Ruby: capture output of system() AND print it in terminal

Question

I am trying to run a system command (rspec) and print its output to the terminal in real time and in colour, but I also want to capture its printed output in a string to use later.

At the moment it looks something like this:

cmd = "rspec #{ARGV.join(' ')}"
system cmd
raw = `#{cmd}`

This works, but since RSpec takes several seconds to complete, doubling that time is not insignificant. Is there any way to keep the system call printing to the terminal, but also have its output available for capture?

That doesn't print the output in colour or in real time. — PJSCopeland
– PJSCopeland, Commented Dec 16, 2014 at 2:58

PJSCopeland · Accepted Answer · 2019-04-10 21:15:38Z

2

It turns out there is a PTY.spawn method that is very similar to Open3#popen2e but also does colour. Here's my whole block:

require 'pty'

raw = ''
PTY.spawn(cmd) do |stdout_err, stdin, pid|
  begin
    while (char = stdout_err.getc)
      raw << char
      print char
    end
  rescue Errno::EIO # always raised when PTY runs out of input
  ensure
    Process.waitpid pid # Wait for PTY to complete before continuing
  end
end

edited Apr 10, 2019 at 21:15

answered Apr 28, 2015 at 0:35

PJSCopeland

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Comments

Boris Bera · Accepted Answer · 2014-12-16 03:32:05Z

1

You should probably use Open3#popen2e. This method yields a stream for stdin, a stream for both stdout and stderr (they are merged), and a waiter thread.

You can safely close the stdin stream and ignore the waiter thread. You can read the merged stdout and stderr stream chunk by chunk.

answered Dec 16, 2014 at 3:32

Boris Bera

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1 Comment

PJSCopeland Over a year ago

Useful, but still not in colour.

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