# apt-get install qemu
Reading package lists... Done
Building dependency tree... Done
The following extra packages will be installed:
  bochsbios openhackware proll vgabios
Recommended packages:
  debootstrap vde
The following NEW packages will be installed:
  bochsbios openhackware proll qemu vgabios
0 upgraded, 5 newly installed, 0 to remove and 20 not upgraded.
Need to get 4152kB of archives.
After unpacking 11.9MB of additional disk space will be used.
Do you want to continue [Y/n]? n

Deleting a package

Similarly it should come as no surprise that the following will deinstall example, however it will not remove the dependencies installed above, this will be covered later in deb-orphan

# apt-get remove example
apt-get remove gaim
Reading package lists... Done
Building dependency tree... Done
The following packages will be REMOVED:
  gaim gaim-irchelper
0 upgraded, 0 newly installed, 2 to remove and 310 not upgraded.
Need to get 0B of archives.
After unpacking 4743kB disk space will be freed.
Do you want to continue [Y/n]?
(Reading database ... 174409 files and directories currently installed.)
Removing gaim-irchelper ...
Removing gaim ...

In this last case the configuration files of the package 'gaim' (basically how its setup) were not deleted from the system. You could use them when installing the same package again and it would work with these settings

If you want the config files deleted also, issue this command:

# apt-get remove --purge gaim
Reading package lists... Done
Building dependency tree... Done
The following packages will be REMOVED:
  gaim*
0 upgraded, 0 newly installed, 1 to remove and 309 not upgraded.
Need to get 0B of archives.
After unpacking 4678kB disk space will be freed.
Do you want to continue [Y/n]? Y
(Reading database ... 174405 files and directories currently installed.)
Removing gaim ...
Purging configuration files for gaim ...

Here the file /etc/gaim was not only deleted all config files were also purged/deleted, IF however an application holds or files because they were not created during the installation of the package that we want to delete, they will remain.

After the database is updated, one can find out which packages have new/newer versions:

# apt-show-versions -u |grep upgrade
libpam-runtime/unstable upgradeable from 0.79-1 to 0.79-3
passwd/unstable upgradeable from 1:4.0.12-5 to 1:4.0.12-6
teclasat/unstable upgradeable from 0.7m02-1 to 0.7n01-1
libpam-modules/unstable upgradeable from 0.79-1 to 0.79-3
fonts-ttf-west-european/unstable upgradeable from 1.3-9 to 1.3-1-1
fonts-ttf-decoratives/unstable upgradeable from 1.3-1 to 1.3-1-1
sysv-rc/unstable upgradeable from 2.86.ds1-3 to 2.86.ds1-4
libpam0g/unstable upgradeable from 0.79-1 to 0.79-3
unionfs-utils/unstable upgradeable from 1.0.14+cvs2005092318w-M1 to 1.0.14+cvs2005092908w-M1libpam-runtime/unstable upgradeable from 0.79-1 to 0.79-3
passwd/unstable upgradeable from 1:4.0.12-5 to 1:4.0.12-6
teclasat/unstable upgradeable from 0.7m02-1 to 0.7n01-1
libpam-modules/unstable upgradeable from 0.79-1 to 0.79-3
fonts-ttf-west-european/unstable upgradeable from 1.3-9 to 1.3-1-1
fonts-ttf-decoratives/unstable upgradeable from 1.3-1 to 1.3-1-1
sysv-rc/unstable upgradeable from 2.86.ds1-3 to 2.86.ds1-4
libpam0g/unstable upgradeable from 0.79-1 to 0.79-3
unionfs-utils/unstable upgradeable from 1.0.14+cvs2005092318w-M1 to 1.0.14+cvs2005092908w-M1

The update of a single package including its dependencies can for example (for gcc-4.0) be done with:

apt-get install gcc-4.0
Reading package lists... Done
Building dependency tree... Done
gcc-4.0 is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and xxx not upgraded

Upgrade of an Installed System

If the whole system is to be upgraded this is achieved through "dist-upgrade". It is extremely important to first check which packages will be upgraded:

The upgrade should take place without the GUI in a textconsole (in this case you can first download the packages and after that finish the dist-upgrade in the textconsole.) This small inconvenience can save you hours of work later.

