Safe Startup

function forward(num)
  num = num or 1
  for i = 1, num do
    if turtle.forward() then
      if f == 0 then
        z = z - 1
        update("z",z)
      elseif f == 1 then
        x = x + 1
        update("x",x)
      elseif f == 2 then
        z = z + 1
        update("z",z)
      elseif f == 3 then
        x = x - 1
        update("x",x)
      end
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function back(num)
  num = num or 1
  for i = 1, num do
    if turtle.back() then
      if f == 0 then
        z = z + 1
        update("z",z)
      elseif f == 1 then
        x = x - 1
        update("x",x)
      elseif f == 2 then
        z = z - 1
        update("z",z)
      elseif f == 3 then
        x = x + 1
        update("x",x)
      end
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function up(num)
  num = num or 1
  for i = 1, num do
    if turtle.up() then
      y = y + 1
      update("y",y)
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function down(num)
  num = num or 1
  for i = 1, num do
    if turtle.down() then
      y = y - 1
      update("y",y)
      checkFuel(returning)
      return true
    else
      return false
    end
  end
  return true
end

function left(num)
  num = num or 1
  for i = 1, num do
    turtle.turnLeft()
    f = (f+3)%4
    update("f",f)
  end
end

function right(num)
  num = num or 1
  for i = 1, num do
    turtle.turnRight()
    f = (f+1)%4
    update("f",f)
  end
end

function update(var,value)
    if not tostring(var) then
        error("var needs to be a string")
    end
    value = tostring(value)
    local str = fs.open("info","r").readAll()
    if str:find(var) then
        str = str:gsub(var.."%=.-%;",var.."="..value..";")
    else
        str = str .."\n"..var.."="..value..";"
    end
    local h = fs.open("info","w")
    h.write(str)
    h.close()
end

--     0 (-z)
-- 3 (-x)  1 (+x)
--     2 (+z)

function home()
  returning = true
  while y ~= 0 do
    if y < 0 then
      turtle.digUp()
      up()
    elseif y > 0 then
      turtle.digDown()
      down()
    end
  end
  if z > 0 then
    while f ~= 0 do
      left()
    end
  elseif z < 0 then
    while f ~= 2 do
      left()
    end
  end
  while z ~= 0 do
    forward()
  end
  if x > 0 then
    while f ~= 3 do
      left()
    end
  elseif x < 0 then
    while f ~= 1 do
      left()
    end
  end
  while x ~= 0 do
    forward()
  end
end

shell.run("info")
home()