856:index expected got nil

local trueCursor={term.getCursorPos()}

local redirectBufferBase = {
    write=
	  function(buffer,...)
	    local cy=buffer.curY
	    if cy>0 and cy<=buffer.height then
		  local text=table.concat({...}," ")
		  local cx=buffer.curX
		  local px, py
		  if buffer.isActive and not buffer.cursorBlink then
		    term.native.setCursorPos(cx+buffer.scrX, cy+buffer.scrY)
		  end
		  for i=1,#text do
		    if cx<=buffer.width then
			  buffer[cy][cx]={char=string.char(text:byte(i)),textColor=buffer.textColor,backgroundColor=buffer.backgroundColor}
		    end
		    cx=cx+1
		  end
		  buffer.curX=cx
		  buffer[cy].isDirty=true
		  if buffer.isActive then
		    buffer.drawDirty()
		    if not buffer.cursorBlink then
			  trueCursor={cx+buffer.scrX-1,cy+buffer.scrY-1}
			  term.native.setCursorPos(unpack(trueCursor))
		    end
		  end
	    end
	  end,
	
    setCursorPos=
	  function(buffer,x,y)
	    buffer.curX=math.floor(x)
	    buffer.curY=math.floor(y)
	    if buffer.isActive and buffer.cursorBlink then
		  term.native.setCursorPos(x+buffer.scrX-1,y+buffer.scrY-1)
		  trueCursor={x+buffer.scrX-1,y+buffer.scrY-1}
	    end
	  end,
	
    getCursorPos=
	  function(buffer)
	    return buffer.curX,buffer.curY
	  end,
	
    scroll=
	  function(buffer,offset)
	    for j=1,offset do
		  local temp=table.remove(buffer,1)
		  table.insert(buffer,temp)
		  for i=1,#temp do
		    temp[i].char=" "
		    temp[i].textColor=buffer.textColor
		    temp[i].backgroundColor=buffer.backgroundColor
		  end
	    end
	    if buffer.isActive then
		  term.redirect(term.native)
		  buffer.blit()
		  term.restore()
	    end
	  end,
	
    isColor=
	  function(buffer)
	    return buffer._isColor
	  end,
	
    isColour=
	  function(buffer)
	    return buffer._isColor
	  end,
	
    clear=
	  function(buffer)
	    for y=1,buffer.height do
		  for x=1,buffer.width do
		    buffer[y][x]={char=" ",textColor=buffer.textColor,backgroundColor=buffer.backgroundColor}
		  end
	    end
	    if buffer.isActive then
		  term.redirect(term.native)
		  buffer.blit()
		  term.restore()
	    end
	  end,
	
    clearLine=
	  function(buffer)
	    local line=buffer[buffer.curY]
	    local fg,bg = buffer.textColor, buffer.backgroundColor
	    for x=1,buffer.width do
		  line[x]={char=" ",textColor=fg,backgroundColor=bg}
	    end
	    buffer[buffer.curY].isDirty=true
	    if buffer.isActive then
		  buffer.drawDirty()
	    end
	  end,
	
    setCursorBlink=
	  function(buffer,onoff)
	    buffer.cursorBlink=onoff
	    if buffer.isActive then
		  term.native.setCursorBlink(onoff)
		  if onoff then
		    term.native.setCursorPos(buffer.curX,buffer.curY)
		    trueCursor={buffer.curX,buffer.curY}
		  end
	    end
	  end,
	
    getSize=
	  function(buffer)
	    return buffer.width, buffer.height
	  end,
	
    setTextColor=
	  function(buffer,color)
	    buffer.textColor=color
	    if buffer.isActive then
		  if term.native.isColor() or color==colors.black or color==colors.white then
		    term.native.setTextColor(color)
		  end
	    end
	  end,
	
    setTextColour=
	  function(buffer,color)
	    buffer.textColor=color
	    if buffer.isActive then
		  if term.native.isColor() or color==colors.black or color==colors.white then
		    term.native.setTextColor(color)
		  end
	    end
	  end,
	
    setBackgroundColor=
	  function(buffer,color)
	    buffer.backgroundColor=color
	    if buffer.isActive then
		  if term.native.isColor() or color==colors.black or color==colors.white then
	    term.native.setBackgroundColor(color)
		  end
	    end
	  end,
	
    setBackgroundColour=
	  function(buffer,color)
	    buffer.backgroundColor=color
	    if buffer.isActive then
		  if term.native.isColor() or color==colors.black or color==colors.white then
	    term.native.setBackgroundColor(color)
		  end
	    end
	  end,
  
