Getting started with ACPI
Tags: Fedora
Hardware stuff, especially laptops is usually window$/manufacturer specifc so getting some of it work work can be a bit of a pain. In FC1 low batteries & standby is handled by APM, in FC2 upwards APM is replaced with ACPI & this is a quick text on getting going.
There are two folders & a service :
- /etc/acpi/actions
- /etc/acpi/events
- /etc/init.d/acpid
The service , I think runs in the background monitoring the hardware, you can then put events into /etc/acpi/events and get things to happen when the deamon spots them .
I originally tested this on an HP Omnbook 6000, but now I’m using a compaq nc6000 . To replace apm I need to
- Monitor Battery / Power Status
- React when power button / standby pressed
- Do Something when the lid is closed
Documentation :
http://acpi.sourceforge.net/documentation/index.html
To get started I had a look at the documentation 
also there is an example in FC3 ( /etc/acpi/events/sample.conf )
Sample.conf
# This is a sample ACPID configuration event=button/power.* action=/sbin/shutdown -h now
the Documentation explains that all acpi “real time” info is stored in /proc/acpi .
If you look /proc/acpi there is a directory “button” and in there “power” , so sample.conf seems to suggest if anything under “power” changes do action (in this case shutdown the machine).
Armed with this I could “test” with something a little quicker than switching on/off the machine; like opening / closing the lid….
So, in /proc/acpi/button/lid/C139 (the C139 is laptop specific) there is a file called state, and when it will tell you if the lid is open or closed (just in case you didn’t know ), now it is this “lid” directory that acpi will be looking for, so what you do is create a file called /etc/acpi/events/lid and put in it
/etc/acpi/events/lid
event=button/lid.*
action=/etc/acpi/actions/lid “%e”
You’ll notice that the action is set to /etc/acpi/actions/lid “%e” this will be a script, but you could point it to any script/executable you like. In /etc/acpi/actions/lid I have put
/etc/acpi/actions/lid
# Make sure we recieve something !
if [ "$1" = "" ]
then
echo “Script Failed, NO Input !!!”
exit
fi
# what we got from the acpi deamon
acpi_in=”$1″
# what the lid code was…
lidstatus=`echo “$acpi_in” | awk ‘{print $4}’`
# the lid code in dec
dec_lidstatus=`printf “%d\n ” 0x$lidstatus`
#prepare for the maths
x=$dec_lidstatus
y=2
# was the lid code odd or even ?
mod=$(($x % $y)) # get the remainder of x / y and assign it to variable mod
# If %mod is 0 then the number is even, if it is 1 then the number is odd !
if [ $mod = 0 ]
then
# even numbers mean the lid is open !!!
# wall lid open
exit
else
# odd means closed !!!
# wall lid closed
# so lets lock the desktop !
su - nick -c “xscreensaver-command -lock”
fi
Now, this needs a little explaining…..
There a couple of things you need to know about acpi
- acpi runs as root, so any event get’s executed with root priveleges
- %e is an incremental number, odd numbers for lid closed & even for lid open
- To get these changes to work you need to restart the acpi service
So as you can see my lid open/close script gets a little complicated. To play if you remove the comments in from of the “wall” commands, save an open an empty shell, when you open & close the lid you get broadcast messages from root ! 
or if you just leave it as it is it’ll try and run “nick”’s screen saver
Now my old laptop had both a standby or sleep button and a power button, but getting events to happen is exactly the same, simply create a script doing what you want it to do, create a file ine /etc/acpi/events and set the event to event=button/power.* (or event=button/sleep.*) and voila !
Now finally, monitoring battery status, thankfully gnome already has a nice applet
or if you want to check it your self /proc/acpi/battery/C138/state (again C138 is laptop specific) this file will tell you !
I hope you’ve found this of some use !
