If there's no output from the pipe open, there'll be nothing read from PROCS, so your loop won't execute. You could set a flag:

my $output;
while (<PROCS>)
{
    $output++;
    # do something else
}

print "Couldn't find $process\n" unless $output;
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The a Proc::ProcessTable module might be another approach.

Update: I forgot to mention that your loop will not execute if a line evaluating to false in boolean context is read — that may not be a problem in this case, but it's worth remembering.

I forgot to mention that your loop will not execute if a line evaluating to false in boolean context is read

Actually, no - if you read the perlop 'I/O Operators' entry carefully, you'll find that using <FILEHANDLE> inside a while (or for(;;)) construct is treated specially - a defined test is done for you, to avoid exactly the problem you mention...

I case you're too lazy to look it up, I quote:

In these loop constructs, the assigned value (whether assignment is automatic or explicit) is then tested to see whether it is defined. The defined test avoids problems where line has a string value that would be treated as false by Perl, for example a "" or a "0" with no trailing newline. If you really mean for such values to terminate the loop, they should be tested for explicitly:

           while (($_ = <STDIN>) ne '0') { ... }
           while (<STDIN>) { last unless $_; ... }
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In other boolean contexts, "<filehandle>" without an explicit "defined" test or comparison elicit a warning if the "use warnings" pragma or the -w command-line switch (the "$^W" variable) is in effect.

HTH

--
3dan

You're right; I just confirmed this with B::Deparse. Thanks!

$ perl -MO=Deparse
while (my $in = <STDIN>)
{
    print $in;
}
^D
while (defined(my $in = <STDIN>)) {
    print $in;
}
- syntax OK
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Very nice. Quick and easy...and I woudln't have thought of it. Thanks!

You're not totally off. You're looking right at it. If the process doesn't exist, you'll get nothing on standard output. Here's why.

  while ( my $proc = <PROCS> ) {
      # if the grep filters out everything, this is a
      # zero-trip loop.
  }
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  my $found_one = 0;
  while ( my $proc = <PROCS> ) {
      ...
      $found_one = 1;
  }
  if ( $found_one ) {
      print "looks okay here\n";
  } else {
      print "couldn't find $process\n";
  }
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Thanks dws. Also informative. ONe thing though: I needed to make a small change.

while ( my $proc = <PROCS> ) {
      ...
      $found_one = 1;
  }
  if ( $found_one ) {
      print "looks okay here\n";
      found_one = 0;
  } else {
      print "couldn't find $process\n";
  }
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W/o resetting the $found_one to 0, it would never alert me when the other processes (anything after 1) weren't running. As for the greps externally - I recall finding this in a perl tutorial many moons ago, and never learned it another way. Might you have a link that explains it internally?

As for the greps externally - I recall finding this in a perl tutorial many moons ago, and never learned it another way. Might you have a link that explains it internally?

You'd find that described under "regular expressions". As an example,

  open(PROCS, "ps -a | grep $process |");
  while ( my $proc = <PROCS> ) {
      # ...
  }
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can be rewriten (assuming $process holds an alphanumeric string) as

  open(PROCS, "ps -a |");
  while ( my $proc = <PROCS> ) {
      next if $proc !~ /$process/;
      # ...
  }
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Regular expressions are your friends. Learning about them pays off in many ways.