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ibnSaadoosh
Asked: 2019-08-11 04:43:59 +0800 CST

Automatically dim brightness if battery is low (command fail)

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I was trying to apply these instructions on how to automatically dim brightness when battery is low, but in the answer, this command didn't work with me 

> percent_left=$((100 * `cat /sys/class/power_supply/BAT0/charge_now` / ` cat /sys/class/power_supply/BAT0/charge_full`))

It said 

cat: /sys/class/power_supply/BAT0/charge_now: No such file or directory
cat: /sys/class/power_supply/BAT0/charge_full: No such file or directory
bash: 100 *  / : syntax error: operand expected (error token is "/ ")

Any solution!! I'm a beginner Ubuntu user

power-management battery brightness system-settings 19.04

2 Answers

  1. Best Answer

    Install acpi:

    $ sudo apt install acpi

$ acpi

    View the battery percentage:

    $ echo $(acpi | cut -f2 -d, | cut -c2- | cut -f1 -d%)%

    Automatically dim brightness if battery is low:

    $ A=$(acpi | cut -f2 -d, | cut -c2- | cut -f1 -d%)
$ B=15 # if low battery is <=15.
$ if [ "$A" -le "$B" ]; then xrandr --output VGA-1 --brightness .5; fi;

    Here, VGA-1 is the screen. Value of brightness is 50%. (--brightness .5)

    I need your feedback ;)

    • 0

  2. I have the same problem on my system: 

    $ cat /sys/class/power_supply/BAT0/charge_now
cat: /sys/class/power_supply/BAT0/charge_now: No such file or directory

    The reason is sometimes it is BAT0 and sometimes it is BAT1. The solution is to search for either one like this:

    $ cat /sys/class/power_supply/BAT*/charge_now
6216000

    The BAT* wildcard lets it find either BAT0 or BAT1.

    The complete command then:

    $ percent_left=$((100 * `cat /sys/class/power_supply/BAT*/charge_now` / ` cat /sys/class/power_supply/BAT*/charge_full`))

$ echo $percent_left
100
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