How to check if a certain path/URL exists in the generated output by jekyll?

1

Hi,

Is it possible to check whether or not a certain path or URL will be generated by jekyll?

The reason I need this is for multilingual sites: here I have a language changer in the navigation area of the site. Some pages are not available in all languages. So if a certain page does NOT exist in the alternative language, I do not want to show the language buttons.
The default language pages are at, say, /foo/ or /bar/, while the alternative language pages are at /da/foo/ or /da/bar/.

So on /foo/, I do not want to show the language buttons if /da/foo/ is not to be generated.

Is something like this possible?

Here is the relevant code:

Language buttons:

        <button id="language-btn" class="btn btn-light text-muted" type="button" >
          {% if page.lang == "en" %}
          <svg width="22" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 37 28"><path fill="#c60c30" d="M0,0H37V28H0Z"/><path fill="#fff" d="M0,12H12V0H16V12H37V16H16V28H12V16H0Z"/></svg>
          <!--
          Dansk
          -->
          {% else %}
          <svg width="24" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 60 30" >
          <clipPath id="s">
                  <path d="M0,0 v30 h60 v-30 z"/>
          </clipPath>
          <clipPath id="t">
                  <path d="M30,15 h30 v15 z v15 h-30 z h-30 v-15 z v-15 h30 z"/>
          </clipPath>
          <g clip-path="url(#s)">
                  <path d="M0,0 v30 h60 v-30 z" fill="#012169"/>
                  <path d="M0,0 L60,30 M60,0 L0,30" stroke="#fff" stroke-width="6"/>
                  <path d="M0,0 L60,30 M60,0 L0,30" clip-path="url(#t)" stroke="#C8102E" stroke-width="4"/>
                  <path d="M30,0 v30 M0,15 h60" stroke="#fff" stroke-width="10"/>
                  <path d="M30,0 v30 M0,15 h60" stroke="#C8102E" stroke-width="6"/>
          </g>
          </svg>
          <!--
          English
          -->
          {% endif %}
        </button>

And the javascript to make the buttons work:

// Locales
const path = window.location.pathname,
    langUrl = path.indexOf('/da/') !== -1
// Language switch
document.getElementById('language-btn').onclick = () => {
    if ( path == "/da/") {

        // Redirect to /index.html instead of '/' in order not to trigger language detection on the server
        window.location = "/index.html"
    } else {
        window.location = langUrl ? path.replace('/da', '') : '/da'.concat(path);
    }
    document.getElementById('language-img').src = '';
}
2

Given that you know the name of the expected file, you can check if the file in another language exists during build. I created a plugin to check if a file exists in my template site.

module Jekyll
  class FileExistsTag < Liquid::Tag

    def initialize(tag_name, path, tokens)
      super
      @path = path
    end

    def render(context)
      # Pipe parameter through Liquid to make additional replacements possible
      url = Liquid::Template.parse(@path).render context

      # Adds the site source, so that it also works with a custom one
      site_source = context.registers[:site].config['source']
      file_path = site_source + '/' + url

      # Check if file exists (returns true or false)
      "#{File.exist?(file_path.strip!)}"
    end
  end
end

Liquid::Template.register_tag('file_exists', Jekyll::FileExistsTag)

In my case I use it to check if a jupyter notebook exists, and display a message if it doesn’t.

1 Like
3

Not sure I fully understand this code. But am I right that this plugin introduces a new Liquid tag called file_exists ?

And that it works like this {% if file_exists foo.md %}?

If this is the case, how do I load this, and how do I do something like {% if file_exists foo.md %} where foo.md is instead current page’s filename prefixed by /da? That is, if current page is /foo/bar/page/, then it should check if /da/foo/bar/page/ exists.

Also, is there a way to do this without plugins? I currently use no plugins and would really like to keep it simple like this if possible.

4

can you use an extra front matter variable to say which files have other languages and then check that?

5

That might be possible, although this approach seems error prone, because I would have to manually add all those front matter vars and be sure not to have a typo in them. And then going forward, I will have to remember to add this to all new pages without making errors. And if I move/rename a page, I will have to update all references to it… Seems like a lot of manual work that can easily be forgotten or mistakes made here and there… So I would really like an automated way if possible.

