Tor systemd service should have ReadWriteDirectories=/var/run/tor
Tor writes it's pidfile to /var/run/tor/tor.pid by default.
However, https://gitweb.torproject.org/tor.git/tree/contrib/dist/tor.service.in specifies that all but 2 directories are read only. Therefore, when one starts tor using:
systemctl start tor
using the default configuration, this error is logged in the journal:
Jul 26 22:42:32 irrational Tor: Unable to open "/var/run/tor/tor.pid" for writing: Read-only file system
and no pidfile is written.
to the [Service] section fixes the problem.