I would like to see a "Scan this file with avast" as a context menu option, so, like in windows, you can scan select files by right clicking.
Uhm, what's "the context menu"? On Linux, this is something not-as-definite as in the unified world of Windows. Yes, _some_ X-windows window managers offers similar menus, but always in their own style, and thus, such feature must be made for each particular manager in the proper way.
But, some window managers also offer user-definable items in their context menus, which can be scripted for invoking avastcmd, just have a look in the configuration center (usually managers have such configuring utility) for this option.
Which particular X11 window manager are you using?
regards,
pc