By default, aptitude will always perform an apt-get update before installing or
upgrading. You can, however, still issue a command to perform only the update:
$ sudo aptitude update
Get:1 http://security.ubuntu.com feisty-security Release.gpg [191B]
Ign http://security.ubuntu.com feisty-security/main Translation-en_US
Get:2 http://us.archive.ubuntu.com feisty Release.gpg [191B]
...
If you want to upgrade all packages on the system, you can send along the upgrade option with aptitude. This will install any new packages waiting in the repositories (in this example, there were no new packages on hand).
$ sudo aptitude upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Building tag database... Done
No packages will be installed, upgraded, or removed.
0 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B of archives. After unpacking 0B will be used.