• LUASupport

GRUB 2 has integrated Lua as an alternative script engine. Apparently available in SVN (rev 2218) since May 16, 2009.

As of 2009-09-26 Lua is not anymore in official GRUB. It has moved to grub-extras now.

Enter/Exit Lua mode from command line

To enter Lua from command line run simply:

lua

Now the command prompt will change from "grub> " to "lua> ". Press the escape key to exit lua and return to grub's sh.

Run Lua scripts

To run lua scripts, you simply pass the script name to the lua command:

grub.cfg:

lua hello.lua
echo hello from sh

hello.lua:

print("hello from lua")

GRUB library

The GRUB library allows accessing GRUB facilities from Lua. Currently, it contains the following functions:

grub.run

Execute a GRUB command, which is passed as a single string. Return grub_errno, the error message is returned as second value if grub_errno is not 0.

Example:

err_no,err_msg = grub.run("ls /")
print(err_no,err_msg)

grub.getenv

Get GRUB environment variable. Allow multiple input.

Example:

prefix,root = grub.getenv("prefix", "root")

grub.setenv

Set GRUB environment variable.

Example:

grub.setenv("aa", "Hello")
grub.run("set")

Examples

hanoi tower

Here is a script to solve the hanoi problem, it shows basic Lua constructs such as variable, assignment, control structure and (recursive) function.

hanoi.lua:

function hanoi(n,a,b,c)
  if (n == 1) then
    print("move from", a, "to", c)
  else
    hanoi(n-1, a, c, b)
    hanoi(1, a, b, c)
    hanoi(n-1, b, a, c)
  end
end

hanoi(3, 1, 2, 3)

season

Here is a script to demonstrate the interaction between Lua and GRUB. It uses date hook to get the current month, then calculate the season. You can do other tricks based on this, for example, using a different background image for each season.

season.lua:

grub.run("insmod datehook")
m = tonumber(grub.getenv("MONTH"))

if (m >= 3) and (m <= 5) then
  print("spring")
elseif (m >= 6) and (m <= 8) then
  print("summer")
elseif (m >= 9) and (m <= 11) then
  print("autumn")
else
  print("winter")
end

Lua Reference

Complete reference to Lua language [http://www.lua.org/manual/5.1/]