Look into syncthing: https://syncthing.net/

Also note: this should be available in your OS repo/package manager for most linux distros, which is the easiest way to install it there.

You would have to find a way to mount the document directories of all the distros. If they are encrypted, that isn't possible. If they are not, then you might be able to mount all of them and run rsync against each in turn. That would be a lot of messy fstab files, but I suppose it could be done. The easiest way would be to use a separate data partition, which would be mounted by each distro when it boots. That's the way I would go. What I do is use one of my computers as an NFS server, with multiple USB drives attached, and I sync them and keep my data there, backed up. But my situation is different from yours, because I'm running multiple computers with only one OS on each. The most practical way I can think of for you is to use a data partition accessible to all your distros. You would need to keep that backed up of course, and that could be done in many ways.

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IF all of the operating systems were on the same hardware, it would make sense to simply allocate a data partition for this data and always have it mount to whichever distro is booted. No sync needed, you would always be using the same data!

If you have different hardware platforms, then a NAS solution sounds optimal.

In all of the shared solutions backups are important. An error or flaw in any ONE session can destroy data for ALL of them. Rsync does not replace proper backups, but is one level of security.

A suboptimal, but useful, compromise is to rsync ( or using another tool that uses the rsync libs) between two or more locations that might be shared between these distros. This is a really inefficient way to do the job, but as long as your volume is relatively low it should serve. As you try to scale it up the problems begin to become burdensome. Only you, the OP, can determine when the delays become unacceptable.

I just installed syncthing, and it seems to be useful. It conveniently provides a default sync folder, named Sync, in the home directory, into which you put files you want to sync. That's useful for me, sort of, but since I'm already running an NFS server it's not that big. It appears you can sync more folders, but I haven't investigated all the possibilities yet.

IMO it's better to have one data partition for everybody, and back that up to multiple places - locally, in the cloud, or both. If just locally, back up to multiple drives. If the data is really important, keep a copy offsite. It all depends on the value of the data and one's level of paranoia. My data isn't really that important for the most part, but I keep it backed up on multiple local drives and on multiple cloud services. If it's sensitive, it's encrypted before it goes to the cloud.

Syncthing basically will 'mirror' any folder(s) you feed into it, across however many machines you integrate together. It is not meant to be a centralized server, or even a 'backup' as such - in other words there's not good support for redundancy or file versioning in the application itself, but if you overlaid it on redundant storage + had some versioning SOP you could solve that. The default folder is basically just a template. If you really wanted to you could do some really potentially dumb things like sync /home or something, but that can quickly create havoc (as I'm sure you can imagine). The basic concept is to be a 'personal cloud' - so you have a common folder/tree/whatever that exists on every machine in the cluster, on whatever storage device you have it set-up on, and it makes sure the data is equal across all of them (and uses an rsync-like delta implementation to be bandwidth/time friendly after the initial clone). You can run it with a server in the mix too, but that can get silly because say you have 20TB on a server, probably your client machines don't have that much space (hence why you have the server), you aren't really going to sync the whole thing, but if you had a frequently used set of data you could just tag that folder (or folders) and work on it 'locally' on the client machines, knowing it was being reflected back (almost instantly, basically limited only by the speed of your network) to the server (here's where the 'is not a backup' comes into play though - if you delete/bork something, that too is reflected). It does require a different way of thinking about things to make it useful, and I'm not trying to be an evangelist - I played with it for a few months and went back to a more standard 'file server' approach mostly because it was simpler to plot out for myself. That said, I would take syncthing over any 'backup' software that puts files into a non-standard format (e.g. that bundles them up into compressed blobs/formats), because it behaves more like a smarter version of rsync (but in some cases 'too smart for its own good' is probably more apt) which should in theory mean recovery is easier/faster.

Some people tend to use sync and backup interchangeably, although they are not. Dropbox syncs, adding and removing files on both the server and local folders as they change. I need the synchronization of Dropbox, because I need changes to some files there to be available to multiple devices, and they all need to be in sync. That's not the same thing as a backup. Both methods do have their uses, though. I sync with Dropbox, but back up to Google Drive as well as external drives locally. I don't really have an actual use for syncthing now, I just think it has the potential to be useful for some people, and I plan to play with it just for my own education, which will never be finished.

Adding a Linux operating system to your computer has become much easier and more automated than in the past, whether you intend to use it as your sole operating system or install it in a separate partition as an alternative to a Windows or Mac system. Most versions of Linux now come on CD-ROM, DVD, or USB flash drive. Depending on the flavor of Linux, you can either order the discs from the manufacturer at no cost according to the rules of the open-source movement or burn an image of the entire system onto your own discs. Despite the automation, some problems can still happen during an initial Linux installation. These are commonly caused by incorrect communication with hard drive components or peripheral devices. Being able to recognize these issues is the first step in being able to perform some basic corrections that will fix them in the vast majority of cases.

And what, if anything, has that to do with the current topic?

It's a direct copy/paste of a website selling commercial software. It's an ad. Reported.

If I remember right syncthing's genesis was to replace Dropbox/Google Drive/etc with something that you both control and can audit, as opposed to nebulously defined 'cloud services' from parts unknown. If you're already using a commercial/cloud/etc service it would be redundant.

On the other hand there is some comfort in using a utility that cannot be blocked, ended, or locked by decision of some faceless corporation or government totally out of your control! Backups and duplication do not only protect you from HARDWARE issues!

Syncthing is indeed redundant to me for now. I've been using Dropbox for years, and I know it works. I don't yet know enough about syncthing to know if it will fill my needs. I plan to give it a try though, and see. For now I don't need it, but my needs could change. An option I use for some syncing is rclone, which is an rsync frontend with additions to allow one-way syncs to cloud services as well as local drives/directories. I'll investigate syncthing in good time.

I have also used Dropbox for yoars, but I now find it restrictive in the storage allowed and the accounts. With my total of five OS's and the size of storage I need, dropbox is no longer an option. Don't get me wrong I try and pay for what I use; donating to various software company's and Linux systems, but paying $17.50 / month is too steep for me. If they had a one time payment I may consider, but rhere are other less expensive options.

Unfortunately the 'pay just for the sake of paying' is rapidly becoming the business model. Syncthing may be worth a look on your end, as there are no capacity or usage restrictions beyond what your own equipment can support.

Yes thanks; I'll give it a try.