Question

I am trying to create something in Perl that is basically like the Unix tee command. I’m trying to read each line of STDIN, run a substitution on it, and print it. (And eventually, also print it to a file.) This works if I’m using console input, but if I try to pipe input to the command it doesn’t do anything. Here’s a simple example:

print "about to loop\n";
while(<STDIN>)
{
  s/2010/2009/;
  print;
}
print "done!\n";

I try to pipe the dir command to it like this:

C:\perltest>dir | mytee.pl
about to loop
done!

Why is it not seeing the piped input? (I’m using Perl 5.10.0 on WinXP, if that is relevant.)

Answer 1

This is actually a bug in how Windows handles IO redirection. I am looking for the reference right now, but it is that bug that requires you to specify

dir | perl filter.pl

rather than being able to use

dir | filter

See Microsoft KB article STDIN/STDOUT Redirection May Not Work If Started from a File Association:

1. Start Registry Editor.
2. Locate and then click the following key in the registry:
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
3. On the Edit menu, click Add Value, and then add the following registry value:
   • Value name: InheritConsoleHandles
   • Data type: REG_DWORD
   • Radix: Decimal
   • Value data: 1
4. Quit Registry Editor.

C:\Temp> cat filter.pl
#!/usr/bin/perl

while ( <> ) {
    print "piped: $_";
}

C:\Temp> dir | filter
piped:  Volume in drive C is MAIN
piped:  Volume Serial Number is XXXX-XXXX
piped:
piped:  Directory of C:\Temp>
piped:
piped: 2010/03/19  03:48 PM              .
piped: 2010/03/19  03:48 PM              ..
piped: 2010/03/19  03:33 PM                32 m.pm
piped: 2010/03/19  03:48 PM                62 filter.pl