Zotero

The only way to do it for free is to sync the metadata with Zotero servers and sync the files with another software. I use syncthing.

Set up sync

• Create a Zotero account
• Open Zotero
• Go to Settings > Sync > Login > Set up syncing
• Fill the username and password
• Login
• Uncheck:
  • Sync attachment files in My Library using ...
  • Sync attachment files in group librearies using Zotero storage
• Close settings
• Top right corner, there is a button to sync. Press it.
• Wait for sync to finish.

Backup

Copy all of your files from ~/Zotero/storage/

Restore

Copy all of your files to ~/Zotero/storage/