The H has a
four page article
by Lennart Poettering, Kay Sievers and Thorsten
Leemhuis on systemd. From the third page: "The unit files that are associated with systemd and the services are located in the /lib/systemd/system/ directory; if an identically named file exists in /etc/systemd/system/, systemd will ignore the one in the lib directory. This allows administrators to copy and customise a systemd unit file without running the risk that it could be overwritten during the next update – this can happen in SysVinit distributions if one of the init scripts stored in /etc/rc.d/init.d/ has been modified.
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