#!/opt/bin/perl

if (@ARGV != 3)
{
    print "\nUsage: merge <file1> <file2> <file3>\n\n";
    print "Alternately writes a line from <file1> and <file2> to <file3>\n\n";
    exit;
}

$file1 = shift;
$file2 = shift;
$file3 = shift;

unless (open(FILE1, "<$file1"))
{
    die "can't open file '$file1': $!\n";
}

unless (open(FILE2, "<$file2"))
{
    die "can't open file '$file2': $!\n";
}

unless (open(FILE3, ">$file3"))
{
    die "can't open file '$file3': $!\n";
}

$ok = 1;

while ($ok)
{
    $ok1 = ($line1 = <FILE1>);
    $ok2 = ($line2 = <FILE2>);
    print FILE3 $line1 if ($ok1);
    print FILE3 $line2 if ($ok2);
    $ok = $ok1 || $ok2;
}

close(FILE1);
close(FILE2);
close(FILE3);