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

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Top Forums Shell Programming and Scripting Perl: How to check if there is something being piped in
# 1  
Old 10-25-2012
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:
Code:
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!
Last edited by zaxxon; 10-25-2012 at 09:25 AM.. Reason: spelling
# 2  
Old 10-25-2012
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
Code:
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:
Code:
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.
Last edited by elixir_sinari; 10-25-2012 at 09:58 AM..
# 3  
Old 10-25-2012
Quote:
Originally Posted by elixir_sinari
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
Code:
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.
Myself too...i got the prompt back when i piped a empty file
Code:
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
Code:
 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..
# 4  
Old 10-25-2012
Hey,

my problem is to catch the issue, that there is nothing being piped in. Example:
Code:
$ cat mach.pl
#!/usr/bin/perl

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

exit(0);

Yes, feeding it works:
Code:
$ 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:
Code:
$ ./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.
# 5  
Old 10-25-2012
Check my earlier post now.
Code:
$ 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);

These 3 Users Gave Thanks to elixir_sinari For This Post:
msabhi Scott zaxxon
# 6  
Old 10-25-2012
Works perfect, thanks a lot Smilie

Edit:
If not too lengthy/complicated other solutions are welcome. Maybe I grasp a tad more of PERL Smilie
# 7  
Old 10-25-2012
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 Smilie
Code:
$ cat MyScript
if(tell(\*STDIN) == -1) {
  printf( "got file\n\n");
}

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

$
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