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Is there a way how to offline safely synchronize directories between sd card on Android which is not rooted and PC with Ubuntu?

Safely = using secured connection between the devices (for example using ssh).

Offline = without Internet access, but the PC and smartphone are connected through LAN.

Synchronize directories = a process which transfers data between smartphone and PC in a way that at the end of the process, both devices contain exactly same data in selected directories (preferably, but not necessarily, excluding filtered files). The process is able to detect deleted data so it doesn't copy file which was originally on both devices and later deleted on one of them.

My systems

  • Android 6.0.1 (not rooted)
  • Xubuntu 18.04 64 bit PC

What I already tried/checked (without success)

Unison

Unison for Android

  • https://play.google.com/store/apps/details?id=com.nerdysoftware.unison
  • Not secure - from application description: "No SSH access is necessary - clients connect directly via socket. Because Unison clients connect to your device over a direct socket connection instead of over SSH, the server should only be run when your Android device is connected to a trusted network, like a home WiFi. Running the server on other networks could expose files on your device to untrusted parties."
  • Also seems to use older version of Unison (this application requires exactly same versions on server and client to allow data transfer).

Rsync

SSHDroid

  • https://play.google.com/store/apps/details?id=berserker.android.apps.sshdroid
  • For using SSH with another software (see bellow)
  • Cannot write/modify SD card using the ssh connection. I went to settings -> Applications -> SSHDroid -> Permissions: storage is enabled. There I opened context menu -> all permissions: reading SD card available. Modification/deleting data on SD card available. In other words, I checked the application has permission to modify the SD card: it has.

SSHFS with SSHHelper

SSHFS with SimpleSSHD

What I already tried (with mixed/partial success)

Combination: Unison (on PC) + some SSH server (Android) + SSHFS (on PC)

  • In case I could find working SSH server; use SSHFS to mount remote Android device locally and then use Unison for synchronization.
  • Results:
    • When the SSH is actually rw, then it works, but it's so slow that it's impractical - it must ask for every single file and directory over the network. Of course, other apps (like Unison) must do that too, but they don't ask for every single file sequentially (not: client send request for single file - latency - server checks and sends response - client receive response and go to next file).
    • Unison requires permission to rename files; it always creates temporary files and directories and when everything is downloaded and double-checked, then old files (if any) are deleted and new files renamed. So if renaming is not allowed, it doesn't work.

SSH/SFTP Server - Terminal

  • https://play.google.com/store/apps/details?id=net.xnano.android.sshserver
  • This actually works but has two problems;
    • It requires entering password several (>10) times when mounted as sshfs (I tried trivial one and and made sure it's not typo) and doesn't support ssh keys. I was able to solve this problem with a hack: zenity --password | sshfs user@host:/ "$PWD/data" -o reconnect,password_stdin, but I'm not sure how secure it is.
    • This ssh is unbelievably slow. Much slower when compared with SSHDroid and SSHHelper (not only with sshfs, but also when I simply connect using ssh and terminal; typing and responses are really, really slow). Maximum speed is 2 MB/s, but most of the time it's ~500kB/s.
    • Cannot rename files (permission error) so it makes it unusable with Unison (on PC).

Syncthing

  • https://play.google.com/store/apps/details?id=com.nutomic.syncthingandroid
  • Problem: cannot modify SD card (on non-rooted systems) and devs are not going to fix this anytime soon.
  • The only solution is to use obscure, hard-to-reach deep directory structure: /storage/XXXX-XXXX/Android/data/com.nutomic.syncthingandroid/files.
  • But for this particular directory, it works. It's speed is about 5 - 10 MB/s and I suspect that it's being throttled down by SD card itself (or Android). For now, I'm using this solution, but the directory is really inconvenient.
  • Smaller problems:
    • It doesn't support following symlinks and until someone (other than current devs) implement it, it's not going to be supported.

What problem I'm trying to solve

I "just" want to synchronize data on file level (some pdfs, images, etc.) between my PC and Android smartphone. I use multiple devices (desktop, laptop, etc.) and use Unison (note: I do not insist on this app) to synchronize data between them. Because I switch environment quite offten, I need it to work on one button press and to be fast as possible (which, for example, Unison does greatly). Because I work on different environments, I need to cover my cases: sometimes I'm without Internet access. Other times I'm connected to untrusted network.

This is also reason why I cannot use cloud (like Dropbox): it's slower than two devices communicating directly and sometimes I'm without Internet connection so I cannot synchronize even when the devices are connected on same network.

The reason why I want to sync with SD card is simply because it has bigger capacity so my files fits.

  • 1
    Upvoted for thorough spade work :) Welcome to our site – beeshyams Nov 3 '18 at 18:48
  • 1
    Not tried, but maybe installing Nextcloud on your Linux machine and the Nextcloud app on your Android device? Nextcloud comes with a sync client AFAIK. Alternatively, if Playstore is acceptable, FolderSync works via SFTP (ie. SSH) and wouldn't require additional software on your Linux machine. – Izzy Nov 3 '18 at 20:23
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    No, it's not the fastest – but that never bothered me. It syncs in bg at 3am while I sleep, and is done when I get up. Conflicts are the real bad thing here indeed – which I usually notice when having copied an entire folder to a new device (FS seems unable to detect their state and doesn't even seem to bother trying CRC or something else to find out). But unfortunately, no better ideas here. Be welcome to check my corresponding app listing, there are many candidates I didn't try myself ;) – Izzy Nov 4 '18 at 16:01
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    @Izzy I guess it depends on our workflows - I usually do some work to the last minute, then sync, take my things and run to catch a train. (About FS - yeah, it seems to me exactly as you wrote.) Your list of apps is really interesting, thanks for sharing. I'll try to find some time to check out some apps from it :). – Tom Nov 4 '18 at 16:11
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    @Izzy Well, it seems good, except it doesn't work on SD card; see links in my question (I mention the app there). – Tom Nov 4 '18 at 17:09

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