How to write startup script for Systemd?

There are mainly two approaches to do that:

With script

If you have to run a script, you don’t convert it but rather run the script via a systemd service:

Therefore you need two files: the script and the .service file (unit configuration file).
Make sure your script is executable and the first line (the shebang) is #!/bin/sh. Then create the .service file in /etc/systemd/system (a plain text file, let’s call it vgaoff.service).
For example:

1. the script: /usr/bin/vgaoff
2. the unit file: /etc/systemd/system/vgaoff.service

Now, edit the unit file. Its content depends on how your script works:

If vgaoff just powers off the gpu, e.g.:

exec blah-blah pwrOFF etc 

then the content of vgaoff.service should be:

[Unit]
Description=Power-off gpu

[Service]
Type=oneshot
ExecStart=/usr/bin/vgaoff

[Install]
WantedBy=multi-user.target

If vgaoff is used to power off the GPU and also to power it back on, e.g.:

start() {
  exec blah-blah pwrOFF etc
}

stop() {
  exec blah-blah pwrON etc
}

case $1 in
  start|stop) "$1" ;;
esac

then the content of vgaoff.service should be:

[Unit]
Description=Power-off gpu

[Service]
Type=oneshot
ExecStart=/usr/bin/vgaoff start
ExecStop=/usr/bin/vgaoff stop
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Without script

For the most trivial cases, you can do without the script and execute a certain command directly:

To power off:

[Unit]
Description=Power-off gpu

[Service]
Type=oneshot
ExecStart=/bin/sh -c "echo OFF > /whatever/vga_pwr_gadget/switch"

[Install]
WantedBy=multi-user.target

To power off and on:

[Unit]
Description=Power-off gpu

[Service]
Type=oneshot
ExecStart=/bin/sh -c "echo OFF > /whatever/vga_pwr_gadget/switch"
ExecStop=/bin/sh -c "echo ON > /whatever/vga_pwr_gadget/switch"
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Enable the service

Once you’re done with the files, enable the service:

systemctl enable vgaoff.service

It will start automatically on next boot. You could even enable and start the service in one go with

systemctl enable --now vgaoff.service

as of systemd v.220 (on older setups you’ll have to start it manually).
For more details see systemd.service manual page.

Troubleshooting

• How to see full log of a systemd service?
• systemd service exit codes and status information explanation