Where is the "sites/all" folder for an ubuntu linux installation? I have installed drupal 7 from the ubuntu repositories.  All works well but I cannot locate the folder to install additional modules.

All the documentation says I need to upload files to the "sites/all" folder. This either doesn't exist or is somewhere I have yet to look.

Any pointers would be much appreciated.

ETA: I had to create the "/all/modules/contrib" subfolders manually, under the "/usr/share/drupal7/sites" absolute path.
It's referring to the "sites/all" folder in your website's root folder. 

The location of your site's files will have been set by your web server software (probably Apache, in this case).

Have a look here:  /var/www/

and see if there are any directories in there that look like "httpdocs" or anything with your site's name in it or anything.