The SCO Group has a clear message for companies that have enjoyed the low-cost benefits of running Linux as an alternative to Unix: It's time to pay up. The company on Monday introduced a plan to charge Linux users for UnixWare licenses designed to support run-time, binary use of Linux based on kernel version 2.4 and later. Just in case this doesn't appeal to the Linux community, SCO Group also says it now has the Unix System V source code copyright registrations needed to do legal battle against Linux users who resist.

Although SCO Group hasn't determined pricing for these new UnixWare licenses, the cost to companies that use Linux will be determined by the number of servers running the open-source operating system, with volume discounts available. SCO Group contends that the licenses are a way to compensate the company for unlicensed use of its copyrighted material.

In May, SCO Group sent letters to 1,500 companies worldwide, warning them that use of Linux might interfere with SCO's Unix intellectual property. "Today, we confirm that claim," says Darl McBride, SCO Group's president and CEO. With its copyrights in hand, "the legal fairway that we're working with just got wider."

SCO Group claims that its licensed Unix source code was used to improve Linux, without any compensation to SCO. Although this claim hasn't been proven conclusively, SCO Group has invited media and analysts to see the Unix and Linux code side by side so they can judge for themselves. Dozens of people have come to SCO Group's Lindon, Utah, facilities, McBride says, and "everyone who's come out there has come to the same conclusion--that there's a problem with Linux."

SCO Group claims that Unix has been used to accelerate the development of Linux in two key ways--line-by-line copying of Unix System V source code into the Linux kernel and copying derivative Unix code that enables multiprocessing capabilities. This isn't a matter of changing a line or two of code, McBride says. If Unix were removed, he adds, Linux "would have little multiprocessor capabilities."

Although IBM initially raised SCO Group's suspicions regarding use of the Unix source code to improve Linux, SCO Group legal counsel David Boies says the lawsuit against IBM doesn't have to be resolved for SCO Group to litigate against Linux users for copyright infringement.

SCO Group has a lot to gain by officially obtaining the Unix System V source code copyrights. Research firm IDC estimates that there are 2.4 million Linux servers in the business world. Each proposed UnixWare license would be payable by Linux customers, as opposed to Linux distributors.