Bash Prompt HOWTO:
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11.11. Laptop Power

If you have a laptop with APM installed, try the following PROMPT_COMMAND to create an environment variable ${battery} you can add to your prompt. This will indicate if AC power is connected and percentage power remaining. AC power is indicated by a "^" (for on) and a "v" (for off) before the percentage value.

function prompt_command {
	#  As much of the response of the "apm" command as is 
	#  necessary to identify the given condition:
   NO_AC_MESG="AC off"
   AC_MESG="AC on"

   APMD_RESPONSE="$(apm)"
   case ${APMD_RESPONSE} in
      *${AC_MESG}*)
         ACstat="^"
         ;;
      *${NO_AC_MESG}*)
         ACstat="v"
         ;;
   esac

   battery="${temp##* }"
   battery="${ACstat}${battery}"
}

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