I highly recommend reading this list of built-in macOS terminal commands: https://ss64.com/mac/

Some commands I use often:

- `pbcopy` and `pbpaste` to copy and paste data via the clipboard

- `date -u` to give the date in UTC

- `networkQuality` is speedtest but built-in to macOS

- `caffeinate` prevents your computer from going to sleep (it's the same as the old Caffeinate program, but built-in now)

- `open ...` to open a file, as if you double-clicked it in Finder

- `security` gives you command-line access to the Keychain.