
Add the service account of the source bucket to the destination bucket.You may download JSON key if you need to use it later, but this is optional. Create a service account for your destination bucket.Take note of the path to this file as you will need it later. Make sure you create a JSON key and download this to a safe place on your computer. Create a service account for your source bucket.However, there are a few things you should setup properly to prevent issues. See more configuration options in the official docs. If you try to transfer individual files, you will get an error.

Please note that /dir refers to a directory under the main bucket. This is the quickest ways to do it: gsutil -m rsync -r gs://bucket-source/dir gs://bucket-destination/dir
