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If you want to modify anything in the file, the best way to do this is to create a file named /etc/systemd/system/shibd.service.d/shibd.conf and include any overridden lines you want to merge into the file. You can name the file itself anything that ends in the "conf" extension, but do not create that directory and leave it empty.
Note that any time you modify the unit file definition, you will need to tell systemd about the change using systemctl daemon-reload