====== Unusual cron job settings ======

cron jobs can be set up to run at various times, such as:

  * X minutes past every hour
  * Y minutes past hour Z
  * X,Y and Z minutes past hours A,B and C
  * Every Monday at a particular time
  * The Nth day of each month at a particular time

Some cron systems can also be set to run jobs:

  * 3 times per hour (at 0, 20 and 40 minutes past)
  * 4 times per hour plus an offset (eg: at 3, 18, 33 and 48 minutes past)

However, there are some things that standard cron configurations can't cope with, such as:

  * the first Monday of each month
  * the last Friday of each month
  * the 14th of each month provided it's a Tuesday

**Note:** It may at first seem simple to set up a job to run on the 14th of the month when it's a Tuesday, but it turns out that if you do specify this, the job runs **both** every Tuesday and also on the 14th of the month.  It's the isolated instance of a logical OR between timings, when all others are a logical AND.

<code>34    12    14    *    Tue    command.sh</code>

Anyway, back to the topic...

===== How do you set up a cron job to run on the first Monday of each month? =====

There are two pretty much equally simple ways of going about this (and probably several others which are not so simple):

  - Set up a job to run every Monday, and check whether it's the first one in the month
  - Set up a job to run on the first seven days of each month, and check whether the day is a Monday

For example (assuming the job should run at 03:45 on the appropriate day) either of the following will work:

<code>45    3    *    *    Mon    test `date +%e` -lt 8 && command.sh
45    3    1-7  *    *      test `date +%u` -eq 1 && command.sh</code>

Note that I used "date +%u" to get the numerical day of the week (1=Monday, 7=Sunday) instead of "date +%a" (for Mon, Tue, Wed, etc) to avoid any internationalisation problems - you want the command to work on a machine whose system language is French or German just the same as it works in English.

For the next example...

===== How do you set up a cron job to run on the last Friday of each month? =====

This not so simple, because there's no easy way of knowing which dates in a month represent the final seven days.

In this case, we run a check every Friday to find out whether the current month is the same as the month in seven days' time:

<code>45    3    *    *    Fri    test `date +%m` -eq `date +%m -d +7days` || command.sh</code>

So, every Friday, if the month number equals the month number in 7 days' time, we do not run the command, otherwise we do.

===== How do you set up a cron job to run on every Tuesday when it's the 14th of the month? =====

By now this should be obvious - use either of:

<code>45    3    *    *    Tue    test `date +%e` -eq 14 && command.sh
45    3    14   *    *      test `date +%u` -eq 2 && command.sh</code>
===== Backticks =====

Some people might change **`...`** into **$(...)** - feel free to use this syntax if you prefer it.

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