By Ivan Noble BBC News Online technology staff

Four firms selling the Linux computer operating system are banding together to create UnitedLinux.

The move seems to address the criticisms of software developers who have to contend with a variety of different versions of the alternative, cooperatively developed system.

SuSE, Caldera, Turbolinux and Conectiva say they will release Linux products based on a common core system towards the end of 2002, making life easier not just for developers, but for computer equipment makers who want to have their products certified Linux-compatible.

But the UnitedLinux group does not include the dominant player in the Linux market, Red Hat.

"UnitedLinux will make global corporate Linux computing a reality," said SuSE chief executive Gerhard Burtscher.

Open invitation

The four firms say Red Hat and other firms would be welcome to join them.

Red Hat is a rare success story in an industry used to seeing financial failure follow ambitious start-up announcements.

It may prefer to try to expand its own position and establish its own version of Linux as a de facto standard.

Red Hat's immediate response was that UnitedLinux was good for Linux overall.

Caldera CEO Ransom Love says Red Hat and Mandrake have been invited to join the group and will respond over the next few weeks.

Taking on Microsoft

Linux is a computer operating system which competes primarily with server products from firms like Microsoft and Sun Microsystems.

Server operating systems are unglamorous pieces of software running on machines generally kept out of sight, but setting them up and keeping them running is big business.

Servers host and deliver every one of the billions of web pages on the internet, store countless files for millions of companies, and route documents from users to printers.

Linux is perhaps the most famous example of open source software, developed by programmers who allow public access to the instructions, or source code, used to put the system together.

Flexibility versus compatibility

Open source advocates say openly developed software is better than products from big commercial companies who vigorously defend their ownership of the "recipes" for their software.

They say bugs are found more quickly and users can adapt the system to their own ends.

But this flexibility had led to the creation of countless different versions of Linux with varying degrees of mutual compatibility.

A program packaged to run on Red Hat Linux, for example, might well fail on, say, SuSE Linux.

The UnitedLinux four say they will build a common core system where such irritations are ruled out.