\documentclass[answers]{exam}

\usepackage{expl3}

\ExplSyntaxOn

% Flag to enable Latexmk
\bool_new:N \g__diaa_use_latexmk_bool

% Uncomment the following line to use Latexmk
% \bool_gset_true:N \g__diaa_use_latexmk_bool


% Flag to output two files
\bool_new:N \g__diaa_build_two_files_bool

\legacy_if:nT { printanswers }
  {
    \bool_gset_true:N \g__diaa_build_two_files_bool
  }

% Uncomment the following line to force build two files
% \bool_gset_true:N \g__diaa_build_two_files_bool


% Answers jobname variable
\str_new:N \g__diaa_solved_jobname_str

% LaTeX engine and options variables
\str_new:N \g__diaa_latex_cmd_str
\str_const:Nn \g__diaa_latex_options {-interaction=nonstopmode }

% Latexmk engine and options variables
\str_const:Nn \g__diaa_latexmk_str { latexmk }
\str_new:N \g__diaa_latexmk_engine_str

% Final build commands
\str_new:N \g__diaa_cmd_questions_str
\str_new:N \g__diaa_cmd_solved_str


% Build the answer jobname
\cs_new:Nn \__diaa_build_solved_jobname:
  {
    \str_gset:Nx \g__diaa_solved_jobname_str { \c_sys_jobname_str }
    \str_gremove_all:Nn \g__diaa_solved_jobname_str { " }
    \str_gput_left:Nn \g__diaa_solved_jobname_str { "[solved]~ }
    \str_gput_right:Nn \g__diaa_solved_jobname_str { " }
  }

% Set engines based on current engine
\cs_new:Nn \__diaa_set_engine:
  {
    \sys_if_engine_luatex:T
      {
        \str_gset:Nn \g__diaa_latex_cmd_str { lualatex }
        \str_gset:Nn \g__diaa_latexmk_engine_str { -lualatex ~ -g ~ -interaction=nonstopmode }
      }
    \sys_if_engine_pdftex:T
      {
        \str_gset:Nn \g__diaa_latex_cmd_str { pdflatex }
        \str_gset:Nn \g__diaa_latexmk_engine_str { -pdf ~ -g ~ -interaction=nonstopmode }
      }
    \sys_if_engine_xetex:T
      {
        \str_gset:Nn \g__diaa_latex_cmd_str { xelatex }
        \str_gset:Nn \g__diaa_latexmk_engine_str { -xelatex }
      }
  }

% Build LaTeX questions command
\cs_new:Nn \__diaa_build_latex_questions_cmd:
  {
    \str_gset:Nx \g__diaa_cmd_questions_str
      {
        \g__diaa_latex_cmd_str \c_space_tl
        -jobname=\c_sys_jobname_str \c_space_tl
        \g__diaa_latex_options \c_space_tl
        "\string\AtBeginDocument{\string\printanswersfalse}" \c_space_tl
        "\string\input{\c_sys_jobname_str}"
      }
  }

% Build LaTeX answers command
\cs_new:Nn \__diaa_build_latex_solved_cmd:
  {
    \str_gset:Nx \g__diaa_cmd_solved_str
      {
        \g__diaa_latex_cmd_str \c_space_tl
        -jobname=\g__diaa_solved_jobname_str \c_space_tl
        \g__diaa_latex_options \c_space_tl
        "\string\AtBeginDocument{\string\printanswerstrue}" \c_space_tl
        "\string\input{\c_sys_jobname_str}"
      }
  }

% Build Latexmk questions command
\cs_new:Nn \__diaa_build_latexmk_questions_cmd:
  {
    \str_gset:Nx \g__diaa_cmd_questions_str
      {
        \g__diaa_latexmk_str \c_space_tl
        \g__diaa_latexmk_engine_str \c_space_tl
        -usepretex="\string\AtBeginDocument{\string\printanswersfalse}" \c_space_tl
        \c_sys_jobname_str
      }
  }

% Build Latexmk answers command
\cs_new:Nn \__diaa_build_latexmk_solved_cmd:
  {
    \str_gset:Nx \g__diaa_cmd_solved_str
      {
        \g__diaa_latexmk_str \c_space_tl
        \g__diaa_latexmk_engine_str \c_space_tl
        -usepretex="\string\AtBeginDocument{\string\printanswerstrue}" \c_space_tl
        -jobname=\g__diaa_solved_jobname_str \c_space_tl
        \c_sys_jobname_str
      }
  }


% Build the files
\sys_if_shell_unrestricted:T
  {
    % Set up engines
    \__diaa_set_engine:

    % Build answers jobname
    \__diaa_build_solved_jobname:

    % Set up compile commands
    \bool_if:NTF \g__diaa_use_latexmk_bool
      {
        \__diaa_build_latexmk_questions_cmd:
        \__diaa_build_latexmk_solved_cmd:
      }
      {
        \__diaa_build_latex_questions_cmd:
        \__diaa_build_latex_solved_cmd:
      }

    % Build questions
    \sys_shell_now:x
      {
        \str_use:N \g__diaa_cmd_questions_str
      }

    % Build answers
    \bool_if:NT \g__diaa_build_two_files_bool
      {
        \sys_shell_now:x
          {
            \str_use:N \g__diaa_cmd_solved_str
          }
      }
    \stop
  }

\ExplSyntaxOff

\begin{document}
\begin{questions}
  \question One of these things is not like the others; one of these things is
  not the same. Which one is different?
  \begin{oneparchoices}
    \choice John
    \choice Paul
    \choice George
    \choice Ringo
  \end{oneparchoices}
	\begin{solution}
	the solution
	\end{solution}
\end{questions}
\end{document}