Systemd - Run before shutdown/reboot

run-before-shutdown.service

# systemd unit file
# /etc/systemd/system/run-before-shutdown.service
# TimeoutStartSec - set a timeout for your task or set "0" to disable.
#                   When the timeout is turned off and the task "hangs", 
#                   the process will never be interrupted and the system
#                   shutdown will stop
# Before - Specify the services before which the task should run

# after add unit file in systemd/system catalogue, "run systemctl daemon-reload"
# and enable new service "systemctl enable run-before-shutdown.service"

[Unit]
Description=My task
DefaultDependencies=no
Before=shutdown.target

[Service]
Type=oneshot
ExecStart=/path_to/script.sh
TimeoutStartSec=0

[Install]
WantedBy=shutdown.target