AT&T sued Berkeley over 186 lines of Unix code, and the delay handed the market to Linux

Published Sep 12, 2026, 8:00 PM EDT I’m Adam Conway, an Irish technology fanatic with a BSc in Computer Science and I'm XDA’s Lead Technical Editor. My Bachelor’s thesis was conducted on the viability of benchmarking the non-functional elements of Android apps and smartphones such as performance, and I’ve been working in the tech industry in some way or another since 2017. In my spare time, you’ll probably find me playing Counter-Strike or VALORANT, and you can reach out to me at adam@xda-developers.com, on Twitter as @AdamConwayIE, on Instagram as AdamConwayIE, or u/AdamConwayIE on Reddit. Sign in to your XDA account Back in 1992, AT&T paid an expert witness to spend over 400 hours assessing Berkeley's version of Unix line by line, and what he returned with amounted to 186 lines of code, when Berkeley's own release amounted to roughly 1.5 million lines. That was what was considered to be the meaningful code overlap at the center of a lawsuit that dragged on for 22 months and, arguably, decided which free Unix we all ended up running. The case was UNIX System Laboratories v. Berkeley Software Design, filed in New Jersey on April 20, 1992 and settled on February 4, 1994. For most of those two years, anyone thinking about building on the free BSDs had to decide whether they were happy to be adjacent to an AT&T lawsuit. Especially when Linux had no such problem, given that there was nothing in Linux for AT&T to sue over. I've often seen the claim that this lawsuit is the reason for why Linux exists, and it was actually a version of that claim that made me go looking for what really happened. In short: it doesn't hold up, given that Torvalds posted his Linux announcement to the comp.os.minix newsgroup in August 1991; eight months before AT&T filed anything. It was also approximately seven months before Bill Jolitz released the first version of 386BSD that anyone could actually run. By this stage, Linux was already out there. But there's an important caveat: the lawsuit did contribute to Linux as we know it existing today. That lawsuit bought Linux the two years it needed to grow and become a mainstay, something that Torvalds himself has acknowledged in his autobiography Just for Fun. The whole case came down to 186 lines of code Or about 0.012 percent of Net/2 Berkeley had been shipping its own Unix under an AT&T licence since the 1970s, and over the course of the 1980s the Computer Systems Research Group set out to replace the AT&T-derived parts piece by piece. In July 1991 came Networking Release 2, often simply referred to as Net/2. It was a nearly complete operating system that Berkeley thought it could hand out to anyone, with or without a licence, but it was missing six kernel files. Replacements for those six came from Bill and Lynne Jolitz, which is how 386BSD came to be, alongside the commercial BSD/386 that Berkeley Software Design sold. It went on sale in January 1992 for $995, and came with the source included. AT&T's own source licences easily ran into five figures. USL, AT&T's Unix arm, saw a $995 operating system it had never licensed being sold on a phone number that spelled out "it's Unix". AT&T had registered UNIX as a trademark back in 1986, so before the litigation its opening move was a demand that BSDi stop calling the product Unix. When USL went to court, though, the dispute expanded into the code itself. At the first hearing, the judge told USL to restate its complaint around those six Jolitz files alone or he would throw it out, which is when USL widened the case to take in the university itself. Its expert, Professor John Carson, went through Net/2 against AT&T's UNIX/32V and reported that eighty of Net/2's eight thousand files were, in his words, "littered with textual references" to 32V. Judge Dickinson Debevoise wasn't a big fan of that argument; his opinion, published in March 1993, states the following: "Net2 has far outgrown 32V and now weighs in at nearly ten times the size of its parent. The alleged overlaps between parent and child probably amount to less than a percent of the total. Indeed, ignoring header files and comments, the overlap in the critical 'kernel' region is but 56 lines out of 230,999, and the overlap elsewhere is 130 lines out of 1.3 million." Most of what Carson found wasn't code in the sense that most people think of it. A good chunk of it was variable and function names, which are arbitrary until they cross an interface, at which point industry standards dictate them anyway. Another chunk was header files, which the court noted Berkeley had included "as a matter