bump...

Just trying to see if anyone knows IO.readpartial better than me.

Matt

----- Original Message -----
From: "Matt Mencel" <MR-Mencel / wiu.edu>
To: "ruby-talk ML" <ruby-talk / ruby-lang.org>
Sent: Thursday, July 3, 2008 3:07:15 PM GMT -06:00 US/Canada Central
Subject: Reading stdout & stderr from a pipe with popen3

I originally posted this to a Ruby forum at railsforum.com, but I just found this list so thought I'd try here since I haven't gotten a response at railsforum.

OK...so I'm using popen3 to run some command line stuff and be able to get back stdout/stderr...


 $cmdin, $cmdout, $cmderr = Open3.popen3("zmprov") 

zmprov opens a new command prompt to which I can send more commands to and get back output...

 $cmdin.puts("sm username")
 $cmdin.puts("sm")
 count = 0
 $cmdout.each |line|
   count += 1 if line.include?("\n")
   puts line
   break if count == 2 # IF I FIND 2 BLANK LINES IN stdout, BREAK THE LOOP
 end 

Which works just fine.  However, I've run into a snag.  $cmdout and $cmderr are pipes that never close until I send the "quit" command...so in order to break out of my loops I've got to jump through some extra hoops to look for specific patterns in the output.  In the above example I have to send an extra "sm" command without the username, and in that output I can find the two blank lines...so I know it's done and can drop out of the loop.

It's a pain, but I can do that.  The real problem becomes what to do when a command returns something in stderr?  Because stdout returns nothing...there's nothing to check for to drop out of that loop before I check for stderr....check the example below.

 $cmdin.puts("sm BADUSERNAME") # PRODUCES AN ERROR
 $cmdout.each |line|
   puts line
 end
 $cmderr.each |line|
   puts line
 end 

If the command produces an error....I never get out of the stdout loop to grab the error because stdout is returning me nothing...it's just an open pipe with no data in it until I run a command that produces stdout.  So I never get where I can read stderr.

A coworker suggested I run the original "zmprov" command as "zmprov 2>&1" so that stderr is returned in stdout.  Which will work...but I was just wondering if I could get the code to work as designed.  If stdout is returning data...read it and do something....if stderr is returning data then do something with that.  I'm wanting to keep one from blocking my ability to read the other.


After some further research and a little testing, I've found that using IO.readpartial (http://www.noobkit.com/show/ruby/ruby/ruby-core/io/readpartial.html) might be what I'm looking for, but I'm not sure.  So I can do something like...

  $cmdin.readpartial(4096)

...and it will grab all the data in the pipe (up to 4096 bytes I think) and when it runs out of stuff to read it returns control back to the program.  What happens though if there is more data waiting in the pipe than I call for with my IO.readpartial(maxlen) argument?  Will it still return control or will it keep reading 4096 blocks until it runs out and THEN return control?  I think that is how I would want it to handle.

There is almost no examples out there about IO.readpartial, so I'm hoping someone here can help point me in the right direction.

Thanks,
Matt