How can I synchronize a directory on my Windows system to a remote Linux machine, and then update the remote copy efficiently when files in the directory are changed? Nowadays a common answer is to use Dropbox or the like - install it on both systems and let it do its magic. It works, but what are other options without using a third-party serivce?

One way is to set up a git repository on Linux, a working directory on Windows and then git push changes to the repo. A drawback is that a commit must be done before each synchronization, and every version of each file will be stored in the repository which is not always necessary.


Another approach is to use the rsync tool originally created for Unix-like OS but a couple of Windows ports exist such as cwRsync (proprietary) and DeltaCopy (presumably open source). Both are cygwin-based, although do not require a complete cygwin installation being bundled with the required dlls.

Here is the command line I ended up using to synchronize a directory to a remote linuxhost where myuser is an existing user on that system for the ssh connection:

rsync --stats --delete --chmod=Du=rwx,Dg=rx,Do=rx,Fu=rw,Fg=r,Fo=r --modify-window=5 --exclude=".git" -c -r /cygdrive/C/win/path/ myuser@linuxhost:/home/user/linux_path

--stats provides some stats about the transfer, which I find useful:

Total bytes sent: 3,573
Total bytes received: 130

sent 3,573 bytes  received 130 bytes  1,481.20 bytes/sec
total size is 341,524  speedup is 92.23

--delete is required to ensure files deleted from local directory are deleted remotely as well. Otherwise, a file synced once and then removed from the local directory will not be deleted remotely.

--chmod specifies permissions assigned to directories and files created during synchronization.

--modify-window=5 is a workaround for differences in handling timestamps between Linux and Windows file systems. Otherwise, rsync thinks all files have changed and sends the entire contents. Stats above indicate this actually works.

--exclude=".git" excludes the .git sub-directory from synchronization. My local directory is a git repo and I only need the contents synced, without the metadata.

-c skips files from synchronization based on checksum instead of the default timestamp/size. I first thought that it should resolve the issue described in modify-window, but it didn’t.

-r for recursive mode, of course.

ssh authentication

The rsync command above works over an ssh connection, and thus requires to authenticate each time it is run which is not extremely convenient. In case of key-based authentication which I am using, it prompts for a passphrase

Enter passphrase for key '/cygdrive/c/Users/winuser/.ssh/id_rsa':

Now, this can be avoided by either setting an empty passphrase or using an ssh-agent (pageant for PuTTY). Empty passphrase adds risk of identity theft if the key file is exposed. Using ssh-agent is somewhat better, but this utility is not included in the rsync distributions I mentioned. My git distribution has ssh-agent.exe in it (msys-based), but for some reason it didnt’t work with cwRsync when I tried. Maybe will find something later, typing the passphrase is not that big of a deal for now.

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