You can find out what
systemd reload nginx will do by looking at the
ExecReload= option in the
[Service] section in the
nginx.service unit file (located at
/usr/lib/systemd/system/nginx.service on my system):
$ systemctl cat nginx | grep ExecReload=
Or by running:
$ systemctl show nginx.service --property=ExecReload
On my system, I get:
ExecReload=/usr/bin/kill -HUP $MAINPID
-s signal Send a signal to the master process. The argument signal
can be one of: stop, quit, reopen, reload. The following
table shows the corresponding system signals:
systemctl reload nginx and
nginx -s reload will, almost, do the same thing.
The differences are:
systemctl reload nginx will execute the command in a clean environment (and not the current user environment);
systemctl reload works for any service that allows it (which has it configured in the unit file). No need to remember service specific commands. This is even more interesting if you have several instances of a service.
init.d scripts are legacy/deprecated ways of doing the same thing. Although they might work, they are not supported nor recommended anymore on a systemd based system.