A super basic command-line string interpolator in Ruby

string-interp.rb

#!/usr/bin/env ruby
require 'optparse'

class StringInterpolator

  def initialize(inFile, vars)
    @in = File.open(inFile).read
    @vars = vars
  end

  def generate
    @in.each_line do |line|
      @vars.each do |key,val|
        line.gsub!("${#{key}}", val || "")
      end
      puts line
    end
  end
end

if __FILE__ == $0
  file = ''
  vars = {}
  OptionParser.new do |opts|
    opts.banner = "Usage: string-interp.rb [options]"
    opts.on('-fFILE', 'The template file to interpolate.') do |v|
      file = v
    end
    opts.on('-vPAIR', 'A variable substitution name=value pair.') do |v|
      parts = v.split('=')
      vars[parts[0]] = parts[1]
    end
  end.parse!

  StringInterpolator.new(file, vars).generate
end

Example usage:

# Input file: vars.yml

---
name: ${name}
cats_owned: ${cats}
crazy: ${crazy}

# Interpolate
$ string-interp.rb \
  -f vars.yml \
  -v name=Ted \
  -v cats=1024 \
  -v crazy=yes \
  > vars_out.yml

# Output file: vars_out.yml

---
name: Ted
cats_owned: 1024
crazy: yes