How do I run a program via Ruby and access its standard input and output

Question

I have a completely separate Ruby file that reads from Standard Input and writes to Standard Output.

I have certain test cases that I want to try. How do I pass my inputs to Standard Input to the file, and then test the Standard Output against the expected results?

As an example, here's the stuff I've already figured out:

There's a file that reads a number from standard input, squares it, and writes it to standard input

square.rb:

#!/usr/local/bin/ruby -w

input = STDIN.read

# square it
puts input.to_i ** 2

Complete the pass_input_to_file method of test.rb:

require 'minitest/autorun'

def pass_input_to_file(input)
  # complete code here
end

class Test < Minitest::Test
  def test_file
    assert_equal pass_input_to_file(2), 4
  end
end

Macro · Accepted Answer · 2018-09-17 18:23:45Z

2

You can use the Ruby Open3 Library to submit STDIN when calling a ruby script.

require 'open3'

def pass_input_to_file(input)
  output, _status = Open3.capture2('path_to_script', :stdin_data => input)
  output
end

answered Sep 17, 2018 at 18:23

Macro

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Comments

Chris Hall · Accepted Answer · 2018-09-17 16:26:44Z

1

The easiest way to test this would probably be to have your program look to see if it was passed any arguments first. Something like this:

#!/usr/local/bin/ruby -w

input = ARGV[0] || STDIN.read

# square it
puts input.to_i ** 2

and then you can shell out to test it:

def pass_input_to_file(input)
  `path/to/file #{input}`.to_i
end

Otherwise, I would reach for something like expect to automate a subshell.

As an aside, for more complicated programs, using OptionParser or a CLI gem is probably better than looking at ARGV directly.

answered Sep 17, 2018 at 16:26

Chris Hall

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1 Comment

Amin Shah Gilani Over a year ago

Thank you for your answer Chris! :) Unfortunately, the square.rb file was a stand-in for any kind of file (even non-Ruby) that reads and writes STD IN and OUT, respectively. One that we don't have control over. The backticks to execute shell code idea was something I started using immediately after posting this question, but it's more Bash in Ruby that Ruby itself :). Still a valid way to do it though, and thank you for help!

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