How to find max value from specific XML element using a Perl one liner?

Question

I have an XML file with the following syntax

<element>
  <id>0</id>
  <tag1>something</tag1>
  ...
  <tagn>something</tagn>
</element>
<element>
  <id>1</id>
  <tag1>something</tag1>
  ...
  <tagn>something</tagn>
</element>

What Perl one-liner would find the largest value from the 'id' elements?

I am a Perl newbie, yet, I know I can get the values of the id elements by doing:

perl -wne 'print $1 if /<id>(\d+)<\/id>/'

which produces "0123456789" (a string with just the numbers?).

Based on this answer https://unix.stackexchange.com/a/130903/374251 I could try to store the matches in a list and print the max value from the list. However, I don't know how to do so in a one-liner.

Answer 1

Standalone perl script:

#!/usr/bin/perl

use strict;
use XML::LibXML;
use List::Util qw(max);

my $filename = './input.xml';
my $dom = XML::LibXML->load_xml(location => $filename);

my @ids = map { $_->to_literal() } $dom->findnodes('/data/element/id');

print max(@ids), "\n";

Uglier, harder-to-understand and harder-to-edit one-liner version:

perl -MXML::LibXML -MList::Util=max -e '
  $dom = XML::LibXML->load_xml(location => shift);
  @ids = map { $_->to_literal() } $dom->findnodes("/data/element/id");
  print max(@ids), "\n";' input.xml

Note: both of the above assume that the <element>s are wrapped inside a <data> path. If not, adjust the xpath in the findnodes() function call to suit your actual data.

I ran them both with the following input.xml file:

<?xml version='1.0' encoding='UTF-8'?>
<data>
<element>
  <id>0</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
<element>
  <id>1</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
<element>
  <id>2</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
<element>
  <id>3</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
<element>
  <id>4</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
<element>
  <id>5</id>
  <tag1>something</tag1>
  <tagn>something</tagn>
</element>
</data>

and both got the correct result of 5.

---

BTW, either version can be made to read STDIN by changing the lines with location => $filename or location => shift to:

my $dom = XML::LibXML->load_xml(IO => *STDIN);

The my is optional in the one-liner version without use strict, but is required in the stand-alone version.

---

BTW, it's also easy to modify either script so that both the input filename and the xpath can be specified on the command line. so that you have a generic tool for getting the max() value of any xpath element. e.g.

#!/usr/bin/perl

use strict;
use XML::LibXML;
use List::Util qw(max);

my $dom = XML::LibXML->load_xml(location => shift);
my @ids = map { $_->to_literal() } $dom->findnodes(shift);
print max(@ids), "\n";

Run as, e.g.,

$ xml-max.pl input.xml /data/element/id
5