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get rid of Processing triggers for man-db taking too long time
method 1 (remove auto-update to prevent rebuild cache)
The Processing triggers for man-db step is only executed if the file /var/lib/man-db/auto-update exists. This is an empty file with the sole purpose of controlling this behavior, so it can be safely removed to disable this time-consuming and arguably unnecessary process. Ref
method 2 (upgrade man-db)
This bug was fixed in the package man-db - 2.10.0-2 Ref
method 3 (remove man-db)
sudo apt-get remove --purge man-db
method 4 (config man-db to not re-build cache)
echo "set man-db/auto-update false" | sudo debconf-communicate && sudo dpkg-reconfigure man-db
method 5 (re-index man-db cache)
sudo rm -rf /var/cache/man
sudo mandb -c