The hacker ethos and community as I have described it here long predates the emergence of Linux after 1990; I first became involved with it around 1976, and, its roots are readily traceable back to the early 1960s. But before Linux, most hacking was done on either proprietary operating systems or a handful of quasi-experimental homegrown systems like MIT's ITS that were never deployed outside of their original academic niches. While there had been some earlier (pre-Linux) attempts to change this situation, their impact was at best very marginal and confined to communities of dedicated true believers which were tiny minorities even within the hacker community, let alone with respect to the larger world of software in general.
What is now called "open source" goes back as far as the hacker community does, but until 1985 it was an unnamed folk practice rather than a conscious movement with theories and manifestos attached to it. This prehistory ended when, in 1985, arch-hacker Richard Stallman ("RMS") tried to give it a name â€” "free software". But his act of naming was also an act of claiming; he attached ideological baggage to the "free software" label which much of the existing hacker community never accepted. As a result, the "free software" label was loudly rejected by a substantial minority of the hacker community (especially among those associated with BSD Unix), and used with serious but silent reservations by a majority of the remainder (including myself).
Despite these reservations, RMS's claim to define and lead the hacker community under the "free software" banner broadly held until the mid-1990s. It was seriously challenged only by the rise of Linux. Linux gave open-source development a natural home. Many projects issued under terms we would now call open-source migrated from proprietary Unixes to Linux. The community around Linux grew explosively, becoming far larger and more heterogenous than the pre-Linux hacker culture. RMS determinedly attempted to co-opt all this activity into his "free software" movement, but was thwarted by both the exploding diversity of the Linux community and the public skepticism of its founder, Linus Torvalds. Torvalds continued to use the term "free software" for lack of any alternative, but publicly rejected RMS's ideological baggage. Many younger hackers followed suit.
In 1996, when I first published this Hacker HOWTO, the hacker community was rapidly reorganizing around Linux and a handful of other open-source operating systems (notably those descended from BSD Unix). Community memory of the fact that most of us had spent decades developing closed-source software on closed-source operating systems had not yet begun to fade, but that fact was already beginning to seem like part of a dead past; hackers were, increasingly, defining themselves as hackers by their attachments to open-source projects such as Linux or Apache.
The term "open source", however, had not yet emerged; it would not do so until early 1998. When it did, most of hacker community adopted it within the following six months; the exceptions were a minority ideologically attached to the term "free software". Since 1998, and especially after about 2003, the identification of 'hacking' with 'open-source (and free software) development' has become extremely close. Today there is little point in attempting to distinguish between these categories, and it seems unlikely that will change in the future.
It is worth remembering, however, that this was not always so.