When using Google Sites, there is currently no way to make a backup of your site, or download the site so you can host it on another server.
This command uses a tool called
wget to spider through a website
and download all the public files to the local computer. Unix users will
most likely have the wget tool already installed (if not, you can
install it via your preferred package manager), while Windows users can
get it from here.
Once wget is installed, run it with the following parameters:
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#Downloads all public pages on a Google Site wget -e robots=off -m -k -K -E -rH -Dsites.google.com http://sites.google.com/a/domain/site/
This tells wget to spider through all the links on your site and download the html files and linked content (such as images). Note that pages that aren't linked from anywhere on the site won't be downloaded.
This technique will also work for websites other than the ones hosted on Google Sites.