use strict;

die "Usage: $0 port#\n (where port# is a number > 9 and < 65536\n"
    if ( @ARGV != 1 or $ARGV[0] !~ /^[1-9]\d+$/ or $ARGV[0] >= 65536 );

my $sshport = shift;
my $filename = "sshd_config";

open (SSHD_CONFIG,$filename)
    or die ("can't open $filename for reading: $!");
open (SSHD_CONFIG_NEW,">","$filename.new")
    or die ("can't open $filename.new for writing: $!");

while (<SSHD_CONFIG>){
    s/^(#?)port.+/$1Port $sshport/i;
    print SSHD_CONFIG_NEW;
}
( rename $filename, "$filename.old" and
  rename "$filename.new", $filename )
    or die "rename failed: $!"