/etc/systemd/system/

[Unit]
Description=executed at shutdown
Conflicts=reboot.target
After=network.target

[Service]
Type=oneshot
ExecStart=/bin/true
ExecStop=/path/to/script
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

#!/bin/sh
echo 'the script is executed' > test.txt

bash test_script.sh

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

[Service]
Type=oneshot
ExecStart=/bin/true
ExecStop=[whatever you need to run]
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

[Service]
User=osmc
ExecStart=/bin/true
ExecStop=/path/to/script
RemainAfterExit=yes