• 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, use the following command:

parser.lua

Now command prompt would change from "sh:grub > " to "lua:grub > " to indicate the current script engine is Lua. To switch back to sh mode, use the following Lua command:

grub.run("parser.sh")

Run Lua scripts

GRUB 2 support the use of #! to specify parser for current file. At the end of the script, it would switch back to default parser automatically. You can use this feature to run Lua script from the main configuration file grub.cfg. For example:

grub.cfg:

configfile hello.lua
echo hello from sh

hello.lua:

#!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:

#!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:

#!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/]