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1 Introducing mcron

The mcron program represents a complete re-think of the cron concept originally found in V7 Unix, ratified by POSIX, and re-realized by Paul Vixie for the free world. The original idea was to have a daemon that wakes up every minute, scans a set of files under a special directory, and determines from those files if any shell commands should be executed in this minute.

The new1 idea is to read the required command instructions, work out which command needs to be executed next, and then sleep until the inferred time has arrived. On waking the commands are run, and the time of the next command is computed. Furthermore, the specifications are written in Scheme, allowing at the same time simple command execution instructions and very much more flexible ones to be composed than the POSIX format allows. This has several useful advantages over the original idea.

A full discussion of the design and philosophy of mcron can be found in the white paper at



With retrospect, not so new: AT&T and Berkeley Unices had a cron program which also implemented this ’new’ idea, though Dale Mellor was unaware of this at the time of mcron’s conception and initial development. The Wikipedia page for cron appears quite authoritative and is a good source of information about the history of cron.

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