There are two ways to do this:
-y, --yes, --assume-yes
Automatic yes to prompts; assume "yes" as answer to all prompts
and run non-interactively. If an undesirable situation,
such as changing a held package, trying to install a unauthenticated
package or removing an essential package occurs then apt-get will abort.
Force yes; this is a dangerous option that will cause apt to continue
without prompting if it is doing something potentially harmful.
It should not be used except in very special situations.
Using force-yes can potentially destroy your system!
yes - output a string repeatedly until killed [...]
Repeatedly output a line with all specified STRING(s), or 'y'.
yes can't know anything about
apt-get internals, it is probably equivalent to
--force-yes and thus dangerous. If you still want to do it, you pipe the output of
yes | apt-get update