darkomen has asked for the wisdom of the Perl Monks concerning the following question:

I want to write some perl code that can detirmine when it has something in STDIN to process.

Ex: echo a | test.pl

That should process STDIN...

Ex: ./test.pl

That should exit without wanting a character entered into STDIN. Whenever you try and read from STDIN, if it is empty, it waits til it has something in it and gets an EOF... I want to basically be able to test.... Is there something in STDIN? Yes - process the contents of STDIN... No - Skip processing STDIN and exit....

Am I being clear at all? Thanks for any advice...

Replies are listed 'Best First'.
Re: Testing for the existence of STDIN?
by wog (Curate) on Jan 24, 2002 at 03:22 UTC
    In addition to the code above, you could use the -t filetest operator to check if STDIN was opened to a terminal (and thus not a pipe, etc): 

      if (-t STDIN) {
    # we're running from a shell, without input being
    # piped to us.
  } else {
    # ...
  }

    [download]
    Or you could also try a non-blocking read of STDIN, which most directly answers your original question of seeing if there is any input waiting without blocking: 
      use IO::Handle;
  STDIN->blocking(0);
 # and try to read from it, like you would normally.
  my $line = <STDIN>;
  if (defined $line) {
    # we have input, read succeeded
  } else {
    # no input at the moment.
  }

    [download]
    (Note that this technique may give false negatives if pipes take some time to start writing data.)

[reply]
[d/l]
[select]
Re: Testing for the existence of STDIN?
by Zaxo (Archbishop) on Jan 24, 2002 at 03:28 UTC

    Four argument select 1, undef, undef, $timeout is what you're after.

    After Compline,
    Zaxo

[reply]
(jeffa) Re: Testing for the existence of STDIN?
by jeffa (Bishop) on Jan 24, 2002 at 02:58 UTC
    This is expected behaviour. If you ask for something, Perl waits until it gets it. Your alternative is to pass arguments to the script, like the name of a file to open. If the file is empty, then you could exit cleanly.

    UPDATE:
    Actually, you _could_ use a time out with $SIG{ALRM}: 

    $SIG{ALRM} = sub { die 'STDIN' };

eval {
   alarm(1);
   print while <>;
   alarm(0);
};

print "no STDIN\n" if $@ =~ /STDIN/;

    [download]
    You cannot get finer grained than one second with this way, but you can with select.

    Or .... just use wog's much more elegant solution. wog++

    jeffa

    L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
B--B--B--B--B--B--B--B--
H---H---H---H---H---H---
(the triplet paradiddle with high-hat)
[reply]
[d/l]
      I have a piece of working code using sigalrm, i was just hoping there was more of a clean way to do so... It would be nice if you could use the -s test operator on STDIN...
[reply]

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