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How can we determine the version of the running kernel?

The official version of an Ubuntu kernel is found in the /proc/version_signature file. This file contains both the full Ubuntu version of the kernel and the mainline version on which it is based. The first field is always Ubuntu, the second field is the Ubuntu kernel version, and the final field is the upstream version:

• $ cat /proc/version_signature
  Ubuntu 2.6.35-6.9-generic 2.6.35-rc3
  $