of necessity" so that software written for Unix would still compile. And some of it was comments, which Debevoise clearly didn't care about, given that he also pointed out that they "have no role whatsoever in software performance." Stripping out all of that leaves you with just 186 lines that anyone actually cared about. That 186 doesn't cover everything in Net/2 that came from AT&T, though. Marshall Kirk McKusick ran the Computer Systems Research Group, and had spent hundreds of hours going through Net/2 against 32V with Keith Bostic when the suit was filed. McKusick's January 1993 declaration, halfway through it, states that the 1,494 lines making up the cpio utility was "identical to AT&T proprietary source code." That's eight times the size of the other 186 lines in dispute, and both sides agreed it was identical to AT&T's proprietary code. McKusick's account of how it got there, though, is that USL asked for it. The declaration refers to cpio as a special case that USL had requested Berkeley add to its distributions. USL didn't accept that version, and Debevoise noted the disagreement: the cpio copying was the one example of direct copying the defence didn't dispute, but the parties disagreed over whether Berkeley had copied it on their own initiative or at AT&T's behest. It didn't do a great deal for USL either way, because the same opinion lumps CPIO in with BREAD as instructions that "might perhaps be trade secrets," and then says Berkeley's experts had argued persuasively that they were either in the public domain or otherwise exempt. AT&T could not prove it still owned 32V It didn't matter how many it was sold to The reason USL failed to get its injunction isn't actually the reason most people assume; it wasn't that Berkeley's rewrite was clean enough, though it more or less was. No, instead, it was that AT&T had spent well over a decade handing out 32V without a copyright notice on it, in an era when that was fatal. Decades ago, a published work in the United States of America could still be considered free of copyright if you were careless about it. For anything published between 1978 and the start of March 1989, you had to attach a proper notice, and if you didn't, section 405(a) of the Copyright Act gave you three narrow ways out, but AT&T didn't fit any of them. It hadn't registered 32V until 1992, which the judge noted was "well over five years after 32V was published," and it hadn't made any effort to go back and add notices to the copies that had already been distributed. USL, in response, tried to claim that it had only ever released 32V to a carefully chosen group, which would make it a limited publication rather than a general one. Debevoise wasn't having it: "Plaintiff essentially seeks what any lessor of property seeks: someone who will take good care of the property and pay the bills as they fall due. If this were enough to create a selected group, then practically anyone who leased property would belong to a selected group." What this means is that AT&T, on its own deposition evidence, licensed 32V source to anyone who wanted it, could pay for it, reasonably needed it, and would keep it to themselves. For all intents and purposes, that was a general publication. On Debevoise's reading, that left USL unable to establish an enforceable copyright in 32V. There's one detail I repeatedly saw in my research that turns out to be misreported, and Wikipedia repeats the claim as well. Essentially, it boils down to saying that AT&T lost the copyright to 32V because it had shipped thousands of copies without a notice attached. To be clear, Debevoise did write that in his opinion published on March 3. However, USL asked him to reconsider, and in a second opinion published on March 30, he agreed that the finding was "without adequate basis in the record" and struck it out, replacing it with a far smaller estimate: about 65 commercial, 220 educational, and an unknown number of government licensees. Even still, he wouldn't order a new hearing, as the number ultimately didn't matter. "The decisive factor in determining whether Plaintiff's publication of 32V was limited was not the number of licensees," he wrote, "but rather the criteria by which AT&T selected the licensees." Basically, even a few hundred licensees can still amount to a general publication if your policy is effectively to license anyone who meets the ordinary commercial criteria. Both March opinions were marked not for publication but were eventually discovered and published anyway, which probably helps explain why so few accounts quote them directly. The trade secret claim had the same result; after a generation of Unix being