latex -----------------> lua first call
       <-------- return full file name "/home/me/etc..../file_name.m"



         (full file name)
 latex -------------------> second lua call
          <---- return final result

documentclass{article}
IfFileExists{luatex85.sty}{usepackage{luatex85}}{}

ifdefinedHCode% detect tex4ht
usepackage[utf8]{luainputenc}
usepackage[T1]{fontenc}
else
usepackage{fontspec}
fi
usepackage{luacode}

begin{luacode*}
require 'lfs'
-- first LUA function
function fullpath(curDir,fileName)
    local pathseparator = package.config:sub(1,1)

    if os.type == "windows" then
       fileName=string.gsub(fileName,"/",pathseparator)
    end

    local name=table.concat({lfs.currentdir(),fileName}, pathseparator);
    print(name)
    tex.print(name)
end

-- second LUA function
function format(fileName)
    --for now, just print it.
    tex.print("\verb|"..fileName.."|")
end
end{luacode*}

newcommandfullpath[2]{luadirect{fullpath(luastring,#2)}}
newcommandformat[1]{luadirect{format(#1)}}

begin{document}
%ask LUA to find full path name of some file. This should work
%on both windows and Linux

%how to save this result in Latex?
%fullpath{jobname.tex}{"sub_folder/foo7_1.tex"}  %causes an error.

%edef causes error
defname{fullpath{jobname.tex}{"sub_folder/foo7_1.tex"}}

%send the name back to lua. causes error if uncomment
%format{name}
end{document}