Running a shell command from Ruby: capturing the output while displaying the output?

Question

I have a problem.

I want to run a ruby script from another ruby script and capture it's output information while letting it output to the screen too.

runner

#!/usr/bin/env ruby
print "Enter your password: "
password = gets.chomp
puts "Here is your password: #{password}"

The script file that I run:

start.rb

output = `runner`
puts output.match(/Here is your (password: .*)/).captures[0].to_s

As you see here there is a problem.

In the first line of start.rb the screen is empty.

I cannot see the "Enter your password: " in runner.

Is there a way to display the output of the runner script before it's finished, and still let me capture it to a string so I can process the information, eg. using match like in this example?

Answer 1

You could use tee to write the contents to a file or a pipe, and read the file afterwards.

Answer 2

Have a look at POpen4. It claims to be platform independent (but I do not think it works in jruby where you can use IO#popen instead).

Answer 3

Have your script do its prompt output to stderr.

echo "Enter something" >&2
read answer
echo "output that will be captured"

This will be done for you if you use read -p to issue the prompt:

read -p "Enter something" answer
echo "output that will be captured"

Answer 4

Try this:

rd, wr = IO::pipe
pid = Process.fork do  
  $stdout.reopen(wr)
  rd.close
  exec("command")
end
wr.close
rd.each do |line|  
  puts "line from command: #{line}"
end
Process.wait(pid)

Similar if you want to capture stderr. If you need to capture both it would a bit more difficult (Kernel.select?)

Edit: Some explanation. This is an ancient Unix procedure: pipe + fork + calls to dup2 (reopen) depending on what you want. In a nutshell: you create a pipe as a means of communication between child and parent. After the fork, each peer close the pipe's endpoint it does not use, the child remaps (reopen) the channel you need to the write endpoint of the pipe and finally the parent reads on the read channel of the pipe.

Answer 5

io = IO.popen(<your command here>)
log = io.readlines
io.close

Now in log variable you have the output of executed command. Parse it, convert it, or do whatever you want.

Answer 6

runner.rb

STDOUT.print "Enter your password: "
password = gets.chomp
puts "Here is your password: #{password}"

Note STDOUT.print

start.rb

require "stringio"

buffer = StringIO.new
$stdout = buffer

require "runner"

$stdout = STDOUT
buffer.rewind

puts buffer.read.match(/Here is your (password: .*)/).captures[0].to_s

output

Enter your password: hello
password: hello

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Read more...

I recently did a write-up on this here: Output Buffering with Ruby