How do I find the path of a template file using ERB?

Question

I am using embedded ruby (ERB) to generating text files. I need to know the directory of the template file in order to locate another file relative to the template file path. Is there a simple method from within ERB that will give me the file name and directory of the current template file?

I'm looking for something similar to __FILE__, but giving the template file instead of (erb).

Answer 1

When you use the ERB api from Ruby, you provide a string to ERB.new, so there isn’t really any way for ERB to know where that file came from. You can however tell the object which file it came from using the filename attribute:

t = ERB.new(File.read('my_template.erb')
t.filename = 'my_template.erb'

Now you can use __FILE__ in my_template.erb and it will refer to the name of the file. (This is what the erb executable does, which is why __FILE__ works in ERB files that you run from the command line).

To make this a bit a bit more useful, you could monkey patch ERB with a new method to read from a file and set the filename:

require 'erb'

class ERB
  # these args are the args for ERB.new, which we pass through
  # after reading the file into a string
  def self.from_file(file, safe_level=nil, trim_mode=nil, eoutvar='_erbout')
    t = new(File.read(file), safe_level, trim_mode, eoutvar)
    t.filename = file
    t
  end
end

You can now use this method to read ERB files, and __FILE__ should work in them, and refer to the actual file and not just (erb):

t = ERB.from_file 'my_template.erb'
puts t.result