Why do I need to run sudo apt-get update [duplicate]

Question

Possible Duplicate:
Why run sudo apt-get update before installing a package?

If I wanted to install some software like below.

Why do I need to run the

sudo apt-get update

before installing the program?

So for example.

sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-4.7

Answer 1

The 'sudo apt-get update' is necessary because apt-get has to know where to download the packages from.

Apt-get doesn't know that you added a new PPA. So, the program has to update the sources.

Answer 2

The 'sudo apt-get update' is not all the time necessary : it tells apt to checks software repository for newer versions and update local cache.

It is not necessary to do it all the time. But if your local software cache is old, and you try to install something new, apt would try to fetch a (old) version that is no longer available and will complain. Then you can do 'sudo apt-get update' to force refresh.

'sudo apt-get update' is also done automatically by desktop "software update" when it check for updates.

This is why you must 'sudo apt-get update' when adding a new PPA or any kind of Debian software repository, if you don't, your local apt cache won't know new available packages.

Just a reminder :

apt-cache policy vim

will show you available and installed versions for package "vim".