Some time ago I wrote the code below to sort my crontab file by time (see sub compare_crons for the sorting criteria). It's very crude, but it could work as a starting point for reading a crontab file in memory. Once you read it in, you can make any changes, write it to a new file, an run the crontab command on it.

This program leaves in each element of @db an array reference containing the 5 time fields, and a text element containing the current line, together with any comments that came before it. Of course, for making any modifications, this second element would have to be split further.

eval 'exec perl -x $0 ${1+"$@"}' # -*-perl-*-
  if 0;
#!perl -w
# Sort a crontab by time
# Each entry (possibly commented) is moved as a block with all the
# comment lines before it.
# Diego Zamboni, March 21, 2001. Happy Spring!

use strict;
use vars qw($curblock $tf @fields @db);

# Regex for a time field in crontab, which can be a number, sequence o
+f numbers
# or an asterisk
$tf='(?:[*]|(?:\d+(?:-\d+)?)(?:,\d+(?:-\d+)?)*)';

while (<>) {
  $curblock.=$_;
  if (@fields=(/^\#?($tf)\s+($tf)\s+($tf)\s+($tf)\s+($tf)\s+(.*)/)) {
    push @db, [[@fields], $curblock];
    $curblock="";
  }
}

foreach (sort compare_crons @db) {
  print "$_->[1]";
  warn "-----------\n" . join(" | ", @{$_->[0]})."\n";
}

# compare by month, day, hour and minute. Ignore weekday
sub compare_crons {
  my @fa=@{$a->[0]};
  my @fb=@{$b->[0]};

  return compare_tf($fa[3],$fb[3]) ||
         compare_tf($fa[2],$fb[2]) ||
         compare_tf($fa[1],$fb[1]) ||
         compare_tf($fa[0],$fb[0]);
}

# Compare two cron fields. Asterisk considered as zero, otherwise
# compare by the first number
sub compare_tf {
  my ($na,$nb);
  $na=($_[0]=~/^(\d*)/)[0]||0;
  $nb=($_[1]=~/^(\d*)/)[0]||0;
  return ($na <=> $nb);
}
[download]

--ZZamboni

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