I can run Google Chrome from the command line using $ google-chrome, but what flag can I pass to open it with developer console already open, preferably open to the
$ google-chrome
The flag you're looking for is --auto-open-devtools-for-tabs. Please note, that you should quit Chrome before this setting will take effect.
This has worked at least since Chrome 55.0.2883.87 m (the latest version as of initial post)