ddcctl

You might want to check this out then: https://haim.dev/posts/2020-07-28-dual-monitor-kvm/ That was on HN earlier this year. Basically it's software that you run and set up to detect when certain peripherals connect/disconnect and have it switch the input on the monitor automatically. I've been using it and, while not always perfect, it actually works pretty well, especially considering the price. If you don't always want the monitor to switch inputs (sometimes I just want to quickly switch my keyboard/mouse to my laptop to do something but don't want to switch monitor inputs) you can also just set up shortcuts or scripts (I created an Alfred workflow, for example) and use something like https://github.com/kfix/ddcctl to switch monitor inputs.

Alongwith ddctrl (https://github.com/kfix/ddcctl), these aliases serve all my external monitor brightness needs: alias verydull="~/Software/ddcctl/ddcctl -d 1 -b 3 -c 15" alias dull="~/Software/ddcctl/ddcctl -d 1 -b 6 -c 35" alias decent="~/Software/ddcctl/ddcctl -d 1 -b 10 -c 40" alias medium="~/Software/ddcctl/ddcctl -d 1 -b 25 -c 50" alias bright="~/Software/ddcctl/ddcctl -d 1 -b 30 -c 50" alias morebright="~/Software/ddcctl/ddcctl -d 1 -b 35 -c 60" alias superbright="~/Software/ddcctl/ddcctl -d 1 -b 100 -c 80"

I have been using this command line tool to control my external monitor from my Mac. I created few Automator scripts calling it to make the monitor either dimmed or bright. https://github.com/kfix/ddcctl

Thank you for that! The built in Mac display brightness controls only seem to work for the Apple Cinema Displays - they do nothing on my Asus monitor. I just tried ddcctl ( https://github.com/kfix/ddcctl ) - and it worked perfectly over HDMI through a HDMI switch. I didn't even know what to look for - so this comment was a lot of help.

For a command-line option, ddcctl (https://github.com/kfix/ddcctl) works nicely in my experience, with a Dell Ultrasharp connected via USB-C. I have a script hacked together to fetch the internal display's brightness once a minute and apply it, with a scaling factor, to the external, so that they're both effectively controlled by the laptop's ambient light sensor. It's really nice for working in a room with lots of windows. A more batteries-included alternative, which appears to also provide brightness sync, is Lunar (https://lunar.fyi/). I haven't had a chance to try it, but the Github repo makes it look pretty solid.