ruby -- string interpolation from file\pattern -

i using ruby 1.9.3 , trying achieve string interpolation follows:

• declare , assign variable string value.
• read file has string interpolation "pattern".
• do string interpolation.

for instance:

params.txt

"#{str}" 

test.rb

str = "my string" content = file.open("params.txt", "r").read() puts("#{content}") #result in "#{str}" , not "my string", returns content of file 

how can achieve string interpolation?

update example of tyring achieve:

str = "my string" pat = '#{str}' puts(pat)      # returns nil puts("#{pat}") # returns nil eval(pat)      # returns nil eval("#{pat}") # returns nil 

this easier if use erb instead.

params.txt:

<%= str %> 

test.rb should like:

str = "my string" erb.new("params.txt").result(binding) 

erb docs here.