documentclass{minimal}
makeatletter
defapp@exe{immediatewrite18}
definputAllFiles#1{%
  app@exe{ls #1/*.txt | xargs cat >> jobname.tmp}%
  InputIfFileExists{jobname.tmp}{}
  AtEndDocument{app@exe{rm -f #1/jobname.tmp}}}
makeatother
begin{document}

inputAllFiles{.}% from the current dir

end{document}

documentclass{article}

newcommand*{MaxNumOfChapters}{10}% Adjust these two settings for your needs.
newcommand*{MaxNumOfSections}{6}%

usepackage{pgffor}%

begin{document}
foreach c in {1,2,...,MaxNumOfChapters}{%
    foreach s in {1,2,...,MaxNumOfSections}{%
        IfFileExists{Chapterc/Sections} {%
            input{Chapterc/Sections}%
        }{%
                % files does not exist, so nothing to do
        }%
    }%
}%
end{document}

newcommand*{ListOfFiles}{%
    Chapter1/Section1,
    Chapter1/Section2,
    Chapter1/Section3,
    Chapter2/Section1,
    Chapter2/Section2
}%

documentclass{article}%
usepackage{pgffor}%

input{ListOfFiles}%

begin{document}%
foreach c in ListOfFiles {%
    input{c}%
}%
end{document}

directlua{dofile("inputall.lua")}

bye

function dirtree(dir)

  if string.sub(dir, -1) == "/" then
    dir=string.sub(dir, 1, -2)
  end

  local function yieldtree(dir)
    for entry in lfs.dir(dir) do
      if not entry:match("^%.") then
        entry=dir.."/"..entry
          if lfs.isdir(entry) then
            yieldtree(entry)
          else
            coroutine.yield(entry)
          end
      end
    end
  end

  return coroutine.wrap(function() yieldtree(dir) end)
end


for i in dirtree(lfs.currentdir()) do
  local filename = i:gsub(".*/([^/]+)$","%1")
  tex.sprint("\input " ..  filename .. " ")
end