Revision Numbers for the Linux Kernel

The Linux kernel is numbered in three parts: major revision, minor revision, and step. The minor revision has special meaning.

An even minor (for example, the "4" in "2.4.18") revision indicates a stable kernel revision. "Stable" means "stable in most cases",so your mileage may vary.

On the other hand, an odd kernel revision (e.g., the "5" in "2.5.35") indicates a development kernel, where all bets are off concerning stability and, for that matter, timeliness. New development kernels can be released daily, so tracking the "current spec" can be an interesting proposition on a slow link. Development kernels tend to track the "latest features", some of which may or may not make it into the stable kernels.

The step is simply indicative of the primary releases a given minor revision has gone through, i.e., 2.0.35 is the 36th formal release of the 2.0 kernel (because 2.0.0 counts, too). Note that most kernel releases go through a "pre-release" cycle, too, so you could be running "2.4.19 prerelease 9" or something similar. (If you are, you probably know all this stuff anyway, but it had to be said.)