How can I dim the screen from Terminal?

You can do it via an Applescript and run it as such:

Dimmer:

tell application "System Events"
        key code 144
end tell

Brighter:

tell application "System Events"
        key code 145
end tell

You can save these as .script files and then run them from the CLI like this:

osascript ~/Path/to/File

You may refer to this.

You can also do it this way, but that requires more setup.

You can install this command line tool from github.com/nriley/brightness. The install is pretty easy if you have homebrew:

brew install brightness

And usage is straightforward:

• Set 100% brightness: brightness 1
• Set 50% brightness: brightness 0.5

Unfortunately it doesn't work for external monitors. See Does Apple support DDC/CI for 3rd party displays via Apple's Thunderbolt to DVI adapter for background on why.

I've created a node module for this that supports setting the brightness using the CLI. See https://github.com/kevva/brightness-cli.

npm install --global brightness-cli

And then simply just run brightness from your command line.