Today, we shall take a look at the Linux kernel version number system.
The version number of the Linux kernel currently consists of four numbers, following a recent change in the long-standing policy of a three-number versioning scheme. For illustration, let it be assumed that the version number is composed thus: A.B.C[.D] (e.g. 2.2.1, 2.4.13 or 18.104.22.168).
* The A number denotes the kernel version. It is changed least frequently, and only when major changes in the code and the concept of the kernel occur. It has been changed twice in the history of the kernel: In 1994 (version 1.0) and in 1996 (version 2.0).
* The B number denotes the major revision of the kernel. Even numbers indicate a stable release, i.e. one that is deemed fit for production use, such as 1.2, 2.4 or 2.6. Odd numbers are development releases, such as 1.1 or 2.5. They are for testing new features and drivers until they become sufficiently stable to be included in a stable release.
* The C number indicates the minor revision of the kernel. In the old three-number versioning scheme, this was changed when security patches, bugfixes, new features or drivers were implemented in the kernel. With the new policy, however, it is only changed when new drivers or features are introduced; minor fixes are handled by the D number.
* A D number first occurred when a grave error, which required immediate fixing, was encountered in 2.6.8's NFS code. However, there were not enough other changes to legitimate the release of a new minor revision (which would have been 2.6.9). So, 22.214.171.124 was released, with the only change being the fix of that error. With 2.6.11, this was adopted as the new official versioning policy. Bugfixes and security patches are now managed by the fourth number, whereas bigger changes are only implemented in minor revision changes (the C number).