Understanding path units

You can use a path unit to have systemd monitor a certain file or directory to see when it changes. When systemd detects that the file or directory has changed, it will activate the specified service. We'll use the Common Unix Printing System (CUPS) as an example.

In the /lib/systemd/system/cups.path file, we see this:

[Unit]
Description=CUPS Scheduler
PartOf=cups.service
[Path]
PathExists=/var/cache/cups/org.cups.cupsd
[Install]
WantedBy=multi-user.target

The PathExists= line tells systemd to monitor a specific file for changes, which in this case is the /var/cache/cups/org.cups.cupsd file. If systemd detects any changes to this file, it will activate the printing service.