Adding Fallback to convert

ubuntu uses IM6 by default which doesn't support magick

Author: MUmarShahbaz

lib/jekyll_picture_tag/parsers/image_backend.rb

@@ -34,6 +34,12 @@ def magick_formats
                               .first
                               .delete_prefix('Delegates (built-in):')
                               .split
+        elsif command?('convert')
+          @magick_formats ||= `convert -version`
+                              .scan(/Delegates.*/)
+                              .first
+                              .delete_prefix('Delegates (built-in):')
+                              .split
         else
           @magick_formats = []
         end
@@ -56,7 +62,7 @@ def error_string(format)
                else
                  'Libvips is not installed.'
                end
-        str << if command?('magick')
+        str << if command?('magick') || command?('convert')
                  "Imagemagick (installed) supports: \"#{magick_formats.join(', ')}\"."
                else
                  'Imagemagick is not installed.'

test/test_helper.rb

@@ -49,7 +49,8 @@ def temp_dir(*descendents)
   # them doesn't support some image format. This returns an array of image
   # formats which we care about, and which are locally supported.
   def supported_formats
-    output = `vips --list` + `magick --version`
+    magick_command = command?('magick') ? `magick` : `convert`
+    output = `vips --list` + `#{magick_command} --version`
 
     formats = %w[jpg png webp gif jp2 avif].select do |format|
       output.include? format
@@ -60,4 +61,13 @@ def supported_formats
 
     formats
   end
+
+  def command?(command)
+    is_windows = RbConfig::CONFIG['host_os'] =~ /mswin|mingw|cygwin/
+    if is_windows
+      system("where #{command} > NUL 2>&1")
+    else
+      system("which #{command} > /dev/null 2>&1")
+    end
+  end
 end