dont know if is so cool.. anyway I was playing with cron entries while I discovered Algorithm::Cron and it's cool method next_time that suddenly provoked a oneliner moment: perl -MAlgorithm::Cron -E "say scalar localtime (Algorithm::Cron->new(base => 'local',crontab => $ARGV[0])->next_time(time)) " "0 9 12 4 *" which prints Fri Apr 12 09:00:00 2019

But this was not cool enough. The following program by other hand parse a cron file or the output of cron -l or some input pasted and shows an ordered list of next commands cron will run.

It accepts input from a pipe, from a file with -f filename and if nothing was given it expects some input to be pasted followed by CTRL-Z or CTRL-D (windows or linux).

Instead of the above input you can use -c "crontab-entry" to parse a solitary crontab entry.

And dulcis in fundo, with the -n N parameter the program will show next N occurrences of the scheduled programs

use strict;
use warnings;
use Getopt::Long;
use Algorithm::Cron;

my $file;
my $howmany;
my $crontab;
my $help;
my @lines;

my $helptext = "USAGE:\n $0 [ [-f filename | -c STRING] -n N]\n\n".
            " $0 -f filename\n".
            " $0 -f filename -n 2\n".
            " crontab -l | $0 \n".
            " crontab -l | $0 -n 2\n".
            " cat filename | $0\n".
            " cat filename | $0 -n 3\n".
            " $0 -c 'crontab entry'\n".
            " $0 -c 'crontab entry' -n 5\n".
            " $0  (paste some content followed by CTRL-D or CTRL-Z on 
+a newline)\n".
            " $0  -n 4 (paste some content followed by CTRL-D or CTRL-
+Z on a newline)\n".
            " $0 -h (print this help)\n";
            
GetOptions ("f|file=s" => \$file,
            "n=i"   => \$howmany,
            "c|crontab=s"    => \$crontab,
            "h|help"  => \$help)  
    or die( $helptext );

print $helptext and exit if $help;    

if ( $crontab )    { @lines = $crontab }
elsif ( $file and -e -f -r $file){
    open my $fh, '<', $file or die;
    @lines = <$fh>;
}
else{ @lines = <> }

foreach my $line ( 
            sort { 
                Algorithm::Cron->new( base => 'local',
                crontab => join' ',(split /\s+|\t/,$a)[0..4])->next_ti
+me(time)
                <=>
                Algorithm::Cron->new( base => 'local',
                crontab => join' ',(split /\s+|\t/,$b)[0..4])->next_ti
+me(time)
            } 
            grep { /^(\d|\*)/ } @lines 
){
    my @parts = split /\s+|\t/,$line;
    my $now = time;
    my $repeat = $howmany;
    print scalar localtime ( Algorithm::Cron->new( base => 'local',
                            crontab => join' ', @parts[0..4])->next_ti
+me($now)
    );
    print " => ( @parts[0..4] )",($crontab ? "" : " => @parts[5..$#par
+ts]"),"\n";
    if ( --$repeat ){
        while( $repeat > 0){
            $now = Algorithm::Cron->new( base => 'local',
                        crontab => join' ', @parts[0..4])->next_time( 
+$now );
            print scalar localtime ( Algorithm::Cron->new( base => 'lo
+cal',
                            crontab => join' ', @parts[0..4])->next_ti
+me($now)
            );
            print "\n";
            $repeat--;        
        }
    }
}
[download]

given a sample file the following example show the usage:

cat crontab.txt

# Crontab Environmental settings
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
00 3 * 7 0 /path/to/command
#15 20 * 1-7 * /path/to/command2
*/30 7,21 1-15 1 * /path/to/another/command
# m h dom mon dow user  command
21 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
0,30 6    * * *   root    test -x /usr/sbin/blah
#47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-pa
+rts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/blahblah || ( cd / && run-pa
+rts )


cat crontab.txt | perl cron-explain.pl

Tue Nov 27 14:21:00 2018 => ( 21 * * * * ) => root cd / && run-parts -
+-report /etc/cron.hourly
Wed Nov 28 06:00:00 2018 => ( 0,30 6 * * * ) => root test -x /usr/sbin
+/blah
Sat Dec  1 06:52:00 2018 => ( 52 6 1 * * ) => root test -x /usr/sbin/b
+lahblah || ( cd / && run-parts )
Tue Jan  1 07:00:00 2019 => ( */30 7,21 1-15 1 * ) => /path/to/another
+/command
Sun Jul  7 03:00:00 2019 => ( 00 3 * 7 0 ) => /path/to/command


cat crontab.txt | perl cron-explain.pl -n 3

Tue Nov 27 14:21:00 2018 => ( 21 * * * * ) => root cd / && run-parts -
+-report /etc/cron.hourly
Tue Nov 27 15:21:00 2018
Tue Nov 27 16:21:00 2018
Wed Nov 28 06:00:00 2018 => ( 0,30 6 * * * ) => root test -x /usr/sbin
+/blah
Wed Nov 28 06:30:00 2018
Thu Nov 29 06:00:00 2018
Sat Dec  1 06:52:00 2018 => ( 52 6 1 * * ) => root test -x /usr/sbin/b
+lahblah || ( cd / && run-parts )
Tue Jan  1 06:52:00 2019
Fri Feb  1 06:52:00 2019
Tue Jan  1 07:00:00 2019 => ( */30 7,21 1-15 1 * ) => /path/to/another
+/command
Tue Jan  1 07:30:00 2019
Tue Jan  1 21:00:00 2019
Sun Jul  7 03:00:00 2019 => ( 00 3 * 7 0 ) => /path/to/command
Sun Jul 14 03:00:00 2019
Sun Jul 21 03:00:00 2019


perl cron-explain.pl -c "2-5 9 12 4 *" -n 4

Fri Apr 12 09:02:00 2019 => ( 2-5 9 12 4 * )
Fri Apr 12 09:03:00 2019
Fri Apr 12 09:04:00 2019
Fri Apr 12 09:05:00 2019
[download]

have fun!

L*