Made it work with the jekyll.environment

[iaminamcom]

Often we need to hide/show on the bases of jekyll built-in environment variable jekyll.environment like:

{% if jekyll.environment == 'production' %}
  <!-- Google Tag Manager -->
  <script>
    // .....
  </script>
{% endif %}

So this little adjustment worked really good for me and I thought better if you add it by default as some plugins also need jekyll environment.

Thanks!

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