27 February 2011

synqa: file synch via SSH

synqa is a Ruby gem which can be used to script the uploading of files to a remote server via SSH. Typically you would use it for maintaining a static website.


gem install synqa



I wrote synqa as a simple alternative to rsync, which I couldn't get to work with Cygwin rsync running on Windows Vista talking to my website server on

I also wrote it partly as a way of gaining experience with SSH, putty, Ruby, rake and gems (which is my excuse for not trying really, really hard to solve my original problem with rsync).

How It Works

synqa works by constructing content trees for the local and remote directories which are to be synchronized. Each content tree consists of a list of all sub-directories of the base directory, all files within the base directory, and for each file, the hexadecimal cryptographic hash, and, also the local time that the calculation of the relevant content tree started.

The remote content tree is calculated by calling a hashing utility on the remote system, such as sha256sum (or sha256 -r for some systems), and using find and xargs to loop through all the files.

Files are copied using a specified SCP client (I recommend pscp, but scp should work). Remote commands are executed using a specified SSH client (I use plink, but ssh will work, as long as you don't have a setup where interactive password entry is required). Remote files are deleted by remotely executing the command rm via the SSH client.

A possible future optimisation would be to use psftp for all copying and remote deletion, which would allow all file update operations to occur within a single SSH session, thus minimising the overhead due to SSH session initiation.


To avoid having to calculate the hashes of all local and remote files every time, synqa has the option to specify that the local and remote content trees can be saved to local files.

Given the existence of either of these cached content trees, synqa can then do the following:

If a synqa script has been configured to use cached content trees, and you want to force synqa to fully recalculate either content tree, simply delete the relevant content tree first. (The sample rakefile includes a clean target which deletes both content tree files.)