This is a moderately old paper, but I think it’s worth reading if you want to understand the rationale behind the NetBSD rc.d startup system. I think this is what is referred to on FreeBSD (which has adopted a similar mechanism) as rcNG.
There are many things to like in this design, which is far better than the organic (to put it politely) way in which the system startup sequence of a given Linux box has evolved. For one, it has the following advantages (outlined in the paper, but I’ll detail them here if you don’t want to read it):
- Independence from lexicographical ordering of filenames (no S90foo running before S91foo), which always struck me as having a sort of BASIC-style limitation (i.e. back in the day having to number your code lines in multiples of ten in case you wanted to insert code in between)
- Use of dynamic dependency ordering (via a special header and the rcorder script)
- No reliance upon a special platform-specific "function" library, as is the case in many Linuxes
- Centralized system configuration via /etc/rc.conf — no bloated /etc/sysconfig nonsense as on many Linuxes (but this is a topic for another day)
- Avoidance of mandatory runlevels, which I can never remember on a given Linux or Solaris machine. ("What is runlevel 5 again?")
I could go on, but I urge you to read the paper instead, where Luke demonstrates a solid design methodology and rationale and then executes on the same. This is more than can be said for Linux.