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Here’s a Lua function that returns the percentage of CPU used on Linux. The load includes time in user space and system space. In case you’re curious, I used `top` to get the load for a couple reasons:

1. It’s installed by default on most distros (as opposed to the sysstat tools).
2. It gives the idle time in percentage normalized to 100% (as opposed to /proc/stat which requires knowledge of the jiffy’s interval, which can vary).

function cpu_load(interval)
   interval = interval or 1
   local f = io.popen('top -bp$$ -d ' .. interval .. ' -n 2')
   local s = f:read('*all')

   -- Find the idle times in the stdout output
   local tokens = s:gmatch(',%s*([%d%.]+)%%%s*id')

   -- We're interested in the 2nd 'top' idle time
   local ii = 1
   for idle in tokens do
      if ii == 2 then
         return 100.0 - tonumber(idle)
      end
      ii = ii + 1
   end
end

You can call it from the Lua shell as:

> load = cpu_load()
> print(load)
43.7