Set up a service to trigger FIRST (this would be the shutdown service):
# cat /lib/systemd/system/fakehalt.service [Unit] Description=Fake-Halt Service After=fakevm.service Requires=fakevm.service [Service] Type=simple ExecStart=/usr/local/bin/fakehalt.sh #this will fail until fakevm succeeds ExecReload=/usr/local/bin/fakehalt.sh
And then set up the one that you want to run and complete BEFORE shutdown is permitted:
# cat /lib/systemd/system/fakevm.service [Unit] Description=Fake Service Before=fakehalt.service [Service] Type=simple ExecStart=/usr/local/bin/fake.sh
Create a script to represent the VM shutdown (or any process that you cannot anticipate the duration of)
# cat /usr/local/bin/fake.sh #!/bin/sh test="1" sleep 21 if [ X"$test" = "X1" ]; then echo "vm has shut down" > /tmp/fake.test exit 0 else exit 1 fi
And a script to pass for a shutdown signal:
# cat /usr/local/bin/fakehalt.sh #!/bin/sh sleep 3 cat /tmp/vmfake.test > /tmp/haltfake.test
Start the service you want to happen AFTER the first one:
# systemctl start fakehalt
What "should" happen is that fakehalt will fail to find a file called /tmp/fake.test to cat from, and so everything should go horribly wrong.
What actually happens is that systemd places fakehalt service on hold until it gets an exit 0 signal from the fake service. So if you wait 21 seconds and cat /tmp/fakehalt.test, you see that the cat from a file that did not exist when fakehalt was started - actually succeeded.