How do I take the output of one program and use it as the input of another?

Question

I've looked at this and it wasn't much help.

I have a Ruby program that puts a question to the cmd line and I would like to write a Python program that can return an answer. Does anyone know of any links or in general how I might go about doing this?

Thanks for your help.

EDIT
Thanks to the guys that mentioned piping. I haven't used it too much and was glad it was brought up since it forced me too look in to it more.

Answer 1

If you're on unix / linux you can use piping:

question.rb | answer.py

Then the output of question.rb becomes the input of answer.py

I've not tried it recently, but I have a feeling the same syntax might work on Windows as well.

Answer 2

Pexpect

http://www.noah.org/wiki/Pexpect

Pexpect is a pure Python expect-like module. Pexpect makes Python a better tool for controlling other applications.

Pexpect is a pure Python module for spawning child applications; controlling them; and responding to expected patterns in their output. Pexpect works like Don Libes' Expect. Pexpect allows your script to spawn a child application and control it as if a human were typing commands.

Answer 3

There are two ways (off the top of my head) to do this. The simplest if you're in a Unix environment is to use piping. Simple example:

cat .profile .shrc | more

This will send the output of the first command (cat .profile .shrc) to the more command using the pipe character |.

The second way is to call one program from the other in your source code. I'm don't know how Ruby handles this, but in Python you can run a program and get it's output by using the popen function. See this example chapter from Learning Python, then Ctrl-F for "popen" for some example code.

Answer 4

p = subprocess.Popen(['ruby', 'ruby_program.rb'], stdin=subprocess.PIPE, 
                                          stdout=subprocess.PIPE)
ruby_question = p.stdout.readline()
answer = calculate_answer(ruby_question)
p.stdin.write(answer)
print p.communicate()[0]  # prints further info ruby may show.

The last 2 lines could be made into one:

print p.communicate(answer)[0]

Answer 5

First of all check this out: [Unix piping][1]

It works on windows or unix but it's slighly dufferent, first the programs:

question.rb:

puts "This is the question"

answer.rb:

question = gets
#calculate answer
puts "This is the answer"

Then the command line:

In unix:

question.rb | answer.rb

In windows:

ruby question.rb | ruby answer.rb

Output:

This is the question
This is the answer