are then executed sequentially. (REQUIRED)
RemainAfterExit=true # required by the oneshot setting (REQUIRED)
Environment=ONE='one' "TWO='2" # you can set some environment variables, that may be necessary to pass as arguments
ExecStart=/bin/true # because is a shutdown script nothing is done when this service is started
ExecStop=/bin/bash /usr/local/bin/my-custom-script.sh ${ONE} ${TWO} # < --*********** change to the script full path ************ (REQUIRED)
TimeoutStopSec=1min 35s # Configures the time to wait for stop.
[Install]
WantedBy=multi-user.target # When this unit is enabled, the units listed in WantedBy gain a Want dependency on the unit. (REQUIRED)
4. Place The Service File Into The Expected Service Collection Directory
Lets place the custom script in service collection directory i.e. /etc/systemd/system/:
# cp my-custom-script.sh /etc/systemd/system/
5. Enable The Script For Future Reboots
Similar to the chkconfig from earlier versions, the service must be enabled. Since a new service was added, notify the systemd daemon to reconfigure itself:
# systemctl enable my-custom-script.service # systemctl daemon-reload