Perl: How to check if there is something being piped in

Hi,

I am somewhat new to Perl and currently checking it out. I have a problem testing, if there is nothing being piped in to that script.

I am reading input from STDIN this way:

while( defined($line = <STDIN>) ) {
        chomp($line);
        if( $line =~ m/($ARGV[0])/g ) {
                $count++;
        }
}

In other words, my script works fine but when there is nothing being piped in, it just stops and waits for input.
I have tried several things I found on different pages but I wasn't able to get it to work.
Is there a simple way to check and avoid this?

Thanks in forward!

Hi zaxxon,

are you sure that this is the problematic code fragment? I am asking because when I tried this with a command line such as

gzip some_file|perl -e '<your code>' some_arg

, the perl process did not wait (the supposed pipe-writer is not writing anything to the pipe) and I got my prompt back.

EDIT: Are you trying to check if the perl script is reading from a pipe?
In that case, may be you could use the file test operator -t with STDIN like:

if (-t STDIN) { die "Input from terminal not allowed\n" }

at the beginning, to ensure that the standard input is not associated with a terminal.

Myself too...i got the prompt back when i piped a empty file

cat temp7 | perl -e 'while( defined($line = <STDIN>) ) {
        chomp($line);
        if( $line =~ m/($ARGV[0])/g ) {
                $count++;
        }
} print $count;' "END"

But when simply executed like below without piping anything

 perl -e 'while( defined($line = <STDIN>) ) {
        chomp($line);
        if( $line =~ m/($ARGV[0])/g ) {
                $count++;
        }
} print $count;' "END"

It waits and i guess its just waiting for input..i believe this is per design..

Hey,

my problem is to catch the issue, that there is nothing being piped in. Example:

$ cat mach.pl
#!/usr/bin/perl

while( defined($line = <STDIN>) ) {
        chomp($line);
        print $line . "\n";
}

exit(0);

Yes, feeding it works:

$ echo yo | ./mach.pl
yo

This hangs and waits for input, like a grep in a shell waiting for input. Can only break it with Ctrl+c:

$ ./mach.pl

...

I am looking for a way to notice, that there is nothing to be read from STDIN so I can end the script with some error message.

Check my earlier post now.

$ cat mach.pl
#!/usr/bin/perl

if (-t STDIN) { die "Input from terminal not allowed\n" }

while( defined($line = <STDIN>) ) {
        chomp($line);
        print $line . "\n";
}

exit(0);

Works perfect, thanks a lot

Edit:
If not too lengthy/complicated other solutions are welcome. Maybe I grasp a tad more of PERL

I've used this in the past, but not sure if it's a good way. Like you, I'm quite new to the language

$ cat MyScript
if(tell(\*STDIN) == -1) {
  printf( "got file\n\n");
}

$ ./MyScript
$
$ cat X | ./MyScript
got file

$

Hey, I am learning this magical beast, too.