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Crontab recipe Hints


WARNING:  '%' is a special character in crontabs! -- escape it '\%'
This is especially important if you use "date" in a cron script.

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Who to mail

Add
   MAILTO="user@email.domain"
to have crontab mail to your specific address

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Run first Saturday of Month

  0 0 1-7 * * [ `date '+\%u'` -eq 6 ] && CMD

OR swaping what tests what

  0 0 * * 6 [ `date '+\%d'` -le 7 ] && CMD

also equivelent to

  0 0 * * 6 [ `date '+\%e'` -le 7 ] && CMD

NOTE solaris cron needs the single quotes for it to work properly

This does NOT work....

  0 0 1-7  6  CMD

will acutally run the command on days 1 to 7 AND Saturdays!!!
It is not just the first saturday of the month.

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Run on the LAST saturday of the month
A 'decent into madness'
  http://arstechnica.com/civis/viewtopic.php?f=16&t=34360

First attempts all fail...

 # july would run twice in 2010
 1 0 24-31 * 6 /some/script.sh

 # april would not run at all in 2010
 1 0 25-31 * 6 /some/script.sh

Note that...
  echo `cal | awk 'NR>2{print $7}'`
gets the days of saturdays for this month, all onto one line

So getting the day of the last saturday for this month is...
Each line compacting the previous line
  cal | awk '{print $7}' | grep -E '[0-9]' | tail -n 1
  cal | awk '$7~/[0-9]/{print $7}' | tail -n 1
  cal | awk '$7{l=$7} END{print l}'

The recipe for a last saturday test...
  [ `date +%d` -eq `cal|awk '$7{l=$7}END{print l}'` ] && CMD

Alternative using GNU date (depends on version)

To see if the day (saturday) plus 7 days is at the start of a month
 0 0 * * 6 [ `date +%d -d '7 days'` -le '7' ] && CMD

Or see if next Saturday is in a different month...
 1 0 * * 6 [ `date +$m` -eq `date +%m -d "next Saturday"` ] && CMD

WARNING: some versions of "cal" starts on mondays, rather than sundays
use "cal -s" on those systems.

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These special time specification "nicknames" which replace the 5 initial time
and date fields, and are prefixed with the '@' character, are supported,
according to rhel5 manpage.

  @reboot    :    Run once after reboot.
  @yearly    :    Run once a year, ie.  "0 0 1 1 *".
  @annually  :    Run once a year, ie.  "0 0 1 1 *".
  @monthly   :    Run once a month, ie. "0 0 1 * *".
  @weekly    :    Run once a week, ie.  "0 0 * * 0".
  @daily     :    Run once a day, ie.   "0 0 * * *".
  @hourly    :    Run once an hour, ie. "0 * * * *"

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