percona-toolkit/docs/user/environment.rst

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ENVIRONMENT
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The environment variable \ ``PTDEBUG``\  enables verbose debugging output to STDERR.
To enable debugging and capture all output to a file, run the tool like:

.. code-block:: perl

    PTDEBUG=1 pt-table-checksum ... > FILE 2>&1

Be careful: debugging output is voluminous and can generate several megabytes
of output.