How to Test For Empty Piped Input

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

I have a test program as shown below. When I pipe data to it, the output displays as expected. When I run the program stand-alone, it silently waits for input. I need a way to test for this condition and exit the program, but so far I can find none. Does anyone know a way of testing for this condition?

Here is the program which I named test.pl:

#!/usr/bin/perl
use strict;
use warnings;
while( <> ) {
    print $_;
}
[download]

Note: I'm running on Windows, hence the use of "echo" below to pipe data.

Test #1:
echo a | test.pl (This works as expected)

Test #2:
test.pl (This silently waits for input. I need to test for this condition and exit the program)

"It's not how hard you work, it's how much you get done."

Comment on How to Test For Empty Piped Input Download Code

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Re: How to Test For Empty Piped Input by parv (Parson) on Sep 02, 2024 at 04:57 UTC
Before the loop, you could test if standard input connected via pipe (and/or if connected to a tty) via -p (or -t) operator (respectively) (see `perldoc -f -x`). Ah, just noticed that you are running on MS Windows; in that case I have not tested if `-[pt]` function work reliably or at all. You should see some results if you were to search for how to check if the program is connected via pipe. Read more... trivial program that tests pipyness (868 Bytes)	[reply] [d/l] [select]
Re: How to Test For Empty Piped Input by roho (Bishop) on Sep 02, 2024 at 08:22 UTC
Thank you for your answers. -t STDIN ... is what I was missing. Working great now! Thanks again. "It's not how hard you work, it's how much you get done."	[reply]
Re: How to Test For Empty Piped Input by etj (Priest) on Sep 02, 2024 at 07:10 UTC
Untested, at start (see https://perldoc.perl.org/perlfunc#-X-FILEHANDLE): `BEGIN { die "No input given and STDIN is tty" if -t STDIN and !@ARGV }` [download]	[reply] [d/l]
Re^2: How to Test For Empty Piped Input by hippo (Archbishop) on Sep 02, 2024 at 08:32 UTC
Tested now (although not on MSWin32). Works as intended and doesn't even need the BEGIN block. `#!/usr/bin/env perl use strict; use warnings; die "What no STDIN?\n" if -t; print while <>;` [download] Runs like this: `$ ./stdintest.pl What no STDIN? $ echo foo \| ./stdintest.pl foo $` [download] 🦛	[reply] [d/l] [select]