I have a laptop with Mint and Fedora; a PC with Mint, Fedora and Windows 10. I've gone through a lot of reviews on suncing software and haven't come up with anything. I'm not against paying for the service, but would like a one time fee and not monthly. I have Insync now and it's not too bad, but uses MS One drive and Google drive. I would like to get away from being tied to either and just use a dedicated file manager.
TIA.

What all, or what exactly, are you wanting to keep in sync?

Just my personal files/documents

Between Windows and Linux or Linux to Linux? Makes a huge difference. Linux to Linux I use rsync. I don't keep any data at all on Windows so have no idea there, other than having a central storage location like a NAS or a cloud service.

+1 for rsync, it will work with many work flows.

A file server / NAS might be more appropriate depending on work flow. That could be as simple as SFTP via the file manager or as fancy as Ceph. Or rsync over SSH or SSHFS to the NAS. I'd avoid SMB/CIFS though. If you are worried about the NAS being always on, depending on your router and netork specifics, it might be set to power off and then activated through Wake-on-LAN packets.

A commercial option, more like tar than rsync, would be Tarsnap. The client is not quite open source but the source is available, ostensibly.

I use rclone. It will sync with most cloud services, including Dropbox, Google Drive, and Microsoft's offerings. I used Insync for some time, but it just doesn't work well enough for me. Rsync does the job after configuring it correctly. I don't know if rsync is in the Fedora repositories, but it is in Debian, and thus probably for Mint. It uses rsync, but adds the ability to easily sync with cloud services as well as local syncs.

I do have Windows, but all I do with it is keep it updated and spend 99.9% of my computer time on Linux. I have just added Ubuntu 20.4 so now have on my PC Ubuntu, Mint, Fedpra amd W10. On my laptop Fedora and Mint; no Windows.
I'll check our rsync. I don't bother wiht any docs on Windows so like you it's all between Linux's. At present I am using insync, but that uses Google driive or Windows One Drive. I want to get away from relying on MS or Google and synk folders and files from my file manager be it Caja or what ever. Thanks for the help.

I hope to avoid using MS Google or Dropbox in favour of limiting the syncing mm stuff to within my own network.

Yeah I had a NAS once, but wasn't all that happy with it; mind you it was a few years ago.

So what do you plan to sync to? Local drives, something in the cloud, what? You haven't provided much information. Rclone will sync to most cloud services and any local drives. There are other packages available for local sync if that's what you want. The basic sync package is rsync, and most other sync packages are just frontends for it, but it won't do cloud sync by itself. Without more information about where you want to sync to, it's hard to make an informed recommendation.

I sync to my local Synology NAS, which I absolutely love. It was pricey and I probably could have built one but I am lazy so this was easier. All my Linux VMs, FreeBSD Vms and Macs get backed up to it. Linux and FreeBSD use rsync exclusively to back up user documents or configuration files.

Not exclusively. Tar is commonly used for backups.

Sorry, I wasn't clear: I use rsync exclusively to back up user files and configs. I rarely use tar because I have so much disk space, compression is not needed. I meant in my use case, not in general; poorly worded on my part!

I also rarely use tar, but it's often useful. I probably should use it more. Several backup programs use it, but I seldom use it directly. Linux offers lots of choices for almost eveything, sometimes too many ISTM. But too many is always better than too few.

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All I wish to do is sync my documwns between two computers with a combined total of five Linux OS's
My reasoning is if I switch between OS's, my documuments will be identical.
i.e. If I edited a file on Fedora and reboot to Mint or Ubuntu that same file will be updated to the recent edit.
I back up docs to USB sticks and Backup OS's with Acronis True Image so I'm totally covered for data loss; so backing up isn't the issue.
Actually insync works pretty well. I just wanted to get away from Google Drive and One Drive.