taught, dissected, and reimplemented in universities worldwide, the judge said he was "unable to ascertain whether any aspect of Net2 or BSD/386, be it an individual line of code or the overall system organization, deserves protection as Plaintiff's trade secret." No injunction issued on March 3, 1993, and none issued when USL asked again on March 30. Berkeley went on the offensive after that, suing USL in June 1993 in California, and arguing that AT&T had helped itself to BSD code in System V without giving the university the credit its own licence required. Novell took USL over that same month, and it had a Unix business to run rather than a lawsuit to win. All of it was dropped eight months later. Nobody ever ordered BSD off the internet It still existed for a while BSD didn't actually vanish from the internet, though you'd be forgiven for assuming that was the case. USL lost its injunction twice, so nothing was ever legally required to take it off the market, and the free BSDs kept coming out throughout the case. These included NetBSD 0.8 in April 1993 and FreeBSD 1.0 that November, both built on the supposedly toxic Net/2 code. What the lawsuit did, though, was make BSD look toxic as a whole. If you were a developer with little time and you were faced with a choice between contributing to a codebase that AT&T's lawyers were currently arguing about in federal court, or contributing to a kernel from a Finnish student that nobody had any claim on, it's an easy call to make. Jordan Hubbard, one of FreeBSD's founders, later said the episode "effectively ceded the territory to Linux during a critical period in which there was a leadership vacuum and lots of Fear, Uncertainty and Doubt in the BSD community." It didn't help that the settlement forced the BSDs into a rebase that consumed most of another year on top of the time already lost. Berkeley had agreed to produce a cleaned-up release, 4.4BSD-Lite, and BSDI agreed in its own announcement to "substitute a port of the University of California's forthcoming new release" for what it had been shipping. That meant the free projects had to tear out their Net/2 foundations and rebuild on Lite. FreeBSD threw out its entire 1.x line to do it, and FreeBSD 2.0 didn't arrive until November 22, 1994. One other common claim I came across was that BSDI announced Lite when the settlement was agreed, but that isn't what happened. Rob Kolstad's announcement on February 6 calls it "the University of California's forthcoming new release," and Keith Bostic’s letter to prospective buyers is dated March 1, 1994 and describes "our March 1994 release," and he didn't post it publicly until April 20. This meant that the BSDs spent two more months after the settlement without the settlement-cleaned Berkeley release to build on, followed by the better part of a year rebasing onto it. To be fair, the BSDs had problems that had nothing to do with AT&T. 386BSD had a real coordination issue, which is why NetBSD and FreeBSD both forked away from it in 1993 rather than waiting on Jolitz to merge the patchkit. Alan Cox also pointed out that 386BSD 0.1 needed a floating point unit and was awkward to compile, while Linux 0.12 ran fine without one, which was a big deal if you were a student buying whatever hardware you could afford. The lawsuit wasn't the whole story, but those were still two years lost at probably the worst possible time to lose them. Linux spent those two years growing up Time to mature uninterrupted When looking at the timelines for both Linux's rise and BSD's AT&T-induced purgatory, the overlap is almost uncomfortable. AT&T filed in April 1992, a couple of months after Torvalds had relicensed Linux under the GPL with version 0.12. Then, when the settlement was signed on February 4, 1994, Linux 1.0 shipped on March 14, 1994, just over a month later. A lot of the infrastructure that turned Linux from a kernel into something anyone could use appeared inside of that two-year window, too. SLS, Slackware, and Debian all popped up during the case, and Red Hat followed that October. Distribution, install media, and package management were all being figured out in the Linux world, and Linux was accumulating users and contributors while BSD was spending part of the same period under a legal cloud. What's funny is that Torvalds was already defending Linux publicly before AT&T filed anything, which shows how Linux was already starting to grow. On the comp.os.minix newsgroup across late January to early February 1992, Andrew Tanenbaum told Torvalds that "LINUX is obsolete" while also saying "be thankful you are not my student, you would not get a high grade for such a design." Torvalds