rclone

rsync for object storage: one tool that speaks S3, B2, Drive, Dropbox, SFTP and dozens more.

What it does

Copies, syncs and moves files between any two configured remotes, with checksums, bandwidth limits and a dry-run mode. rclone mount exposes a remote as a local filesystem, and the crypt wrapper encrypts names and contents before upload so the provider stores nothing readable.

rclone config                       # interactive, once per remote
rclone sync ~/Documents b2:my-bucket/documents --progress --dry-run
rclone mount gdrive: ~/mnt/gdrive --vfs-cache-mode writes

Notes

sync makes the destination match the source, deletions included. Run it with --dry-run first, every time.
It mirrors rather than versions: yesterday’s file is gone once today’s sync runs. For history, back up with restic — which can use an rclone remote as its backend — and keep rclone for moving bulk data.
mount needs a FUSE implementation; see macFUSE.

Alternative to

Alternative	Type	Trade-off
rsync	Open source	The same shape for SSH hosts rather than cloud storage
Cyberduck	Open source	A window with files in it, rather than a scriptable tool
restic	Open source	Versioned snapshots, not a mirror
Syncthing	Open source	Continuous peer-to-peer sync, no cloud account anywhere