How can I remove all installed packages and keep only base system packages?

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One why to do this is by dumping all the packages from a clean minimal installation (you can use a vm for that)

root@tempvm# rpm -qa --qf "%{NAME}\n" > file1

and then look for all the un-common packages.

cat file1 file2 | sort | uniq -u

this will give you all the extra packages that where installed over the base system. then you can pipe this to xargs and remove all the packages.

cat file1 file2 | sort | uniq -u | xargs dnf remove [-y**]

** add the -y only after you made sure nothing critical will be removed.