From: Robert Klemme <shortcutter@...>
Date: 2009-11-07T02:50:05+09:00
Subject: Re: reading from an external process with IO.popen

On 11/06/2009 04:01 PM, Robert Citek wrote:
> On Fri, Nov 6, 2009 at 5:15 AM, Robert Klemme
> <shortcutter@googlemail.com> wrote:
>> 2009/11/5 Robert Citek <robert.citek@gmail.com>:
>>> However, if I change the cat command to a sed command, the ruby
>>> version no longer works.  The command-line equivalent does work, but
>>> the ruby version waits forever and has to be interrupted:
>> That's probably because you do not close the write end of the pipe in
>> Ruby code.
> 
> Perhaps, but what if I don't want to close the pipe?  That is, I would
> like to keep the pipe open so that I can send some data, read some
> data and work on it, send some more data, read some more data and work
> on it, etc. much like the process was a service, e.g. database.  I am
> trying to code the equivalent of a Call and Response.  My examples
> using cat and sed are just stand-ins for the real program.

If the program you are using does not cooperate you're out of luck.  For 
example, if it assigns a huge read buffer then you might have to send 
hundreds of lines before it even starts processing the first one.  I 
have no idea how the implementation of sed that you are using does it 
but if you for example think of sort you _cannot_ get any output before 
the last line has been written and the write end of the pipe has been 
closed.

> BTW, the cat example works as expected, but the using sed doesn't
> work.  That is, there is no output from sed until the pipe closes.
> There seems to be some buffering going on.  I'm guessing it's from the
> Ruby side since I don't see this when run from the shell.  But that's
> just a guess.

The shell closes the pipe as well.  It is sed that is doing the 
buffering and you have no control over it unless it provides an option 
to control this.

> Of course, it's entirely possible that IO.popen is not the "right" way
> to tackle this and I have not discovered the Ruby way, yet.

No, it's the right way but your expectations cannot be met in all cases.

Kind regards

	robert


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