When I wrote my entry on some differences between various versions of ed(1), I forgot to check the Single Unix Specification to see if it has anything to say about ed. It turns out that it does, and ed is part of the 'Shell & Utilities' section.

SUS ed is mostly the same as FreeBSD ed, which is kind of what I think of as 'the baseline modern ed'. SUS ed requires that s support a bunch of flags for printing the results (only GNU ed documents supporting them all), but it doesn't require a z command (to print paginated output). Interestingly, SUS requires that ed support a prompt and extra help, and that it print warnings if you try to do something that would lose a modified buffer. SUS ed is only required to support SUS Basic Regular Expressions, while all modern eds go at least somewhat beyond this; FreeBSD ed supports '\<' and '\>', for example.

One area of ed's history that I don't know very much about is how it evolved in System III and System V. SCO's website (of all people) has a PDF version of the System V Interface Definition online here, and for as long as it lasts you want to look in volume 2 for the manpage for ed. The SVID ed is mostly the same as FreeBSD ed, and in particular it has '\<' and '\>' in its regular expressions, unlike SUS ed. Its s command doesn't support the l, n, or p flags (required by SUS but not in FreeBSD). It does have prompting and help. I think that ed is the only editor required by the SVID, which may explain why the System V people enhanced it over the V7 and BSD eds; in BSD, the development of vi (and before it ex) probably made ed a relatively secondary editor.

Conveniently, the SUS ed documentation includes a rationale section that includes a discussion of historical differences between BSD and SVID behavior, commands supported, and so on. Of course, 'BSD' in the SUS basically means 'UCB BSD through 4.4 or so', as I don't think the SUS looks at what modern FreeBSD and OpenBSD are doing any more than it looks at Linux or GNU programs.

(My previous entry and this one only look at obvious differences like commands supported. I'm sure that there are plenty of subtle behavior differences between various versions of ed, both old and modern ones; the SUS ed rationale points out some of them, and GNU ed's --traditional switch suggests other points of difference for it.)