With the recent release of macOS Sierra, running react-native init app{:sh} hangs until the process is exited. After spending some time diagnosing the issue, I narrowed it down to watchman hanging. To verify, run watchman version{:sh}.

To fix the issue, you need to delete watchman{:sh} and reinstall it:

rm -rf /usr/local/var/run/watchman/ && brew uninstall watchman && brew install watchman