There is a standard program called "tee" that does this. There all sorts of versions of this that you can download in a directly executable format. Below is a simple version written in Perl. Typical output buffer is about 4K bytes. You don't need to turn output buffering off if your program is generating lots of output - buffer gets flushed when it is full. There are various options on standard "tee" to append, overwrite, etc. This is just an example of what it does.

basic sytanx: $someprog | tee somefile

#file: tee.pl
use strict;
use warnings;
$| = 1;      # **** turn off output buffering *****

sub usage () {
print "TEE USAGE:\n tees stdout to a file and to stdout\n".
      " program | tee outfile\n".
      " sends stdout from program to outfile\n";
      exit;
}


my $filename = shift @ARGV;
usage unless $filename;

open (OUTFILE, ">>$filename") || die "Can't open OUTFILE: $!";

while (<>){
  print;
  print OUTFILE;
}
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