Reads and expands a crontab into a 24hour timeline

crontab-timeline.sh

#!/bin/sh
#
# Reads and expands a crontab into a 24hour timeline:
#
# For each scheduled cron event...
# ... outputs the event's time (hh:mm) followed by the event's crontab entry
#
# This output is sorted into a cron event timeline (however it is still
# up to the reader to decode/determine which events run on which days).
#
# Idea is based on http://staff.washington.edu/corey/tools/crontab-timeline
# by Sherwin Daganato, 20201002

perl -alne '
BEGIN {

    # shamelessly stolen from Set::Crontab
    sub expand
    {
        my ($spec, $range) = @_;
        my @list;

        # 1,2-4,*/3
        foreach (split /,/, $spec) {
            my @pick;
            my $step = $1 if s#/(\d+)$##;
 
            # 0+"01" == 1
            if    (/^(\d+)$/)       { push @pick, 0+$1;          }
            elsif (/^\*$/)          { push @pick, @$range;       }
            elsif (/^(\d+)-(\d+)$/) { push @pick, 0+$1..0+$2;    } 
 
            if ($step) {
                my $i;
                @pick = grep { defined $_ if $i++ % $step == 0 } @pick;
            }
 
            push @list, @pick;
        }

        return \@list;
    }
}

next if /^ *(#|$)/;

my $minutes = expand($F[0], [0 .. 59]);
my $hours = expand($F[1], [0 .. 23]);

foreach my $h (@{ $hours }) {
    foreach my $m (@{ $minutes }) {
        printf("%02d:%02d\t%s\n", $h, $m, $_)
    }
}
' $* | sort