Just downloading

One of the little known , however one of the best options is the -d flag:

apt-get dist-upgrade -d

The -d allows you to download the dist-upgrade without installing it, giving you the option to proceed or abort like this

apt-get dist-upgrade -d
xxx upgraded, 30 newly installed, 25 to remove and 2 not upgraded.
Need to get 342MB of archives.
After unpacking 3916kB of additional disk space will be used.
Do you want to continue [Y/n]?
Y
(Y will download the upgraded packages without touching the installed system)

After the 'dist-upgrade -d' has downloaded, and wish to complete the dist-upgrade please follow these instructions:

Download and install

# apt-get dist-upgrade
Reading package lists... Done
Building dependency tree... Done
Calculating upgrade... Done
The following packages will be REMOVED:
  checkmem-kanotix ddcxinfo-kanotix etcskel-kanotix gprsconnect-kanotix
  kanotix-dsl kanotix-graphics kanotix-kernelhacking kanotix-menu
  kanotix-ndiswrapper kanotix-screenres kanotix-scripts kanotix-setpassword
  kanotix-udev-config kdeservicemenus-kanotix modemlink-kanotix
  netcardconfig-kanotix networkconfig-kanotix pdf-printerconfig-kanotix
  rebuildfstab-kanotix rootshell-kanotix sambastart-kanotix
  scanpartitions-kanotix smbconf-kanotix sshstart-kanotix user-profile-kanotix
The following NEW packages will be installed:
  busybox-sidux configure-dsl configure-freenx configure-gprs
  configure-modemlink configure-ndiswrapper configure-networkcard
  configure-screenres configure-xawtv ddcxinfo-sidux etcskel-sidux genisoimage
  kdeservicemenus-sidux libdns22 libenchant1c2a libsexy2 networkconfig-sidux
  ppp-scripts rebuildfstab rootshell scanpartitions sidux-kernelhacking

NEVER EVER do a dist-upgrade nor upgrade whilst in X. Always check Current Warnings on the sidux main web site. These warnings are there for a reason and because of the nature of Debian sid/unstable which is updated on a daily basis. Log out of KDE. Go to Textmode by doing Ctrl+Alt+F1 ; logon as root, and then type init 3 ; apt-get update ; apt-get dist-upgrade ;run fix-fonts ; run fglrx or nvidia script if xorg updated and you have such a graphics adapter otherwise do ; init 5 && exit [NEVER DIST-UPGRADE [or UPGRADE] with adept or synaptic]

If you do not go to init 3, well tough for you ,, you have been warned

Searching for a package with apt-cache

Another very useful command in the APT-system is apt-cache; it searches the APT-Database and shows information on packages; e.g. a list of all packages, that contain or address a "screensaver" is obtained by:

# apt-cache search screensaver
brightside - Add reactivity to the corners and edges of your GNOME desktop
chbg - tool for changing the desktop background image in X11
cmatrix - Console Matrix simulates the display from "The Matrix"
electricsheep - screensaver showing collective dream of sleeping computers
epplets - The Epplets for the Enlightenment Window Manager
gkrellshoot - Plugin for gkrellm to lock the screen and make screenshots
kscreensaver - additional screen savers released with KDE
kscreensaver-xsavers - KDE hooks for standard xscreensavers
ktux - Tux screensaver for KDE
lg-issue24 - Issue 24 of the Linux Gazette.
lockvc - screensaver to lock your Linux console(s)
rss-glx - Really Slick Screensavers GLX Port
shermans-aquarium - Sherman's aquarium applet for GNOME 2
wmmatrix - View The Matrix in a Window Maker dock application
xscreensaver - Automatic screensaver for X
xscreensaver-gl - GL(Mesa) screen hacks for xscreensaver
ksetisaver - SETI screensaver for KDE

