popen(...) public

Runs the specified command string as a subprocess; the subprocess’s standard input and output will be connected to the returned IO object. If cmd_string starts with a ``-’’, then a new instance of Ruby is started as the subprocess. The default mode for the new file object is ``r’’, but mode may be set to any of the modes listed in the description for class IO.

If a block is given, Ruby will run the command as a child connected to Ruby with a pipe. Ruby’s end of the pipe will be passed as a parameter to the block. At the end of block, Ruby close the pipe and sets $?. In this case IO::popen returns the value of the block.

If a block is given with a cmd_string of ``-’’, the block will be run in two separate processes: once in the parent, and once in a child. The parent process will be passed the pipe object as a parameter to the block, the child version of the block will be passed nil, and the child’s standard in and standard out will be connected to the parent through the pipe. Not available on all platforms. 

   f = IO.popen("uname")
   p f.readlines
   puts "Parent is #{Process.pid}"
   IO.popen ("date") { |f| puts f.gets }
   IO.popen("-") {|f| $stderr.puts "#{Process.pid} is here, f is #{f}"}
   p $?

produces: 

   ["Linux\n"]
   Parent is 26166
   Wed Apr  9 08:53:52 CDT 2003
   26169 is here, f is
   26166 is here, f is #<IO:0x401b3d44>
   #<Process::Status: pid=26166,exited(0)>
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