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The migration tool requires a terminal to stay open while the migration proceeds. This may be cumbersome if the migration takes a very long time. In that case, do the following:
1. Start the migration tool running in a terminal as instructed in the link.
2. Once devices and gateways start getting migrated, type Ctrl Z to pause.
3. Type bg followed by Enter to resume the operation in the background. The operation will resume but will still output messages to stdout.
4. You can close your terminal, but the migration will continue.