The command to add an external is:

  1. Navigate to the directory you want the external brought into.
  2. sudo svn propset svn:externals 'http://{path to repo}/ {name of dir}' .
  3. Commit then update to receive the external files.

Note the quotes around the {path to repo} and {name of dir}. Also note the dot at the end of the line to indicate the property setting on the current directory.

My initial playing with this proves changes made & committed in the externally included files will arrive in the external repo when it is updated. I’m impressed. With great power comes great responsibility – be warned.