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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def do_with_ruby(ruby, cmdline, runas=None):
    '''
    Execute a ruby command with rbenv's shims using a specific ruby version

    CLI Example:

    .. code-block:: bash

        salt '*' rbenv.do_with_ruby 2.0.0-p0 'gem list bundler'
        salt '*' rbenv.do_with_ruby 2.0.0-p0 'gem list bundler' runas=deploy
    '''
    if not cmdline:
        # This is a positional argument so this should never happen, but this
        # will handle cases where someone explicitly passes a false value for
        # cmdline.
        raise SaltInvocationError('Command must be specified')

    try:
        cmdline = salt.utils.args.shlex_split(cmdline)
    except AttributeError:
        cmdline = salt.utils.args.shlex_split(six.text_type(cmdline))

    env = {}
    if ruby:
        env['RBENV_VERSION'] = ruby
        cmd = cmdline
    else:
        cmd = cmdline

    return do(cmd, runas=runas, env=env)