[Newbies] Re: PipeableOSProcess incremental output

David T. Lewis lewis at mail.msen.com
Fri Oct 28 17:41:04 UTC 2011

On Fri, Oct 28, 2011 at 09:28:25AM -0700, Sean P. DeNigris wrote:
> Thanks David. Is there a way to query whether output/errors are available? I
> saw some mention of an event driven mechanism, but wasn't clear how to use
> it.

The event-driven stuff is "under the covers". It basically watches
for available data, for example when data is available on the
stderr stream of the external process it is automatically pushed
into the errorPipelineStream.

The easiest way to see if data is available is to read from the
stream and see what you get. For example, if you do #upToEnd and
get an empty string, then there is no data available. Note that
#upToEnd gives you all of the currently available data (there
may be more on the way, for example if the external process is
something that runs continuously and puts out data once in a
while). If you are dealing with an external program that you
expect to run once to completion, then you can use #upToEndOfFile
to read everything until the process is complete.

The output and error streams are just streams of data containing
whatever happens to come out of the external process. If you are
interested in error conditions, you can check exit status of the
external process after it runs to completion. Note that the
external process may not exit until after you have read its
output data (it will block on writing to the pipe if it needs
to write a lot of data and you have not read any of it yet),
so in general you will want to read the output data, then check
exit status afterwards (and maybe read the errorOutputStream


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