The letters, which were sent earlier this week to companies around the world, warned that commercial users of Linux may face legal liability for using the operating system without a license from SCO.
The move follows the filing of a $1 billion lawsuit in March against IBM for allegedly taking bits of Unix code and transferring them to Linux. IBM dismissed the lawsuit as unfounded.
If the latest tactic is successful, it could undermine a tenet of the Linux movement, which has been growing in large part because of the software's very low cost and freedom from the hassles of proprietary software, such as Microsoft Windows.
But even if SCO fails to convince Linux users to pay for licenses, it has raised fear, uncertainty and doubt- or "FUD"- about Linux and whether end users can be held liable if other portions are found to be what amounts to stolen property.
"When SCO's own UNIX software code is being illegally copied into Linux, we believe we have an obligation to educate commercial users of the potential liability that could rest with them for using such software to run their business," Chris Sontag, a SCO senior vice president, said Wednesday.
Lindon, Utah-based SCO acquired control of Unix intellectual property from Novell, which had bought the rights in 1992 from AT&T. AT&T's Bell Laboratories created Unix for minicomputers in the late 1960s and commercialized it in the 1980s.
Linux, a Unix derivative first developed in the early 1990s by Finnish college student Linus Torvalds, has in recent years gained in popularity because of its low cost, reliability and ability to run on inexpensive computer hardware.
In an e-mail interview Wednesday, Torvalds said he has not heard what components of Linux might be infringing.
"I'd dearly love to hear exactly - what - they think is infringing, but they haven't told anybody," he said. "Oh, well. They seem to be more interested in FUD than anything else."
SCO also announced it was pulling its own version of Linux, which may be significant because it was distributed under the same license as other versions. That license allows for the free redistribution of the software.
"I suspect the current letter is because their lawyers finally noticed that as long as they ship Linux, they are themselves bound by the license under which it ships," Torvalds said.
Some observers believe SCO sued IBM in an effort to be bought _ a charge SCO denies. SCO's stock price has nearly doubled since it filed the lawsuit, said Bruce Perens, an open-source software advocate and consultant. He said he believes it's a tactic of SCO's investors to recoup their investment.
"They don't care who or what they hurt," he said.
Darl McBride, SCO's chief executive, said the company is not trying to find a buyer but rather informing users of the problem. He said most Linux distributors leave their customers liable.
"Everyone is protected except the users," he said. "We're trying to alert users there are these problems. We're trying to do this in a favorable fashion. ... The hot potato is being passed. We're trying to keep them from being burned."
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