Why shouldn't I use apt-get install --qq without a no-action modifier?

From man apt-get:

Note that quiet level 2 implies -y; you should never use -qq without a no-action modifier such as -d, --print-uris or -s as APT may decide to do something you did not expect.

Those options all, as suggested, don't actually install anything. What's wrong with using -qq as a quieter alternative to -y ?

1 Answer

When you use apt-get without a no-action modifier, it means you are going to perform some actual operation with packages like install, remove, upgrade etc.

Single -q means quiet. It produces output suitable for logging, omitting progress indicators. But when you use -qq which is quiet level 2, it implies quit with -y (--assume-yes). In this case no output will be produces except error.

Now why it is considered as dangerous?

Sometime apt-get removes some packages to satisfy the dependency. Using -qq in such condition may remove some package which is important for you or for the system without informing you.

So it is not considered as safe because such operation can make your system unstable.