Systemd : How to execute script at shutdown only (not at reboot)

I've finally found how to do that.

It's a bit hackish thought, but it works.

I've used some part of this thread : https://stackoverflow.com/questions/25166085/how-can-a-systemd-controlled-service-distinguish-between-shutdown-and-reboot

and this thread : How to run a script with systemd right before shutdown?

I've created this service /etc/systemd/system/shutdown_screen.service

[Unit]
Description=runs only upon shutdown
Conflicts=reboot.target
After=network.target

[Service]
Type=oneshot
ExecStart=/bin/true
ExecStop=/bin/bash /usr/local/bin/shutdown_screen
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Which will be executed at shudown/reboot/halt/whatever. (don't forget to enable it)

And in my script /usr/local/bin/shutdown_screen I put the following :

#!/bin/bash
# send a shutdown message only at shutdown (not at reboot)    
/usr/bin/systemctl list-jobs | egrep -q 'reboot.target.*start' || echo "shutdown" | nc 192.168.0.180 4243 -w 1

Which will send a shutdown message to my arduino, whom will shutdown my screen.

According to the systemd.special man-page, you should use Before=poweroff.target.

poweroff.target

A special target unit for shutting down and powering off the system.

Applications wanting to power off the system should start this unit.

runlevel0.target is an alias for this target unit, for compatibility with SysV.

Additionally, as I mentioned in my comment, you should put custom scripts into /etc/systemd/system/. The /usr/lib/systemd/system/ directory is meant to be used for system-provided scripts.

So, maybe something like this:

[Unit]
Description=runs only upon shutdown
DefaultDependencies=no
Conflicts=reboot.target
Before=shutdown.target
Requires=poweroff.target

[Service]
Type=oneshot
ExecStart=/bin/true
ExecStop=/usr/local/bin/yourscript
RemainAfterExit=yes

From reading the answers here there seems to be a lot misunderstanding about how systemd works. Firstly don't use conflicts to exclude a target. It is ment to keep to conflicting services from being run at the same time.

If a unit has a Conflicts= setting on another unit, starting the former will stop the latter and vice versa.

Unit means a .service file ment to launch perticular service not a target to be reach. In other-words Conflicts=reboot.target is meaningless at best and at worst it will prevent you from rebooting. Don't do this. It does not mean don't run this on reboot. It means abort either this service or the reboot.target depending on timing and how systemd interprets this erroneous use of conflicts.

Here is example of a currently setup unit (aka .service file) that runs only on shutdown not reboot:

[Unit]
Description=Play sound
DefaultDependencies=no
Before=poweroff.target halt.target

[Service]
ExecStart=/usr/local/bin/playsound.sh
ExecStop=/usr/local/bin/playsound.sh
Type=oneshot
RemainAfterExit=yes

[Install]
WantedBy=poweroff.target halt.target 

poweroff.target is equivalent to the old systemv run level0 which is only reached at shutdown. halt.target is an alternate shutdown path used by systemd also not reachable by reboot. The install section tells systemd to add this service to the list that must be completed before poweroff.target or halt.target will be considered reached.

This service is installed and running on my system.