My preferred tool for managing bibliographies is JabRef, an Open Source software written in Java. Its GUI makes it convenient to manage bibliographies which are stored in the BibTeX / BibLaTeX format. It interfaces well with LaTeX and Office applications. The tool works well on Linux and is available from the Ubuntu repository.
Unfortunately, newer versions of JabRef from version 4.x don’t work together with Java versions 9 or higher. Since Ubuntu ships Java 11 by default, this has been causing me problems. For now I am using Java 8 (also available in the Ubuntu repo) and sticking to JabRef vers. 3.8.2.
Just now I discovered that JabRef version 5.0-alpha has been released. This version supposedly works with Java 11. I briefly tried it and it does indeed seem to work, although I will stick to 3.8.2 for now until the 5.x-version has matured.
Update April 2020: In the meantime, JabRef version 5.0.1 has been released and I have now switched to that. It’s quite a big change (user interface etc.) but so far it is working well.