Backticks and tests in Perl 6

Perl was created for systems administration, and Perl 6 has all the chops you've come to expect from the brand. Here I needed to use MD5 checksums from my collaborator to verify that I downloaded all their data without errors. Each data "$file" has an accompanying "$file.md5" that looks like this:

$ cat HOT232_1_0770m/prodigal.gff.md5
a36e4adfaa62cc4adb8cea44c4f7825f  HOT232_1_0770m/prodigal.gff

So I need to read the contents of this file, get just the first field, then execute my local "md5" (or "md5sum") program on the file without the ".md5" extension and determine if they are the same. All standard stuff, and I think Perl 6 gives us elegant ways to accomplish all of these, including a dead-simple testing framework. Here's my solution:

#!/usr/bin/env perl6

use File::Find;
use Test;

sub MAIN (Str :$dir=~$*CWD, Str :$md5="md5sum", Str :$ext='.md5') {
    my $rx = rx/$ext $/;
    for find(:dir($dir), :name($rx)) -> $md5-file {
        my $basename    = $md5-file.basename;
        my ($remote, $) = $md5-file.slurp.split(' ');
        my $local-file  = $basename.subst($rx, '');
        my $path        = $*SPEC.catfile(
                          $md5-file.dirname, $local-file);
        my ($local, $)  = run($md5, $path, :out)
                          .out.slurp-rest.split(' ');
        is $local, $remote, $md5-file;


I just love the new MAIN entry point where I can easily establish positional or named (as in this case) arguments with defaults and type checking and USAGE statements for free!

$ ./check-md5.pl6 -h
  check-md5.pl6 [--dir=] [--md5=] [--ext=]

Here I'll default to looking in the current directory which is accessible as the global variable $*CWD. This new variable uses a $* "twigle" (two sigils) to denote a global variable which in this case is actually an IO object. I need to stringify it either by putting it in double-quotes or coercing it with the ~ operator. The "md5" argument is the name of my local "md5sum" binary which is often called just "md5." Lastly, I assume the file extension of the remote checksums is ".md5."

Users of Perl 5 will be pleased to note that "File::Find" is available. I was always more of a fan of "File::Find::Rule," I think the interface for the Perl 6 module is quite nice. The output is a list of IO::Path objects.

As a side note, I first wrote it like this:

for (find(:dir($dir), :name(rx/$ext $/))).kv -> $i, $md5-file {
    printf "%3d: %s\n", $i + 1, $md5-file;

Because I like to know as each file is being processed and how many have gone by. Take any list in Perl 6 and call .kv (key-value) on it to get the index (position) and the value:

> my @list = "foo", "bar", "baz"
[foo bar baz]
> @list.kv
(0 foo 1 bar 2 baz)
> for @list.kv -> $key, $val { put "$key = $val" }
0 = foo
1 = bar
2 = baz

Reading an entire file is done by calling slurp on it. In my case, there's only one line, and I want to immediately split it on spaces:

my ($remote, $) = $md5-file.slurp.split(' ');

Since I'm only interested in the first field (the second one, remember, is the name of the file, which I already know), I can ignore it by assigning the position to an anonymous scalar $. (This is similar to Haskell's _ which itself is sometimes like Perl 6's * "whatever" variable.)

To remove the extension, I use the Str.subst (substitute) method with the regular expression I put into the $rx variable. (The first argument to "subst" can also be a string.) So, if the $md5-file is "/work/03137/kyclark/ohana/HOT/HOT237_2_1000m/readpool.fastq.md5", then the basename is "readpool.fastq.md5" and the result of the "subst" is "readpool.fastq":

my $local-file  = $basename.subst($rx, '');

To piece together the local file that needs to be tested, I can use the global $*SPEC object to get access to OS-dependent methods like "catfile" that will use the proper directory separators to construct the path. Again, the $* is a twigle to set the global variable/object apart from everything else. The local file should be in the same directory as the MD5 checksum file.

my $path = $*SPEC.catfile($md5-file.dirname, $local-file);

To get the local checksum, I need to run the correct "md5" binary and then read STDOUT from that program. (To capture STDERR, I could put ":err" in the call.) I put "backticks" in the title to highlight that this is now the way to call external programs and capture the output. Like the MD5 file, I'm only interested in the first field of output and can throw away the rest. I can accomplish it all in one line:

my ($local, $)  = run($md5, $path, :out).out.slurp-rest.split(' ');

Finally, I brought in the "Test" module so I can ask:

is $local, $remote, $md5-file;

If they are equal, I get output like this:

$ ./check-md5.pl6
ok 1 - /work/03137/kyclark/ohana/HOT/HOT224_1_0025m/prodigal.gff.md5
ok 2 - /work/03137/kyclark/ohana/HOT/HOT224_1_0025m/readpool.fastq.md5
ok 3 - /work/03137/kyclark/ohana/HOT/HOT224_1_0025m/ribosomal_rRNA.gff.md5

When things don't go OK, I see this:

not ok 193 - /work/03137/kyclark/ohana/HOT/HOT224_1_0045m/readpool.fastq.md5

# Failed test '/work/03137/kyclark/ohana/HOT/HOT224_1_0045m/readpool.fastq.md5'
# at ./check-md5.pl6 line 13
# expected: '4b49aaa2b0c60342e90cf37e30c30886'
#      got: '4cdd81767c2ffed712c0b8ffb6de8b38'

And so I know to get that file again!

It's not necessary to call "done-testing()", but I like to as the Test library will nicely summarize the results.

So, there's a tidy little program packed with loads of Perl 6 goodness. I hope it makes you excited to jump into the language if you haven't already. I'm busy evangelizing to all my labmates and students why they'll be happier, more productive coders in Perl 6. Also, better, more moral and upright citizens.


Nice article, thanks for the Perl 6!

One note I had, the secondary variable marker is called a twigil - the first character is still the sigil (same as Perl 5) - the second character is the twigil. The sigil still indicates the kind of value the variable is holding, and the twigil indicates the scope: * indicates the Dynamic scope.


You can (and should) remove .slurp and .slurp-rest

The IO object has a .split method that doesn't have to read the entire contents of the file before splitting the file up.

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About Ken Youens-Clark

user-pic I work for Dr. Bonnie Hurwitz at the University of Arizona where I use Perl quite a bit in bioinformatics and metagenomics. I am also trying to write a book at Comments welcome.