9th August 2003 IBM, the world’s largest supplier of enterprise software, has finally joined the legal battle surrounding the ownership of the Linux operating system.

Late on Wednesday the computing giant filed a detailed counterclaim against SCO, the small US Unix software company that alleges IBM took parts of its SCO Unix code and redistributed for free as part of Linux. SCO is claiming damages of $3bn (£1.86bn) from IBM.

IBM, which has remained largely silent since SCO made its complaint in March 2003, appears not to be denying that some of SCO’s Unix code found its way into some versions of Linux via IBM’s version of Unix, known as AIX.

However, it claims that SCO is violating the GNU Public Licence (GPL), which stipulates that any code distributed as part of Linux by a software supplier shall be available for use and redistribution by others. SCO has been distributing Linux, including at least some of the disputed lines of code, from its web site since the mid 1990s.

IBM is also claiming that SCO has violated four of IBM’s patents, and that SCO has violated laws that prevent one company from interfering with another company’s customers.

On Monday (August 4th) another Unix distributer, Red Hat, filed a lawsuit against SCO, accusing it of “unfair and deceptive actions” in its campaign against Linux. Red Hat fears that the dispute could frighten some corporate customers away from using Linux.

SCO also said this week that it intends to charge end users customers for using Linux versions that contain SCO code.

IBM’s move will now put intense pressure on SCO, which is already under attack from the open source software movement worldwide. IBM will be able to bring such resources to the legal battle that SCO could be tied up for years in legal battles that the struggling company can ill afford.

Many observers believed SCO’s actions in starting the battle were an attempt to persuade IBM to buy the company. If that was the case, the IBM lawsuit will now weaken SCO’s position.