terraform/website/source/docs/internals/debugging.html.md

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docs	Debugging	docs-internals-debug	Terraform has detailed logs which can be enabled by setting the TF_LOG environmental variable to any value. This will cause detailed logs to appear on stderr

Debugging Terraform

Terraform has detailed logs which can be enabled by setting the TF_LOG environmental variable to any value. This will cause detailed logs to appear on stderr.

To persist logged output you can set TF_LOG_PATH in order to force the log to always go to a specific file when logging is enabled. Note that even when TF_LOG_PATH is set, TF_LOG must be set in order for any logging to be enabled.

If you find a bug with Terraform, please include the detailed log by using a service such as gist.