crontab的用法？？？

Question

有谁了解crontab的作用啊？能否解释一下，如：0 5 7，12，18，23** ，怎么解释阿？

Answer 1

info crontab

...
Commands are executed by cron(8) when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) match the current time (see``Note'' below). cron(8) examines cron entries once every minute. The time and date fields are:

          field          allowed values
          -----          --------------
          minute         0-59
          hour           0-23
          day of month   1-31
          month          1-12 (or names, see below)
          day of week    0-7 (0 or 7 is Sun, or use names)

   A field may be an asterisk (*), which always stands for ``first-last''.

   Ranges of numbers are allowed.  Ranges are two numbers separated with a hyphen.   The  specified  range is inclusive.  For example, 8-11 for an ``hours'' entry specifies execution at hours 8, 9, 10 and 11.

   Lists are allowed.  A list is a set of numbers (or ranges) separated by commas.  Examples: ``1,2,5,9'', ``0-4,8-12''.

   Step  values can be used in conjunction with ranges.  Following a range with ``/<number>'' specifies skips of the number's  value  through  the range.  For example, ``0-23/2'' can be used in the hours field to spec-ify command execution every other hour (the alternative in the V7 stan-dard  is ``0,2,4,6,8,10,12,14,16,18,20,22'').  Steps are also permitted  after an asterisk, so if you want to say ``every two hours'', just  use ``*/2''.

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