shelljoin

local quote_function
--ZFUNC-escapeshellarg-v2
local function escapeshellarg( str ) --> esc

  local function posix_quote_argument(str)
    str = str:gsub( "[$`\"\\]", "\\%1" )
    return '"' .. str .. '"'
  end

  local function windows_quote_argument(str)
    str = str:gsub('[%%&\\^<>|]', '^%1')
    str = str:gsub('"', "\\%1")
    str = str:gsub('[ \t][ \t]*', '"%1"')
    return str
  end

  if not quote_function then
    quote_function = windows_quote_argument
    local shell = os.getenv('SHELL')
    if shell then
      if '/' == shell:sub(1,1) and 'sh' == shell:sub(-2, -1) then
        quote_function = posix_quote_argument
      end
    end
  end

  return quote_function(str)
end

--ZFUNC-isoneshellarg-v1
local function isoneshellarg( str ) --> res
   --ZFUNC-utf8codes-v1
   local function utf8codes( str )
      return str:gmatch( "[%z\1-\127\194-\244][\128-\191]*" )
   end

   local function isspace( str )
      return str == " " or str == "\t" or str == "\r" or str == "\n"
   end

   local inbuff = false
   local escaped, doubleQ, singleQ, backQ = false, false, false, false

   for r in utf8codes( str ) do
      if escaped then-----------------------------------------------------------
         inbuff = true
         escaped = false

      elseif r == '\\' then-----------------------------------------------------
         if singleQ then inbuff = true
         else escaped = true
         end

      elseif isspace( r ) then--------------------------------------------------
         if singleQ or doubleQ or backQ then inbuff = true
         else return false
         end

      elseif r == "`" then------------------------------------------------------
         if singleQ or doubleQ then inbuff = true
         else backQ = not backQ
         end

      elseif r == '"' then------------------------------------------------------
         if singleQ or backQ then inbuff = true
         else doubleQ = not doubleQ
         end

      elseif r == "'" then------------------------------------------------------
         if doubleQ or backQ then inbuff = true
         else singleQ = not singleQ
         end

      else----------------------------------------------------------------------
         inbuff = true

      end
   end

   if escaped or doubleQ or singleQ or backQ then return false end

   if inbuff then return true end

   return false --no argument
end

--ZFUNC-shelljoin-v2
local function shelljoin( args ) --> line
   local tmp = {}

   for _, a in ipairs( args ) do
      if not isoneshellarg( a ) then
         a = escapeshellarg( a )
      end
      table.insert( tmp, a )
   end

   return table.concat( tmp, " " )
end

return shelljoin

local t = require( "taptest" )
local shelljoin = require( "shelljoin" )

line = shelljoin{ "here", "are", "two words" }
t( 'here are "two words"', line )

line = shelljoin{ "John", "says", 'Hello"s World' }
t( 'John says "Hello\\"s World"', line )

t()