From: ara.t.howard@...
Date: 2006-09-28T11:55:57+09:00
Subject: Re: how to determine if pipe is given

On Thu, 28 Sep 2006 gwtmp01@mac.com wrote:

> Hmm. Doesn't cron arrange for stdin and stdout to be /dev/null, in which
> case the read will simply return EOF, correct?
>
> I just tried some experiments on Mac OS X and it seems that cron arranges
> for stdin to be an empty pipe for cron jobs.
>

hmmm.  you are right.  i had to look back at what had given me trouble before
- the crux of it was basically this

   input_on_stdin = not STDIN.tty?

   infile =
     if input_on_stdin
       STDIN.read
     end

so, basically assuming that the user gave input on stdin simply because STDIN
was not a tty, which may or may not be true, as in the case where one is
running under cron or a system call from a daemon, etc.

i guess my point is that

   'stdin is not a tty'

and

   'input was given on stdin'

are two orthogonal ideas which, when merged, have bitten me in the past.  so
that's why i've developed a preference for '-' to signal input on stdin - it's
non-ambiguous.

> In any case, clearly the program has been invoked incorrectly for that
> context (background cron job) and it pretty much doesn't matter what
> structure you choose for the command line arguments it is always possible to
> screw it up and provide the wrong arguments or plumb the wrong input or
> output sources.

indeed.

> I'm not sure I understand.  The OP was suggesting that the program was
> designed to read from a pipe, so it really shouldn't care if it was reading
> from a tty device, should it?

well - that is true.  maybe i confused things by making it more general.  the
only reason i was making a fuss is that it's caused me problems - as i
mentioned.  probably the noise wasn't worth it.

> I think of this like duck typing in Ruby.  As long as you can read lines
> from the file descriptor and your program is designed to read lines of text,
> should you really care if stdin is associated with a file, a pipe, a network
> connection, or a tty device?  Why not just read and process the data?

i think that you are quite right in this case.

cheers.

-a
-- 
in order to be effective truth must penetrate like an arrow - and that is
likely to hurt. -- wei wu wei