[luatex] How to detect option '--shell-escape'?

[Andreas Scherer]

Dear all,

According to the LuaTeX Reference, primitive '\pdfshellescape' has been 
removed and "can be implemented using Lua".

What would be a canonical way to implement this code (from 'pwebmac.tex')

  \ifpdftex \ifnum\pdfshellescape=1 {\let\\=\BS
    \global\countD=\input{|grep -c -e '\\:\\X' \jobname.tex}} \fi\fi

for use with LuaTeX? Without the '\ifnum\pdfshellescape' part, but with 
'luatex --shell-escape', the pipe is correctly executed.

How can I detect option '--shell-escape' in my macro code?

TIA,
Andreas