backing up the Nexus configuration

Using the built-in procedure is risky - to say the least. I was unable to make it work (the restore part, I mean).

You can do this (see https://stackoverflow.com/a/45986526/651288 ):

java -jar /opt/sonatype/nexus/lib/support/nexus-orient-console.jar

CONNECT plocal:/nexus-data/db/component admin admin

#to export, use this:
export database component-export
#to import, use this:
drop database
create database /nexus-data/db/component
import database component-export.json.gz

I also do this:

gunzip component-export.json.gz
python -m json.tool component-export.json > component-exportPRETTY.json


and the JSON file is quite readable, probably you can even stick it into a Git repo and relax.

From an operational point of view, Nexus 3 is simply a joke.