pip install --user git+https://github.com/schwa/half-moon-tagging.git

$ tag -h
tag - Mavericks command line tool for tagging files

Usage:
  tag <path>
  tag --list <path>...
  tag --add <tag> <path>...
  tag --set <tag> <path>...
  tag --remove <tag> <path>...
  tag (-h | --help)
  tag --version

Options:
  -l --list          List all tags at each path
  -a --add=<tag>     Add one or more (comma separated) tags to paths
  -r --set=<tag>     Set one or more (comma separated) tags on paths
  -r --remove=<tag>  Remove one or more (comma separated) tags from paths
  -h --help          Show this screen.
  --version          Show version.
$ touch example.txt
$ tag --list example.txt
example.txt ()
$ tag --set Green example.txt
$ tag --list example.txt
example.txt (Green)
$ tag --add Examples example.txt
$ tag --list example.txt
example.txt (Green, Examples)
$ tag --set Red,Green,Purple example.txt
$ tag --list example.txt
example.txt (Red, Green, Purple)

BSD 2-clause