One cause of Linux's popularity among Unixes
Regardless of what you feel about it, I think that most people can agree that Linux is winning whatever is left of the Unix wars. It isn't the only Unix left but for a fairly long time now it's been the leading one, often the default choice. You can attribute this to good PR if you want to, but I happen to think that that's a mistake. Linux has attracted people partly because it has genuine attractions.
In light of my rant about the waste inherent in building packages yourself, it has struck me that one such advantage has been Linux's general wide availability of packages. As I mentioned, system administrators really appreciate not having to spend their limited time compiling ordinary things and Linux is very good at that; most major Linux distributions will give you a precompiled version of almost any standard Unix program you could want (or at least a precompiled program to do almost any standard job). I don't think it's an accident that one of the long term favorite distributions is Debian, which has one of the biggest package archives going.
(Prepackaged software is not good enough if you need a specific version of something compiled in a specific way. But for many Unix machines you just need a working and reasonably current version of whatever. And there are a lot of packages on many machines where the exact details are not crucial.)
At this point I have to mention FreeBSD's ports collection, which even comes in precompiled packages; logically one would expect this to be just as good a selling point for FreeBSD as a Linux distribution with a similar package selection. However, I'm not convinced that it is in practice, and for why I'll point at the name: 'ports'. Well, more what the name means or is perceived as meaning.
Debian's vast collection of packages are all Debian together. Some of
them are more important than others, but they are all part of the Debian
whole. The dividing line between really important and less and less
important is both relatively opaque to outsiders and somewhat subject
to debate; it can get pushed back and forth if people want. By contrast
at least to an outsider FreeBSD has a relatively sharp dividing line;
you have FreeBSD core and then you have ports. Ports is clearly not the
same and to drive the point home they install things into
FreeBSD is probably at least as committed to ports as, say, Ubuntu
is to packages in
But I'm not convinced that non-FreeBSD sysadmins who are looking
at the situation really believe down in their guts that FreeBSD is
as committed to ports as Ubuntu is to
main (even if it is, and I
don't believe it's that committed to all ports). And I think that
that makes a difference.
(I am talking about non-FreeBSD sysadmins here because these are often the people who are making decisions about whether or not to use FreeBSD. Also that's the situation I'm in myself, so I don't know how it looks from the inside but I can talk with at least a little bit of authority how it feels from the outside.)
PS: I haven't mentioned commercial Unixes here because oh boy package availability on commercial Unixes, that's a funny joke. Provided by third parties at best. Red Hat Enterprise Linux is sort of in the same boat but at least they woke up and I think started doing something with EPEL.