#!/usr/bin/env perl

use strict;
use warnings;

die "usage: $0 <file>\n" if (not @ARGV);

my $rc = 0;
for my $file (@ARGV) {
  open(my $fh, '<', $file) or die "Cannot open \`$file' for read: $!\n";
  while (<$fh>) {
    next if (/^\s*#/);

    my $errors = 0;

    # remove everything between single quotes
    # this will remove too much in case of: echo "var='$var'"
    # and thus miss an opportunity to complain later on
    # also it mangles the input line irreversible
    s/'[^']+'/'___'/g;

    # highlight unbraced variables--
    # unless properly backslash'ed
    $errors += s/((?:^|[^\\]))(((\\\\)+)?\$\w)/$1\033[31m$2\033[0m/g;

    # highlight single square brackets
    $errors += s/((?:^|\s+))\[([^\[].+[^\]])\](\s*(;|&&|\|\|))/$1\033[31m\[\033[0m$2\033[31m\]\033[0m$3/g;

    # highlight double equal sign
    $errors += s/(\[\[.*)(==)(.*\]\])/$1\033[31m$2\033[0m$3/g;

    # highlight tabs mixed with whitespace at beginning of lines
    $errors += s/^( *)(\t+ *)/\033[31m\[$2\]\033[0m/;

    # highlight trailing whitespace
    $errors += s/([ \t]+)$/\033[31m\[$1\]\033[0m/;

    next if (not $errors);
    print "${file}[$.]: $_";
    $rc = 1;
  }
  close($fh);
}

exit $rc;