[shep]

I am interested in open source database for personal use cases. By personal use cases I mean to store address book (simple, I know
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The other needed property of that database is to be Cross-platform, so I can use the same file on Windows.

mail/abook may meet some your needs. The ncurses interface is quick, reliable and easy to learn. It imports and exports to many formats

 --outformat <outputformat> ] [ --outfile <outputfile> ]
              Converts <inputfile> in <inputformat> to <outputfile> in
              <outputformat> (defaults are abook, stdin, text and stdout).

              The following inputformats are supported:
              - abook abook native format
              - ldif ldif / Netscape addressbook
              - mutt mutt alias
              - pine pine addressbook
              - csv comma separated values
              - palmcsv Palm comma separated values
              - vcard VCard addressbook

              The following outputformats are supported:
              - abook abook native format
              - ldif ldif / Netscape addressbook (.4ld)
              - mutt mutt alias
              - html html document
              - pine pine addressbook
              - vcard VCard addressbook
              - csv comma separated values
              - palmcsv Palm comma separated values
              - elm elm alias
              - text plain text
              - spruce Spruce address book
              - wl Wanderlust address book
              - bsdcal BSD calendar
              - custom Custom output format, see below

Although it has mutt leanings, I have also used it with mail/sylpheed and mail/claws-mail by using the following ~/.abook/abookrc entry

set mutt_command = sylpheed --compose

Here are some screenshots.

Another option is to look at both the client and server implementations of ldap. For example databases/openldap. The littlebrother db, databases/lbdb, I believe can utilize several different databases.