And now - Linux
To get to the directory of the binaries (this is the default directory of course, if you have installed Starbound somewhere else, the path should be different):
cd ~/.steam/steam/steamapps/common/Starbound/linux
To unpack:
./asset_unpacker <.pak file> <output directory>
To pack:
./asset_packer <source directory> <output .pak file>
And if you want it even simpler, use alias to turn them into “commands”
alias asset_packer="~/.steam/steam/steamapps/common/Starbound/linux/asset_packer"
alias asset_unpacker="~/.steam/steam/steamapps/common/Starbound/linux/asset_unpacker"
EDIT: (Apart from the paths used, this is no different on macOS)