You removed the package nginx with apt-get remove, but /etc/nginx still contains the old edited configuration and a fresh install picks it up again. Which command discards those configuration files as well?

LPIC-1 Exam 101-500, objective 102. Linux installation and package management medium

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The options

Correct apt-get purge nginx

Correct. purge removes the package and its conffiles, leaving the package in the not-installed rather than config-files state.

Not correct apt-get autoremove

Wrong. autoremove deletes packages that were pulled in as dependencies and are no longer needed; it does not touch conffiles of a package you already removed.

Not correct apt-get clean

Wrong. clean empties the downloaded .deb cache under /var/cache/apt/archives and has no effect on installed or removed packages.

Not correct dpkg -r nginx

Wrong. -r is the low-level equivalent of remove and deliberately keeps conffiles; the purge counterpart is dpkg -P.

Why

Debian distinguishes removing from purging so that a package can be taken away and reinstated with its settings intact. remove (dpkg -r) deletes the program files but preserves files marked as conffiles, leaving the package in state rc, which dpkg -l shows. purge (dpkg -P) deletes those too and runs the package's postrm purge script.

Where this comes from

Cited
manual page apt-get(8)

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