IO.popen(cmd_string, mode="r" )               → io
IO.popen(cmd_string, mode="r" ) {|io| block } → obj

Runs the specified command string as a subprocess; the subprocess’s

standard input and output will be connected to the returned
<code>IO</code> object. If <i>cmd_string</i> starts with a
``<code>-</code>'', then a new instance of Ruby is started as the
subprocess. The default mode for the new file object is ``r'', but
<i>mode</i> 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 <code>$?</code>.
In this case <code>IO::popen</code> returns
the value of the block.

If a block is given with a <i>cmd_string</i> of ``<code>-</code>'',
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 <code>nil</code>, 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 $?

<em>produces:</em>

   ["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)"></process::status:></io:0x401b3d44>

Source Code

/*
*  call-seq:
*     IO.popen(cmd_string, mode="r" )               => io
*     IO.popen(cmd_string, mode="r" ) {|io| block } => obj
*  
*  Runs the specified command string as a subprocess; the subprocess's
*  standard input and output will be connected to the returned
*  <code>IO</code> object. If <i>cmd_string</i> starts with a
*  ``<code>-</code>'', then a new instance of Ruby is started as the
*  subprocess. The default mode for the new file object is ``r'', but
*  <i>mode</i> 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 <code>$?</code>.
*  In this case <code>IO::popen</code> returns
*  the value of the block.
*     
*  If a block is given with a <i>cmd_string</i> of ``<code>-</code>'',
*  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 <code>nil</code>, 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 $?
*     
*  <em>produces:</em>
*     
*     ["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)>
*/

static VALUE
rb_io_s_popen(argc, argv, klass)
   int argc;
   VALUE *argv;
   VALUE klass;
{
   char *mode;
   VALUE pname, pmode, port;

   if (rb_scan_args(argc, argv, "11", &pname, &pmode) == 1) {
       mode = "r";
   }
   else if (FIXNUM_P(pmode)) {
       mode = rb_io_modenum_mode(FIX2INT(pmode));
   }
   else {
       mode = rb_io_flags_mode(rb_io_mode_flags(StringValuePtr(pmode)));
   }
   SafeStringValue(pname);
   port = pipe_open(pname, 0, mode);
   if (NIL_P(port)) {
       /* child */
       if (rb_block_given_p()) {
           rb_yield(Qnil);
           fflush(stdout);
           fflush(stderr);
           _exit(0);
       }
       return Qnil;
   }
   RBASIC(port)->klass = klass;
   if (rb_block_given_p()) {
       return rb_ensure(rb_yield, port, io_close, port);
   }
   return port;
}