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There is an age-old classical saying that happy people don’t watch the clock. There weren’t any programmers or Unix operating systems in those pre-civilisation times, but in our day programmers know for sure that cron is watching the clock for them.

For me, command-line utilities are both a passion and a daily routine. sed, awk, wc, cut and other old programs are run on our servers every day. Many of them are set up run through cron, a scheduler that dates back to the 1970s.

For a long time, I used cron in nothing more than a superficial way; I never went into the details. But one day, coming across an error while running a script, I decided I would get to the bottom of things. That is how this article came to be written and how, in the writing of it, I got to know POSIX crontab, the versions of cron in Linux distributions and what some of them consist of. …

Vladimir Kazanov

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