Linux's file system check utility fsck is little recognised and largely unloved, no doubt because it seems to know whenever you're in a hurry to boot your machine. Then - and why is it only then? - it starts doing what it was designed for: checking the consistency of the filing systems on your hard drives and making any necessary repairs.

Since Ubuntu 9.04 users have at least had the option of hitting the Esc key to cancel the process, (users of earlier versions might like to consider Bonager), but what's actually going on and why does the checking seem so intermittent?

All file systems accumulate errors over time, often the result of application crashes, improper shutdown or bugs. Checking and recovery tools should be run periodically and fsck simply automates the process by setting a counter and checking off how many times each filing system has been booted. (It has to be done at boot as fsck can't be run over a mounted partition.) And that apparent randomness actually isn't. fsck typically sets the default boot count at 30 meaning that with daily boots your file systems will be checked once a month.

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