./ReadAPipe.pl a b c

hangs with an empty pipe
how do I fix that?

hangs with an empty pipe how do I fix that?

By only reading from it if it is a pipe...

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

if( -p STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
[download]

Will prevent that hang:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

But it prevents supplying the data by stdin redirection rather than a pipe:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123 < ReadPipeAndArgs.pl
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

Which would work without the pipe test.

So, perhaps a better way would be to only read from STDIN, if it is not connected to a terminal, which allows both modes of input and prevents the hang if neither is supplied:

C:\test>ReadPipeAndArgs.pl  A B CDE H 123 < ReadPipeAndArgs.pl
This was read from the pipe:
<#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

unless( -t STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
>

This was the read from the parameters:
<A B CDE H 123>

C:\test>type ReadPipeAndArgs.pl | ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<#!/usr/bin/perl

# http://perlmonks.org/?node_id=1176039

use strict;
use warnings;

my $Pipe = '';

unless( -t STDIN ) {
    while( <STDIN> ) {
        $Pipe .= $_;
    }
}

print "This was read from the pipe:\n";
print "<$Pipe>\n\n";

print "This was the read from the parameters:\n";
print "<@{ARGV}>\n";
__END__
>

This was the read from the parameters:
<A B CDE H 123>

C:\test>ReadPipeAndArgs.pl  A B CDE H 123
This was read from the pipe:
<>

This was the read from the parameters:
<A B CDE H 123>
[download]

---

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Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.

In the absence of evidence, opinion is indistinguishable from prejudice.

That's not an empty pipe, just a pipe that has not yet produced a value.