replied that he probably wasn't getting good grades anyway, having recently had an argument with the person who taught operating system design at his university. That's a couple of months before the lawsuit even existed. Tanenbaum made another point in the same thread that's quite funny in hindsight: price wasn't what drove people to Linux. He listed the alternatives and their costs: MINIX at $169, Coherent at $99, and then "4.4 BSD is $800. I don't really believe money is the issue." (4.4BSD would not actually ship for another sixteen months, so whatever he was pricing there, it was not something you could buy at the time.) His point was more about free software ideology, but he also, without meaning to, described a market with a gap in it. Torvalds says the lawsuit gave Linux the room to spread And to take over the market I've often seen people state that Torvalds himself wouldn't have needed to create Linux if it weren't for the BSD lawsuit. I tracked the origin of that quote, and it comes from Kirk McKusick paraphrasing him at a talk in 2005. I couldn't find the original, but it also can't be right in any sense, purely because of the dates. Torvalds tells the story differently in Just for Fun. He talks about the System V and BSD split, gets to 386BSD and the lawsuit, and then says this: "Meanwhile, all the legal haggling had been instrumental in giving a new kid on the block some time to mature and spread itself. Basically, it gave Linux time to take over the market." That's very different from claiming the lawsuit caused Linux to exist. Torvalds described it as giving Linux a window to mature, spread, and take over the market, not as the reason he started writing it. The only thing he's on record as saying about starting Linux, that I could find, is that if a working 386 BSD had existed at the time, he probably wouldn't have bothered. When he said that to Meta Magazine in 1993 he also added that he'd never even looked at 386BSD because it wasn't available when he began. Berkeley got 23 files back and seemingly never used them Three were completely removed I also discovered that the quoted "three files out of eighteen thousand" figure, which traces back to McKusick's own recollection written years afterwards, was incorrect. The three was right, but the eighteen thousand part wasn't. The Net/2 master tape is still hosted by the Unix Heritage Society, and counting what's actually there gives a little over ten thousand files, while the court was working from a figure of eight thousand. It's a small thing, but yet another oddity I discovered during my research. The settlement ultimately restricted 26 files from Net/2, required AT&T copyright notices on another 68 that stayed freely distributable anyway, and USL reviewed and approved 23 of those 26 for reuse in 4.4BSD-Lite. Removing the approved ones, you're left with three files that didn't survive: sys/kern/sysv_shm.c, sys/sys/shm.h, and usr.bin/cpio/cpio.c. System V shared memory and cpio, both of which were AT&T's own contributions to Unix in the first place. USL gave those 23 files back, and Keith Bostic's distribution letter for 4.4BSD-Lite, dated March 1, 1994, said the same subsystems were "still missing" from the release. To me, it reads as if Berkeley never even availed of the permission it was granted in the end. What this would mean is that, after two years of litigation over a codebase in which the judge had identified only 186 lines of meaningful textual overlap, Berkeley finally got permission to put 23 of the disputed files back, but apparently still didn't use them. The lawsuit didn't kill BSD, given that NetBSD, FreeBSD, and OpenBSD are all still here and running on important hardware. What the case took was two years from BSD, in the years when a free Unix could still become the default free Unix, and gave those two years to a project with no question marks or worries hanging over it. Linux had its own run-ins with other people's intellectual property soon enough, and for years afterwards it still couldn't legally play a DVD. By then it had the users, though. AT&T went into that case claiming Berkeley had built its Unix out of AT&T's property, and came out of it unable to show it could defend its copyright in 32V, with its trade secret claim abandoned and just three Net/2 files ultimately kept out of Berkeley's clean release. It spent 22 months and an unknowable pile of legal fees to establish 186 lines of textual overlap. Novell, which had bought USL halfway through, signed it all away in February 1994 and the industry moved on. By then, Torvalds was five weeks from shipping Linux 1.0, and nobody was arguing about who owned that.