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 e 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, 
+) do |io|
	 io.puts input
	 io.close_write
	 return io.read
	 end
	end

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

This technique is commonly used to invoke a command with sudo, which expects the users password on standard input. This code obtains a users 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 Rubys 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 "
New 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 + "
" 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 + "
" if response
	 end

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


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Ruby Cookbook
Ruby Cookbook (Cookbooks (OReilly))
ISBN: 0596523696
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