Recipe 23.1. Scripting an External Program

Problem

You want to automatically control an external program that expects to get terminal input from a human user.

Solution

When you're running a program that only needs a single string of input, you can use IO.popen, as described in Recipe 20.8. This method runs a command, sends it a string as standard input, and returns the contents of its standard output:

	def run(command, input='')
	   
IO.popen(command, 'r+') do |io|
	    io.puts input
	    io.close_write
	    return io.read
	   end
	end

	run 'wc -w', 'How many words are in this string?'      # => "7\n"

This technique is commonly used to invoke a command with sudo, which expects the user's password on standard input. This code obtains a user's password and runs a command on his behalf using sudo:

	print 'Enter your password for sudo: '
	sudo_password = gets.chomp
	run('sudo apachectl graceful', user_password)

Discussion

IO.popen is a good way to run noninteractive commandscommands that read all their standard input at once and produce some output. But some programs are interactive; they send prompts to standard output, and expect a human on the other end to respond with more input.

On Unix, you can use Ruby's standard PTY and expect libraries to spawn a command and impersonate a human on the other end. This code scripts the Unix passwd command:

	require 'expect'
	require 'pty'
	
	print 'Old password:'
	old_pwd = gets.chomp

	print "\nNew password:"
	new_pwd = gets.chomp

	PTY.spawn('passwd') do |read,write,pid|
	  write.sync = true
	  $expect_verbose = false
	
	  # If 30 seconds pass and the expected text is not found, the
	  # response object will be nil.
	  read.expect("(current) UNIX password:", 30) do |response|
	    write.print old_pwd + "\n" if response
	  end

	  # You can use regular expressions instead of strings. The code block
	  # will give you the regex matches.
	  read.expect(/UNIX password: /, 2) do |response, *matches|
	    write.print new_pwd + "\n" if response
	  end

	  # The default value for the timeout is 9999999 seconds
	  read.expect("Retype new UNIX password:") do |response|
	    write.puts new_pwd + "\n" if response
	  end
	end

The read and write objects in the PTY#spawn block are IO objects. The expect library defines the IO#expect method found throughout this example.

See Also

• Recipe 20.8, "Driving an External Process with popen"

• Recipe 21.9, "Reading a Password," shows how to obtain a password without echoing it to the screen