Ah, the age old question "how does one display laptop battery information at the bottom of a screen terminal?"

Well finally, there is an answer. Actually, there are probably quite a few different answers, but this one is my answer. Since I wanted to replicate some of the functionality of my other battery monitor (vattery), this battery monitor can perform commands when the battery level drops to a certain percent. By the way, this requires that acpi is installed and running.

Let's start with the code to read and process the battery info.

#!/bin/sh
#we need a function to run when the battery gets low
low_battery_action ()
{
    echo "the battery is getting low"
}
#what files have our needed data?   
state_file="/proc/acpi/battery/BAT0/state"
info_file="/proc/acpi/battery/BAT0/info"
#by default, the action has not been performed
action_performed=0
#at what percent does the action get run?
action_percent=10
# start the loop
while [ 1 ]; do
    #get info about the battery
    battery_current_charge=`cat $state_file | grep remaining | awk '{print $3}'`
    battery_ac_state=`cat $state_file | grep charging | awk '{print $3}'`
    battery_last_total_charge=`cat $info_file | grep last | awk '{print $4}'`
    battery_percent=$((100*battery_current_charge/battery_last_total_charge))
    #display the info
    echo "battery: $battery_percent% $battery_ac_state"
    if [ $action_performed -eq 1 ] # has the low power action been performed?
    then    # if "yes"
        if [ $battery_ac_state = "charging" ] # is the battery charging?
        then #if "yes"
            #is the battery percent greater than the action percent?
            if [ $battery_percent -gt $action_percent ]
            then
                #reset the action_performed to false
                action_performed=0
            fi
        fi
    else #the low power action has not been performed
        #is the battery percent less than or equal to the action percent?
        if [ $battery_percent -le $action_percent ]
        then 
            #perform the action
            low_battery_action
            #set the action_performed to true
            action_performed=1
        fi
    fi
    # take a two second nap
    sleep 2
done

I have this saved as battery_state.sh in my ~/bin directory, which is in my $PATH. If ~/bin isn't part of your $PATH, don't worry, you can just make a directory called "scripts" and put the file in the "scripts" directory. The file should be set to executable by issuing

chmod +x ~/bin/battery_state.sh

Adjust the command accordingly if your file is not in the ~/bin directory.

Since the code doesn't do anything major when the battery charge drops to 10 percent, change line 5 to whatever command or commands you wish to occur. I'm a big fan of

espeak -g7 -ven-us+f3 "Hello? is anyone there? my battery power is low. please plug in my charger."

To adjust when the command is run, edit line 13 and set the action_percent to the percent of charge that the battery needs to reach for the command to run.

Now it is time to setup screen to run the battery_state.sh script and process the scripts output when screen is run. To do this, the ~/.screenrc file needs to be edited (if the file doesn't exist, just create the file).

The following should be added to the .screenrc file

hardstatus alwayslastline
backtick 1 0 0 battery_state.sh
hardstatus string '%1`'

Now start screen.
If the battery_state.sh file is not in a directory that is part of your $PATH, adjust the backtick 1 0 0 battery_state.sh line to be backtick 1 0 0 /path/to/your/battery_state.sh using the actual path to your battery_state.sh file.

Now quit reading, and go write something.