/etc/rc.local is a thing of the past.

Forget about /etc/rc.local.

/etc/rc.local dates from the time of 4BSD, Seventh Edition Unix, and before. It came about so that system administrators could make "local" modifications to the bootstrap process without (as they had had to) editing the /etc/rc script. /etc/rc invoked /etc/rc.local just over halfway through. Of course, editing /etc/rc.local was just as fraught with the same dangers, and people had war stories about it such as Steve Simmons' tale of putting it under version control, for example.

It was superseded by /etc/inittab and a runlevel-based rc in AT&T Unix System 3 in 1983 (and with a slightly different /etc/inittab in AT&T Unix System 5). The "new" rc system allowed multiple individual scripts to be dropped into script directories organized by run-levels, rather than needing administrators to edit the startup commands for each new piece of local software into a single /etc/rc.local script. Even those (/etc/inittab and run-levels both) are now things of the past.

In NetBSD and FreeBSD, /etc/rc.local is a backwards compatibility mechanism, and not a native part of the system. /etc/rc no longer runs it. Instead, it is run by a Mewburn rc service named /etc/rc.d/local. This reflects the FreeBSD manual's own advice to not use /etc/rc.local but to use the Mewburn rc system directly, placing Mewburn rc scripts for whatever "local" thing that one wants to do in the /usr/local/etc/rc.d/ directory. (This has been the advice for quite a while, Mewburn rc having been introduced by NetBSD 1.5 in 2000 and added to FreeBSD in 2002. In the older FreeBSD /etc/rc, before Mewburn rc, it had been labelled "obsolete" in 1995 and it had been deleted in 1998.)

In the Linux operating system world, /etc/rc.local is at best similarly a backwards compatibility mechanism that is invoked by native mechanisms rather than directly employed. systemd runs it from a systemd service (generated by a service generator, named systemd-rc-local-generator, when an /etc/rc.local file is discovered to exist at bootstrap). van Smoorenburg rc (the clone of AT&T System 5 rc) runs it from a van Smoorenburg rc script, /etc/init.d/rc.local. Other system and service management toolsets do not run it at all, and it is simply a superfluous file in /etc that is ignored.

Indeed, some operating systems have already eliminated these backwards compatibility features from systemd, too. This is the case for Arch Linux, for example. On Arch, the systemd rc-local.service file is present simply because the backwards compatibility flag does not cover it. (This is probably an oversight on the systemd people's parts.) The service is no use without the systemd-rc-local-generator though, which is what enables the service on the fly; and that is not built or included in systemd on Arch.

Even where they exist, these backward compatibility mechanisms can be surprising. They don't run /etc/rc.local in the quite the same way, in quite the same place in the bootstrap, as it used to be in the old Seventh Edition Unix system. (Even that can be surprising, as /etc/rc.local did not, in fact, run last in the old system, as the OpenBSD manual still points out.) Things set up in /etc/rc.local can, depending from the service ordering that happens to take place, end up completely undone by the likes of new udev rules, NetworkManager, systemd-logind, systemd-resolved, or various "Kit"s.

From back in the 1980s when editing services into /etc/rc.local was first superseded by drop-in file mechanisms, the recommendation has been to create proper native service definitions for one's service management system; whether that be a service bundle for the nosh toolset's service-manager and system-control, an /etc/rc.d/ script for Mewburn rc, a service unit file for systemd, an /etc/init/ job file for Upstart, a service directory for runit/s6/daemontools-encore, a service bundle for Solaris SMF, a plist for MacOS launchd, an /etc/init.d/ script for van Smoorenburg rc, an /etc/rc.local.d/ script for FreeBSD's drop-in file mechanism from 1995, or even (as Peter da Silva did in the 1980s and early 1990s) an honest-to-goodness script for AT&T System 5 rc.