Is the information needed to do this automatically simply not available at runtime in jekyll - which is why it needs a plugin like @george-gca suggests? I mean, do I need to escape out of jekyll to do some Ruby code to be able to access the info needed for this?

6

have you tried playing with site.documents or site.pages?

then maybe you loop over that to see if your page exists?

1 Like
7

If @rdyar suggestion doesn’t work, by looping through site.pages for example, the only solution available I believe is doing the plugin.

Yes, it creates a new liquid tag file_exists, and its usage is:
1 - create the file _plugins/file-exists.rb with this content. This directory is the default plugins directory.
2 - I used like this in my code:

{% capture notebook_exists %}{% file_exists assets/jupyter/blog.ipynb %}{% endcapture %}
{% if notebook_exists == 'true' %}

This way I can check at build time if a file exists.

8

Thanks! I tried just this, and I believe this is indeed the right way to go about it.
However, SOMETHING is not working as I expect it.
This is my code:

    {% assign da_page_exists = false %}                                      
    {% for page2 in site.pages %}                                            
      '{{   page2.url }}' == '{{ page.url | prepend: "/da" }}':              
      {% if page2.url     ==     page.url | prepend: "/da" %}                
        {% assign da_page_exists = true %}                                   
        Found corresponding Danish page for '{{ page.url }}' on '{{ "/da" | append: page.url }}'
        {{ page2.url }}                                                      
        {% break %}                                                          
      {% else %}                                                             
        '{{ page2.url }}' does not match '{{ "/da" | append: page.url }}'    
      {% endif %}                                                            
    {% endfor %}                                                             
                                                                             
    {% if da_page_exists %}                                                  
      {{ page.url }}:                                                        
      DANISH PAGE EXISTS ON.                                                 
      {{ "/da" | append: page.url }}                                         
    {% else %}                                                               
        '{{ page2.url }}':                                                   
        '{{ page.url }}':                                                    
      DANISH PAGE DOES NOT EXIST ON.                                         
      {{ "/da" | append: page.url }}                                         
    {% endif %}

It now finds matches on all pages. Even those that do NOT have a Danish page.

This is where it goes wrong:

  {% if page2.url     ==     page.url | prepend: "/da" %}

The issue seems to be that when it encounters e.g., ‘/contact/’, it compares page2 to page. It does not seem to take in the prepend operation before doing the comparison.

How do I fix this?

I have tried using parenthesis like this:

 {% if page2.url     ==     ( page.url | prepend: "/da")  %}

But this does not work as it fails to detect when danish pages actually exist.
It results in output like this:

'/da/contact/' == '/da/contact/':
        '/da/contact/' does not match '/da/contact/'


        '':
        '/contact/':
      DANISH PAGE DOES NOT EXIST ON 
      /da/contact/
9

I think I fixed it using capture:

    {% assign da_page_exists = false %}                                      
    {%- capture da_url -%}                                                   
      {{ page.url |prepend: "/da" }}                                         
    {%-  endcapture -%}                                                      
    {% for page2 in site.pages %}                                            
      {% if page2.url == da_url  %}                                          
        {% assign da_page_exists = true %}                                   
        Found corresponding Danish page for '{{ page.url }}' on '{{ "/da" | append: page.url }}'
        {{ page2.url }}                                                      
        {% break %}                                                          
      {% endif %}                                                            
    {% endfor %}                                                             
                                                                             
    {% if da_page_exists %}                                                  
      {{ page.url }}:                                                        
      DANISH PAGE EXISTS ON.                                                 
      {{ "/da" | append: page.url }}                                         
    {% else %}                                                               
        '{{ page2.url }}':                                                   
        '{{ page.url }}':                                                    
      DANISH PAGE DOES NOT EXIST ON.                                         
      {{ "/da" | append: page.url }}                                         
    {% endif %}

Not sure if this is the best way to do it…

10

I’ve not done something like that before but I guess it looks good.

The main issue is going to be performance - for loops can be slow if you have a lot of pages (100s?) and then call that loop on a lot of pages. If it is just in one place then it probably isn’t too big a deal.

11

If you are creating a multilingual site I would recommend using polyglot to help you handle different languages from your site. Its default solution for this problem is to fallback to your default language, which I think is a nice solution.