#!/usr/bin/env perl

# re! -- rewrite shebangs

use strict;

my $test = 0;
sub usage { die "usage: $0 [-n] file ...\n" }

if (@ARGV and $ARGV[0] eq '-n') {
	$test = 1;
	shift @ARGV;
}
usage if not @ARGV;

for my $file (@ARGV) {
	open my $o, '<', $file or die "could not open $file: $!\n";

	# parse shebang
	my $shebang = <$o>;
	$shebang =~ /^#!/ or die "no shebang: $file\n";
	$shebang =~ /^#!\s*(\S+)\s*(.*)/;
	my ($old, $args) = ($1, $2);

	# validate path
	next if -x $old;

	# get new path
	(my $basename = $old) =~ s,.*/,,;
	chomp(my $new = `which $basename`);
	$new or die "could not find $basename\n";
	next if $old eq $new;

	# print results if test
	if ($test) {
		print "$file: $old -> $new\n";
		next;
	}

	# write new shebang
	open my $n, '>', "$file.tmp" or die "could not open $file.tmp: $!\n";
	print $n "#!$new $args\n";
	print $n $_ while <$o>;

	close for ($o, $n);
	system('mv', "$file.tmp", $file) == 0
		or die "could not overwrite $file\n";
}