To keep the system up to date with the latest fixes and security advisories, upgrades should be done regularly. System upgrade is a two steps process: update the list of available software and, if necessary, upgrade the system. Both operation can be achieved with aptitude, with different options.

Get the latest list of available packages on the repositories:

root@server:~# aptitude update
Get:1 http://ftp.pt.debian.org squeeze Release.gpg [835 B]
Ign http://ftp.pt.debian.org/debian/ squeeze/main Translation-en
Ign http://ftp.pt.debian.org/debian/ squeeze/main Translation-en_US
Get:2 http://ftp.pt.debian.org squeeze Release [89.9 kB]
Get:3 http://security.debian.org squeeze/updates Release.gpg [835 B]
Get:4 http://ftp.pt.debian.org squeeze/main amd64 Packages/DiffIndex [2,038 B]
Ign http://security.debian.org/ squeeze/updates/main Translation-en
Ign http://security.debian.org/ squeeze/updates/main Translation-en_US
Get:5 http://security.debian.org squeeze/updates Release [44.6 kB]
Get:6 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-0256.49.pdiff [10.1 kB]
Get:7 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-0256.49.pdiff [10.1 kB]
Get:8 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-0256.49.pdiff [10.1 kB]
Get:9 http://security.debian.org squeeze/updates/main amd64 Packages [56.3 kB]
Get:10 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-1524.42.pdiff [8,117 B]
Get:11 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-1524.42.pdiff [8,117 B]
Get:12 http://ftp.pt.debian.org squeeze/main amd64 2010-10-22-1524.42.pdiff [8,117 B]
Fetched 215 kB in 3s (71.0 kB/s)

Current status: 5 updates [+5].

There are 5 updates available for our system installation:

Upgrading our system is also very easy:

root@server:~# aptitude safe-upgrade
The following packages will be upgraded:
  libc-bin libc6 locales tasksel tasksel-data
5 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 9,776 kB of archives. After unpacking 176 kB will be freed.
Do you want to continue? [Y/n/?] y
Get:1 http://ftp.pt.debian.org/debian/ squeeze/main tasksel-data all 2.84 [105 kB]
Get:2 http://ftp.pt.debian.org/debian/ squeeze/main tasksel all 2.84 [87.2 kB]
Get:3 http://security.debian.org/ squeeze/updates/main libc-bin amd64 2.11.2-6+squeeze1 [745 kB]
Get:4 http://security.debian.org/ squeeze/updates/main libc6 amd64 2.11.2-6+squeeze1 [4,143 kB]
Get:5 http://security.debian.org/ squeeze/updates/main locales all 2.11.2-6+squeeze1 [4,697 kB]
Fetched 9,776 kB in 20s (480 kB/s)
Preconfiguring packages ...
(Reading database ... 16181 files and directories currently installed.)
Preparing to replace libc-bin 2.11.2-6 (using .../libc-bin_2.11.2-6+squeeze1_amd64.deb) ...
Unpacking replacement libc-bin ...
Processing triggers for man-db ...
Setting up libc-bin (2.11.2-6+squeeze1) ...
(Reading database ... 16181 files and directories currently installed.)
Preparing to replace libc6 2.11.2-6 (using .../libc6_2.11.2-6+squeeze1_amd64.deb) ...
Unpacking replacement libc6 ...
Setting up libc6 (2.11.2-6+squeeze1) ...
(Reading database ... 16181 files and directories currently installed.)
Preparing to replace tasksel-data 2.82 (using .../tasksel-data_2.84_all.deb) ...
Unpacking replacement tasksel-data ...
Preparing to replace tasksel 2.82 (using .../archives/tasksel_2.84_all.deb) ...
Unpacking replacement tasksel ...
Preparing to replace locales 2.11.2-6 (using .../locales_2.11.2-6+squeeze1_all.deb) ...
Unpacking replacement locales ...
Processing triggers for man-db ...
Setting up locales (2.11.2-6+squeeze1) ...
Generating locales (this might take a while)...
  en_US.UTF-8... done
Generation complete.
Setting up tasksel-data (2.84) ...
Setting up tasksel (2.84) ...

Current status: 0 updates [-5].

5 packages upgraded, the system in now up to date.

• Debian Reference, 2.2. Basic package management operations (http://www.debian.org/doc/manuals/reference/ch02.en.html#_basic_package_management_operations)