Changed code in original message.

Thanks!

Try this:

brightness="/sys/class/backlight/intel_backlight/brightness"
if [ -f $brightness ] && [ "$(cat $brightness)" != "3000" ]; then
	tee $brightness 3000
fi

	tee $brightness 3000

...will wait for input instead of writing "3000" to the desired location. And create a file named "3000".

Can you please try your code before publishing it?

echo 3000 > $brightness

Such "tee" tricks typically are used because "sudo echo something >somewhere" still does the output redirection as the calling user because the redirection is done before "sudo" is run. "sudo tee somewhere" writes with "root" rights to "somewhere"  because "tee" opens "somewhere" and that happens with "root" superpowers due to "sudo". This all does not really matter in "rc.local" because the whole script already runs with "root" rights.

Switching the interpreter of "rc.local" to "/usr/bin/zsh" is not a good idea because "zsh" is not an essential package. System scripts should use "/bin/sh".

But ok: It's your system.

brightness="/sys/class/backlight/intel_backlight/brightness"
if [ -f $brightness ] && [ "$(cat $brightness)" != "3000" ]; then
        echo 3000 > $brightness

fi

Edit: Corrected code error.

Translate all BASHisms to standard SH because:

I tried it with just

brightness="/sys/class/backlight/intel_backlight/brightness"
tee $brightness <<< 3000

but that didn't work, i guess im terrible with SH

anyway i solved the problem by changing the rc.local from #!/bin/sh -e to #!/usr/bin/zsh   
thx guys

#!/bin/sh -e

It seems there is the problem, I was running under bash.

(yeti@kumari:23)/tmp$ cat -n meep.sh 
     1  #!/bin/sh -e
     2  
     3  if [[ 1 = 1 ]] ; then
     4          echo 'boo!'
     5  fi
     6  
     7  tee <<< Moo.
(yeti@kumari:23)/tmp$ ./meep.sh 
./meep.sh: 3: ./meep.sh: [[: not found
./meep.sh: 7: ./meep.sh: Syntax error: redirection unexpected

I suggest put something like this on your /etc/rc.local to debug your startup:

date >> /var/log/rc.local.test

if the file its not created, maybe its because /etc/rc.local hasnt execution permissions.

problem is since i compiled a new kernel, my screen backlight will go very dark once open-rc starts booting up
i tried the bootloader thing that people suggest on the internet but it only works randomly sometimes

so i wrote this short script that i want to execute with root rights during startup

brightness="/sys/class/backlight/intel_backlight/brightness"
if [[ -f $brightness ]] && [[ "$(cat $brightness)" != "3000" ]]; then
tee $brightness <<< 3000
fi

i tried to add it into /etc/rc.local but that does nothing...
any ideas how to make it work?