    resize=
	  function(buffer,width,height)
	    if buffer.width~=width or buffer.height~=height then
		  local fg, bg=buffer.textColor, buffer.backgroundColor
		  if width>buffer.width then
		    for y=1,buffer.height do
			  for x=#buffer[y]+1,width do
			    buffer[y][x]={char=" ",textColor=fg,backgroundColor=bg}
			  end
		    end
		  end

		  if height>buffer.height then
		    local w=width>buffer.width and width or buffer.width
		    for y=#buffer+1,height do
			  local row={}		 
			  for x=1,width do
			    row[x]={char=" ",textColor=fg,backgroundColor=bg}
			  end
			  buffer[y]=row
		    end
		  end
		  buffer.width=width
		  buffer.height=height
	    end
	  end,
	
    blit=
	  function(buffer,sx,sy,dx, dy, width,height)
	    sx=sx or 1
	    sy=sy or 1
	    dx=dx or buffer.scrX
	    dy=dy or buffer.scrY
	    width=width or buffer.width
	    height=height or buffer.height
	  
	    local h=sy+height>buffer.height and buffer.height-sy or height-1
	    for y=0,h do
		  local row=buffer[sy+y]
		  local x=0
		  local cell=row[sx]
		  local fg,bg=cell.textColor,cell.backgroundColor
		  local str=""
		  local tx=x
		  while true do
		    str=str..cell.char
		    x=x+1
		    if x==width or sx+x>buffer.width then
			  break
		    end
		    cell=row[sx+x]
		    if cell.textColor~=fg or cell.backgroundColor~=bg then
			  --write
			  term.setCursorPos(dx+tx,dy+y)
			  term.setTextColor(fg)
			  term.setBackgroundColor(bg)
			  term.write(str)
			  str=""
			  tx=x
			  fg=cell.textColor
			  bg=cell.backgroundColor			 
		    end
		  end
		  term.setCursorPos(dx+tx,dy+y)
		  term.setTextColor(fg)
		  term.setBackgroundColor(bg)
		  term.write(str)
	    end
	  end,
	
    drawDirty =
	  function(buffer)
	    term.redirect(term.native)
	    for y=1,buffer.height do
		  if buffer[y].isDirty then
		    term.redirect(term.native)
		    buffer.blit(1,y,buffer.scrX,buffer.scrY+y-1,buffer.width,buffer.height)
		    term.restore()
		    buffer[y].isDirty=false
		  end
	    end
	    term.restore()
	  end,
	
    makeActive =
	  function(buffer,posX, posY)
	    posX=posX or 1
	    posY=posY or 1
	    buffer.scrX=posX
	    buffer.scrY=posY
	    term.redirect(term.native)
	    buffer.blit(1,1,posX,posY,buffer.width,buffer.height)
	    term.setCursorPos(buffer.curX,buffer.curY)
	    term.setCursorBlink(buffer.cursorBlink)
	    term.setTextColor(buffer.textColor)
	    term.setBackgroundColor(buffer.backgroundColor)
	    buffer.isActive=true
	    term.restore()
	  end,
	
    isBuffer = true,
  
  }
  

function createRedirectBuffer(width,height,fg,bg,isColor)
   bg=bg or colors.black
   fg=fg or colors.white
   isColor=isColor~=nil and isColor or term.isColor()
   local buffer={}
   for y=1,height do
	 local row={}
	 for x=1,width do
	   row[x]={char=" ",textColor=fg,backgroundColor=bg}
	 end
	 buffer[y]=row
   end
   buffer.scrX=1
   buffer.scrY=1
   buffer.width=width
   buffer.height=height
   buffer.cursorBlink=false
   buffer.textColor=fg
   buffer.backgroundColor=bg
   buffer._isColor=isColor
   buffer.curX=1
   buffer.curY=1
  
   local meta={}
   local function wrap(f,o)
	 return function(...)
		 return f(o,...)
	   end
   end
   for k,v in pairs(redirectBufferBase) do
	 if type(v)=="function" then
	   meta[k]=wrap(v,buffer)
	 else
	   meta[k]=v
	 end
   end
   setmetatable(buffer,{__index=meta})
   return buffer
end

os.loadAPI("api")
api.createWindow("shellwindow", 10, 10, 10, 10, "rom/programs/shell")
api.run("shellwindow")
api.drawWindows()