what's more effiecient? acpid handler.sh or C-program
 

Hello,

What's more efficient, to use a 100 line C-program to control keyboard backlight or acpid?

That depends on how well the C program is coded, and how many
resources it eats. A shell (bash) will always consumes ~ 2 MB
of RAM, and most likely show a bit more overhead than a well-
written C program.

At the end of the day the only way to find out is to write it
and try ;}