RE: Createing a Crontab for the last day of the month

[(Ted Harding) <Ted.Harding@nessie.mcc.ac.uk>]

On 25-Jun-01 Miller, Jim wrote:
> Hello guys,
>     Is there an easy way to run a script the last day of each month
> using cron.  IE it would handle the 30,31,29/28 issue?

You can't use cron syntax to catch the last day of the month.
What you can do is to run, every day, a script which checks
whether it's the last day today and then runs what you want.

So your crontab entry could be like

  0 0 * * * last_dom

and now the fun starts.

First, the command

  date '+%m %Y %d'

generates the current date as numerical month, year, day.
Just now, it gives

  06 2001 25

Now take the first two of these, and do "cal" with these arguments:

  cal 06 2001

getting

        June 2001
  Su Mo Tu We Th Fr Sa 
                  1  2
   3  4  5  6  7  8  9
  10 11 12 13 14 15 16
  17 18 19 20 21 22 23
  24 25 26 27 28 29 30

The last number of the last non-blank line is the number of the last day
of the current month (allowing for 28, 29, 30 and 31). If this matches the
third element in the output of "date '+%m %Y%d' then today is the last day
of the month. Otherwise it isn't. If it is, the run "whatever".

You can conveniently use awk to do these little jobs ...

You could also write the script so that "whatever" was an argument to
"last_dom", so that you could have crontab entries like

  0 0 * * * last_dom check_bank
  0 0 * * * last_dom pay_electricity
  0 0 * * * last_dom run_payroll
  ...........

which should be intuitive enough for anyone.

A script "last_dom" which somewhat crudely does the above is

  #! /bin/bash
  echo -n "if [ " > lastday.tmp

  echo -n "`cal \`date '+%m %Y'\` | \
  awk '{if(NF) last=$(NF)}END{print last " -eq "}'`" >> lastday.tmp

  echo -n `date '+%d'` >> lastday.tmp

  echo " ] ; then $1 ; fi" >> lastday.tmp

  chmod 755 lastday.tmp
  lastday.tmp

which, when run today as "last_dom ls", generates (and executes)
the file "lastday.tmp" which contains

  if [ 30 -eq 25 ] ; then ls ; fi

(and, by changing "-eq" to "-ne", you can get it to actually do
something today).

Ted.

--------------------------------------------------------------------
E-Mail: (Ted Harding) <Ted.Harding@nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 167 1972
Date: 25-Jun-01                                       Time: 23:10:12
------------------------------ XFMail ------------------------------