/etc/rc.d vs /etc/init.d

Ubuntu uses /etc/init.d to store SysVinit scripts because Ubuntu is based on Debian and that's what Debian uses. Red Hat uses /etc/rc.d/init.d. I forget what Slackware uses. There just isn't a standard location.

Ubuntu briefly switched from SysVinit to Upstart, but has now turned to using systemd.

/etc/init.d was the old historical location for SVR4. I forgot why redhat added the /etc/rc.d/ level. I think to isolate things onto rc.d, but then needed to add a bunch of symlinks anyway for backwards compatibility. So there is /etc/init.d in redhat, just it symlinks elsewhere.

So the standard location is /etc/init.d, though it may be a symlink not a real directory.

There were some really old Linux distros that copied BSD with /etc/rc.local but pretty much no one uses that anymore.

Slackware still uses /etc/rc.d

FreeBSD uses /etc/rc.d and /usr/local/etc/rc.d