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 ed s 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 ed s; 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.)