Add a default value for existing Image alt field I have an image field in article content type and have created hundreds of nodes for article content type, but now I need to add a value for image alt fields for these existing nodes.
Even the default value field for image alt is not showing when I try to edit the field.

How can I add values for the alt attribute of the image field in existing nodes?
Add a node_preprocess function to your theme file. The logic would be to check if the current node has a value in the field for "image alt text", if not then set a default value, else return. 

These are not the exact field names but this should get you started on what to look for in your code.  

 function MYTHEME_preprocess_article(&amp;$variables) {
      if (!field_image_alt_text-&gt;getValue().strlen() = 0 || null ) { set value to something } else {return;}
};