Automatically dim brightness if battery is low (command fail)

Question

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

score 0 · Accepted Answer · 2019-08-11T06:56:39.797

0

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 ;)

edited Aug 11 '19 at 06:56

answered Aug 10 '19 at 13:47

the last command gives me this message : bash: [: 76-le: unary operator expected – zyydoosh Aug 10 '19 at 17:46
@MohamedSaad if [ "$A"-le "$B" ] is missing a space, it should be if [ "$A" -le "$B" ], typos happens... – mook765 Aug 10 '19 at 19:12

score 0 · Answer 2 · answered Aug 10 '19 at 15:05

0

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

answered Aug 10 '19 at 15:05

WinEunuuchs2Unix

102,282

it gives the same massage, either when I write BAT* or BAT0 or BAT1 – zyydoosh Aug 10 '19 at 17:40
@MohamedSaad It might be a 19.04 thing. I'm using 16.04. I'll reboot into 19.04 later and check it out there. – WinEunuuchs2Unix Aug 10 '19 at 19:40
@MohamedSaad I tied Ubuntu Unity 19.04 and it works ok. It might be a Ubuntu Gnome 19.04 problem so I'll reboot into that environment (where desktop icons disappear sigh) and test it. – WinEunuuchs2Unix Aug 10 '19 at 19:48
@MohamedSaad Ubuntu Gnome 19.04 works ok. My last check will be Ubuntu Gnome Wayland (instead of xorg) 19.04. – WinEunuuchs2Unix Aug 10 '19 at 19:58

Automatically dim brightness if battery is low (command fail)

2 Answers2