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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.
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Once devices and gateways start getting migrated, type Ctrl Z to pause.
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Type bg followed by Enter to resume the operation in the background. The operation will resume but will still output messages to stdout.
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You can close your terminal, but the migration will continue.