Data URIs

I’m guessing that a lot of web developers don’t know about data URIs. Most probably won’t even have to know about them. However, I still want to go over them because I think they are clever and could prove useful. The main advantage I see is the possibility to avoid an HTTP Request, especially for a small image.

Wikipedia’s overview is quite thorough. The article also links to a number of places where you can quickly throw together some data URI’s for any mime type. The most well known is the data URI kitchen.

Quickly, the format of a data URI is as follows:


Note that almost every section is optional! The mime type option means you can do some very interesting things. The normal usage might be to to embed a small image, as a datauri, inside the html document. However, another usage might be to embed an entire HTML document inside a link!

I was looking at the code for some command line utilities that make datauri’s and I learned an easy way to find out the mime type of a file is to use the `file` command:

shell> file sym.png
sym.png: PNG image data, 76 x 78, 8-bit/color RGBA, non-interlaced

shell> file -i sym.png
sym.png: regular file

shell> file --mime sym.png
sym.png: image/png

So using `file –mime` and reading whats after the ‘: ‘ you can easily get a mime-type for any file. Then reading and base64 encoding the content you have a quick hack to quickly create datauris! Here goes:

#!/usr/bin/env ruby
# Quick & Dirty data URI for a file
require 'base64'
require 'uri'

# Filename and Contents
filename = ARGV[0]
contents =
base64 = Base64.encode64(contents).gsub("\n",'')

# Mime Type for the Filename
output = `file --mime #{filename}`
mime = output.match( /: (.*)$/ )[1].downcase.gsub(/\s/,'')

# Make Data URI
datauri = "data:#{mime};base64,#{base64}"

# Output
puts datauri

Some better scripts are available. Here is a Perl implementation and a pretty neat Ruby Implementation.

2 Responses


Vikram on December 15, 2010 at 5:09 am  #

Thanks a lot! It worked like a charm!


Cesar Figueroa on May 28, 2012 at 3:20 am  #

Doing `file –mime-type –brief` should remove the need for Regex hackery ;-)

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