[racket] Embedding multiline shell scipts

From: Jay McCarthy (jay.mccarthy at gmail.com)
Date: Mon Feb 7 10:16:16 EST 2011

DrDr has some reusable, robust code for capturing output and sequence



2011/2/6 Neil Van Dyke <neil at neilvandyke.org>:
> Thomas Chust wrote at 02/06/2011 10:14 AM:
> 2011/2/6 Manfred Lotz <manfred.lotz at arcor.de>:
> [...]
> I don't know how to merge stdout and stderr in proper sequence.
> [...]
> There is no such thing as proper sequencing of data flowing through
> different streams.
> Due to buffering in system libraries and the operating system kernel it is
> also entirely impossible to reliably process the output data in the exact
> sequence it was produced by the external process.
> Agreed with Thomas that doing this perfectly is impossible in the general
> case, even when there's a single-threaded producer of the two streams.
> However, if you want to sequence reading stdout and stderr close enough for
> most purposes, keep in mind that these are usually interleaved at the
> resolution of lines rather than characters, so you can get this pretty much
> right, and still do fairly efficient block reads.  So, you can do the
> sequencing efficiently in your reading process by doing raw stdio and
> avoiding any buffering port abstractions in your library, doing a "sync"
> loop on the ports, and implementing your own buffering for each port, out of
> which buffer you pick a complete line at a time.  You also can tweak this to
> better interleave reads from the two streams/buffers.  You can also use time
> or other heuristics to detect when, say, a stdout message like
> "Processing..." without newline should be handled because it was interrupted
> by a subsequent stderr error message.  There might still be buffering
> outside of your control, but I think it doesn't often matter.
> Once you get into "expect"-like interaction with a process using the three
> streams at once, and doing pattern-matching while also capturing the output,
> and doing this all efficiently, it's tricky.  There's no reason that a
> generalized library routine to do this can't be implemented in Racket, but
> I'm not aware of one.  If you hand-implement for a particular purpose, I
> think you'll be getting your hands dirty with "sync", your own buffering of
> both input and output, block I/O, and some strategy for how to do your
> pattern-matching efficiently on the buffers.  IMHO, this should feel like
> systems programming if you're doing it efficiently.  This complication does
> not reflect a limitation of Racket, but instead that Racket is flexible
> enough that this doing this complicated stuff is possible.
> --
> http://www.neilvandyke.org/
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Jay McCarthy <jay at cs.byu.edu>
Assistant Professor / Brigham Young University

"The glory of God is Intelligence" - D&C 93

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