Ruby on Rails Diff Text to HTML <ins> and <del>

November 04, 2008 – tagged as del, diff, html, ins, ruby

This code is perfect if you have 2 text objects in your Rails application and you want to compare their differences in one of your HTML views. It’s 99% pure Ruby too, so if you alter the first line, you can use it for other purposes.

Only one thing to note: you must have diff installed. I’m using: diff (GNU diffutils) 2.8.1.

#set up some variables to reference later
temporary_directory = File.join(Rails.root, "tmp")
max_lines = 9999999 #needs to be larger than the most lines you'll consider
diff_header_length = 3

# text_old and text_new should be the values of the string objects to compare
# these are just example strings to show it works
text_old      = "line1\\ndeleted line2\\nline3\\n\\nline4\\nline5"
text_new      = "line1\\ninserted line2\\nline3\\n\\nline4\\nline5"

# since we're using diff on the file system, we'll save the text we want to compare
# and then run diff against the two files
file_old_name = File.join(temporary_directory,"file_old"+rand(1000000).to_s)
file_new_name = File.join(temporary_directory,"file_new"+rand(1000000).to_s)
file_old      =, "w+")
file_new      =, "w+")

# diff will give provide a string showing insertions and deletions.  We will
# split this string out by newlines if there are difference, and mark it up
# accordingly with html
lines = %x(diff -­-­­­­­­unified=#{max_lines} #{file_old_name} #{file_new_name})
if lines.empty?
  lines = text_new.split(/\\n/)
  lines = lines.split(/\\n/)[diff_header_length..max_lines].
  collect do |i|
    if i.empty?
      case i[0,1]
      when "+"; then "<ins>"+i[1..i.length-1]+"</ins>"
      when "-"; then "<del>"+i[1..i.length-1]+"</del>"
      else; i[1..i.length-1]

#clean up the temporary diff files we created

#return marked up text

If you fire up RAILS_ROOT/script/console and paste that code in, it will return a nicely marked up string like this:

<del>deleted line2</del>
<ins>inserted line2</ins>


Use CSS to make your ins and del tags render however you like.


This article is 8 years old and closed for new comments.

Melvin R. • November 26, 2008

Looks pretty cool. If it works as advertised, this is will fantastic. In combination with the plugin that lets you save multiple variations, it will be very useful. Thank you. I’ll turn it into a plugin when I get to using it in my plugin, unless you’ve already done that… have you?

markmcb • November 26, 2008

No, I’ve not made it a plugin. Please drop a link to yours though if you make it publicly available. Glad this helped!

tkramar • February 19, 2009

Good idea. It might be better to append hash of the file name + current date time than to rely on randomly generated file names. Plus, on my system, a unified diff requires two hyphens

diff –unified instead of diff -unified

markmcb • February 20, 2009

@tkramar, thanks. It is two hyphens … that’s just WordPress thinking for me and deciding that when I type two hyphens that I really want a really long hyphen. I corrected it.

And yeah, the file name could be anything. I’m just a random sort of guy. And I generally avoid time-stamping files that don’t need to be moved around, archived, etc., as the OS does that for me, i.e., it’s redundant info.

Fabrice • March 16, 2009

Hi Mark,

Just a message to say thanks! Works like a charm :-)


MySchizoBuddy • May 31, 2009

can it be modified to show visual diff like this image

Anonymous • June 1, 2009

hi, how do this differ from the differ gem on github?

B Candler • June 1, 2009

Simpler than i[1..i.length-1] is i[1..-1]

Also: it is safer to use the block form of File so that in the case of exceptions, the file is closed. Even better would be to use Tempfile, and/or to put File.delete in an ‘ensure’ block. Tempfile would also reduce the possibility of the same name being used twice concurrently.