Apple - Installing fonts from terminal instead of Font Book?

You can simply copy them into /Library/Fonts (for system wide use) or ~/Library/Fonts (for use by current user).

Install fonts with the following terminal command (replace BRLNSR with your font):

cd ~/Library/Fonts && { 
    curl -O 'https://github.com/FrankHassanabad/ResumeBuilder/blob/master/fonts/src/main/resources/frankhassanabad/com/github/windowsfonts/BRLNSR.TTF'
    cd -; }

This code does the following:

1. cd into the fonts directory
2. curl downloads the font
3. pops back to the original directory

This relies on the very nice fonts github repo from Frank Hassanabad with a bunch of fonts stored - but you could change the curl url to wherever the font you want is located online.

The clever way the cd into a directory, download and pop out again came from user Atle's answer here.