How to get unison to back up an entire file system starting at /?

Question

I'm using unison 2.52.1 both on my linux machine (Ubuntu version 20.04) and my Mac (MacOS version 11.7).

I want to use unison to back up my entire linux filesystem (excluding a few specific directories via "ignore") to a directory on the Mac via an ssh connection.

I know how to set up this ssh-based unison transfer except for one key point: it seems that unison is not permitting the initial "root" of the source machine to be /. However, since I want to back up everything on the linux machine, I need to start at the root of the linux filesystem tree.

Does anyone know of a way to get unison to allow the source "root" to be / under linux?

Thank you very much in advance.

UPDATE: Here is the preferences file which fails, and the error message that I receive ...

root = /
root = ssh://[email protected]//Users/useronmac/backups
force = /
sshargs = -p 6543
[ "ignore" commands go here ]

This is the error I get ...

[ ...  etc. ... ]
Looking for changes
  Waiting for changes from server
Reconciling changes
         error            /  
The path / is a root directory
No updates to propagate

If I use any other name for the first "root" and for "force", it works fine. For example ...

root = /usr
root = ssh://[email protected]//Users/useronmac/backups
force = /usr

That works with no problem.

ANOTHER UPDATE

Per @Edward's suggestion, I tried running with -debug all. Nothing particularly meaningful was printed beyond what I listed above. Here are the significant stdout lines at the end of the -debug all output ...

[ ... etc. ... ]
[update] Updating archives
[update] Saving archive in /root/.unison/scb96ed4facff65e2345cb97206c90f055
[server: update] Saving archive in /Users/useronmac/Library/Application Support/Unison/scce5852e4685b7e41685f5f8f38d86e90
[update] Copying archive /root/.unison/tmb96ed4facff65e2345cb97206c90f055 to /root/.unison/arb96ed4facff65e2345cb97206c90f055
[server: update] Copying archive /Users/useronmac/Library/Application Support/Unison/tmce5852e4685b7e41685f5f8f38d86e90 to /Users/useronmac/Library/Application Support/Unison/arce5852e4685b7e41685f5f8f38d86e90
[update] Removing archive /root/.unison/tmb96ed4facff65e2345cb97206c90f055
[server: update] Removing archive /Users/useronmac/Library/Application Support/Unison/tmce5852e4685b7e41685f5f8f38d86e90
         error            /  
The path / is a root directory
No updates to propagate
Synchronization complete at 18:48:20  (0 items transferred, 1 skipped, 0 failed)
  skipped:  (The path / is a root directory)
[server: remote] Connection closed by the client

Also, I opened an issue on the unison github site, and soon thereafter, they closed it, telling me to read the entire unison docs and then to post any further questions on their mailing list. This implies to me that this root = / behavior is some sort of documented "feature", although I haven't yet found it mentioned in the docs ... although I haven't had time to read the entire documentation yet.

And I requested access to their mailing list, but I haven't been approved yet.

Anyway, I'll post whatever I find out from the unison docs and/or from their mailing list.

YET ANOTHER UPDATE

I couldn't find anything in the unison docs about / being prohibited as a "root". However, I started looking through the latest unison source code on their github site (https://github.com/bcpierce00/unison). It turns out that / is indeed hard-coded to be an illegal "root" under linux.

To see why I am making that claim, go to that unison github site and look at the src/fspath.ml file there. First of all, starting on line 45 of that file, we have this:

let isRootDir d =
(* We assume all path separators are slashes in d                            *)
  d="/" ||
  (Util.osType = `Win32 && Rx.match_string winRootRx d)

This says that on a non-Win32 OS, the isRootDir function returns "true" if the file in question is /, and it returns "false" otherwise.

Then, look at the isRootDirLocalString function starting on line 50 of that same file:

let isRootDirLocalString d =
  let d =
    if Util.osType = `Win32 then Fileutil.backslashes2forwardslashes d else d
  in
  isRootDir ((Fileutil.removeTrailingSlashes d) ^ "/")

This function also will return "true" on non-Win32 OS's for the / pathname.

Then, look at lines 380-382 of that same file:

if isRootDirLocalString realpath then
    raise (Util.Transient(Printf.sprintf
                            "The path %s is a root directory" abspath));

This code will raise an exception if the "realpath" variable is /, and you can see that the text of this exception is the exact error message that I get when I try to specify root = / in my tests.

There is no other place in the source code where this error message is printed.

Therefore, I am concluding that the prohibition of root = / is indeed some sort of intentional feature of unison.

Once I get onto the unison mailing list, I will ask if they would be willing to relax that prohibition if the "root" in question is tagged with "force".

Answer 1

Well, I found the answer to my problem: upgrade to unison 2.53.x !

I had incorrectly thought that the 2.52.x series was the latest, but it turns out that the 2.53.x version has been released for at least a little while, and when I discovered that version and installed it on my hosts, I see that "root = /" along with "force = /" has now started working.

I guess that the unison folks also have recently realized that it's safe to use "root = /" along with "force = /".