: '7th edition Bourne shell aka the V7 shell did not know # as comment sign, yet.'
	: 'Workaround: the argument to the : null command can be considered a comment,'
	: 'protect it, because the shell would have to parse it otherwise.'

##</code><code>##

sub shell_comment_quote {
    return '' unless @_;
    unless (@_ == 1) {
        croak "Too many arguments to shell_comment_quote "
                    . "(got " . @_ . " expected 1)";
    }
    local $_ = shift;
    s/\n/\n#/g;
    return $_;
}

##</code><code>##

sub shell_quote {
    my ($rerr, $s) = _shell_quote_backend @_;
 
    if (@$rerr) {
        my %seen;
        @$rerr = grep { !$seen{$_}++ } @$rerr;
        my $s = join '', map { "shell_quote(): $_\n" } @$rerr;
        chomp $s;
        croak $s;
    }
    return $s;
}

##</code><code>##

 
sub shell_quote_best_effort {
    my ($rerr, $s) = _shell_quote_backend @_;
 
    return $s;
}

##</code><code>##

sub _shell_quote_backend {
    my @in = @_;
    my @err = ();

# ...

    return \@err, '' unless @in;

# ...

        if (s/\x00//g) {
            push @err, "No way to quote string containing null (\\000) bytes";
        }

# ...
 
    return \@err, $ret;
}

##</code><code>##

sub _shell_quote_backend {
    my @in = @_;
    my @err = ();
 
    if (0) {
        require RS::Handy;
        print RS::Handy::data_dump(\@in);
    }
 
    return \@err, '' unless @in;
 
    my $ret = '';
    my $saw_non_equal = 0;
    foreach (@in) {
        if (!defined $_ or $_ eq '') {
            $_ = "''";
            next;
        }
 
        if (s/\x00//g) {
            push @err, "No way to quote string containing null (\\000) bytes";
        }
 
        my $escape = 0;
 
        # = needs quoting when it's the first element (or part of a
        # series of such elements), as in command position it's a
        # program-local environment setting
 
        if (/=/) {
            if (!$saw_non_equal) {
                $escape = 1;
            }
        }
        else {
            $saw_non_equal = 1;
        }
 
        if (m|[^\w!%+,\-./:=@^]|) {
            $escape = 1;
        }
 
        if ($escape
                || (!$saw_non_equal && /=/)) {
 
            # ' -> '\''
            s/'/'\\''/g;
 
            # make multiple ' in a row look simpler
            # '\'''\'''\'' -> '"'''"'
            s|((?:'\\''){2,})|q{'"} . (q{'} x (length($1) / 4)) . q{"'}|ge;
 
            $_ = "'$_'";
            s/^''//;
            s/''$//;
        }
    }
    continue {
        $ret .= "$_ ";
    }
 
    chop $ret;
    return \@err, $ret;
}

##</code><code>##

>bash --version
GNU bash, version 4.3.48(1)-release (x86_64-slackware-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
>echo with some arguments !
with some arguments !
>foo with some arguments !
-bash: foo: command not found
>'!' foo
-bash: !: command not found
>! foo
-bash: foo: command not found
>

##</code><code>##

>bash --version
GNU bash, version 4.3.48(1)-release (x86_64-slackware-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
>perl -MString::ShellQuote=shell_quote -E 'say shell_quote("!","FOO=BAR","baz")'
! FOO=BAR baz
>! FOO=BAR baz
-bash: baz: command not found
>'!' FOO=BAR baz
-bash: !: command not found
>! 'FOO=BAR' baz
-bash: FOO=BAR: command not found
>