I'd like to be able to do the following in the search box:
shell [cmd] [-linux]
to search for "questions containing the 'shell' term AND having the 'cmd' tag AND NOT having the 'linux' tag"
Note that "shell" is not a TAG !
You can if you precede the tag in the search with a minus (-):
shell [cmd] -[linux]
As workaround you can set
linux to your ignore list temporarily (or permanently, if you don't like it at all).