sub bashquote { "'" . shift =~ s/'/'"'"'/gr . "'" }

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    s/'/'"'"'/g;
    return "'$_'";
}

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    defined or die "Argument must not be undef";
    /\x00/ and die "String must not contain NUL characters";
    s/'/'"'"'/g;
    return "'$_'";
}

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    defined or die "Argument must not be undef";
    /\x00/ and die "String must not contain NUL characters";
    s/'/'\''/g;
    return "'$_'";
}

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    defined or die "Argument must not be undef";
    /\x00/ and die "String must not contain NUL characters";
    length or return "''";
    s/'/'\''/g;
    $_ = "'$_'";
    s/^''//;
    s/''$//;
    return $_;
}

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    defined or die "Argument must not be undef";
    /\x00/ and die "String must not contain NUL characters";
    length or return "''";
    s/'/'\''/g;
    s|((?:'\\''){2,})|q{'"} . (q{'} x (length($1) / 4)) . q{"'}|ge;
    $_="'$_'";
    s/^''//;
    s/''$//;
    return $_;
}

##</code><code>##

# untested
sub bashquote
{
    local $_ = shift;
    defined or die "Argument must not be undef";
    /\x00/ and die "String must not contain NUL characters";
    length or return "''";
    # before adding "/" to safe characters: /^[A-Za-z0-9_]+$/ and return $_;
    m|^[A-Za-z0-9/_]+$| and return $_;
    s/'/'\''/g;
    s|((?:'\\''){2,})|q{'"} . (q{'} x (length($1) / 4)) . q{"'}|ge;
    $_="'$_'";
    s/^''//;
    s/''$//;
    return $_;
}

##</code><code>##

# untested
sub shell_quote
{
    return join(' ',map { bashquote($_) } @_);
}