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/fake.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.