If you want more information on a particular package, you can use:

# apt-cache show xscreensaver
Package: xscreensaver
Priority: optional
Section: x11
Installed-Size: 9728
Maintainer: Ralf Hildebrandt 
Architecture: i386
Version: 4.21-7
Depends: libatk1.0-0 (>= 1.9.0), libc6 (>= 2.3.5-1), libglade2-0 (>= 1:2.5.1), libglib2.0-0 (>= 2.8.0), libgtk2.0-0 (>= 2.6.0), libice6 | xlibs (>> 4.1.0), libjpeg62, libpam0g (>= 0.76), libpango1.0-0 (>= 1.8.2), libsm6 | xlibs (>> 4.1.0), libx11-6 | xlibs (>> 4.1.0), libxext6 | xlibs (>> 4.1.0), libxinerama1, libxml2 (>= 2.6.21), libxmu6 | xlibs (>> 4.1.0), libxpm4 | xlibs (>> 4.1.0), libxrandr2 | xlibs (>> 4.3.0), libxrender1 (>> 1:0.9.0-1), libxt6 | xlibs (>> 4.1.0), libxxf86misc1, libxxf86vm1, zlib1g (>= 1:1.2.1)
Recommends: xli | xloadimage, libjpeg-progs, perl5, miscfiles | wordlist
Suggests: netpbm, xdaliclock, xscreensaver-gl, fortune, www-browser, qcam | streamer
Conflicts: funny-manpages (1.3-1), suidmanager (0.50), xscreensaver-gnome (4.09-2), xscreensaver-nognome (4.09-2), gnome-control-center (1:2)
Filename: pool/main/x/xscreensaver/xscreensaver_4.21-7_i386.deb
Size: 3747878
MD5sum: ae0650bafe4834f9c6459d75544d1ddb
Description: Automatic screensaver for X
The purpose of xscreensaver is to display pretty pictures on your screen
when it is not in use, in keeping with the philosophy that unattended
monitors should always be doing something interesting, just like they do
in the movies.

The benefit that this program has over the combination of the xlock and xautolock programs is the ease with which new graphics hacks can be installed: you don't need to recompile this program to add a new display mode, you just change some resource settings. Any program which can be invoked in such a way that it draws on the root window of the screen can now be used as a screensaver without modification. The programs that are being run as screensavers don't need to have any special knowledge about what it means to be a screensaver.

Tag: interface::x11, uitoolkit::gtk, x11::screensaver
Task: gnome-desktop
Package: xscreensaver
Priority: optional
Section: x11
Installed-Size: 10032
Maintainer: Ralf Hildebrandt 
Architecture: i386
Version: 4.21-5
Depends: libatk1.0-0 (>= 1.9.0), libc6 (>= 2.3.2.ds1-21), libglade2-0 (>= 1:2.5.1), libglib2.0-0 (>= 2.6.0), libgtk2.0-0 (>= 2.6.0), libice6 | xlibs (>> 4.1.0), libjpeg62, libpam0g (>= 0.76), libpango1.0-0 (>= 1.8.1), libsm6 | xlibs (>> 4.1.0), libx11-6 | xlibs (>> 4.1.0), libxext6 | xlibs (>> 4.1.0), libxml2 (>= 2.6.16), libxmu6 | xlibs (>> 4.1.0), libxpm4 | xlibs (>> 4.1.0), libxrandr2 | xlibs (>> 4.3.0), libxrender1, libxt6 | xlibs (>> 4.1.0), zlib1g (>= 1:1.2.1)
Recommends: xli | xloadimage, netpbm, libjpeg-progs, perl5, miscfiles | wordlist
Suggests: xdaliclock, xscreensaver-gl, fortune, www-browser, qcam | streamer
Conflicts: xscreensaver-gnome (4.09-2), xscreensaver-nognome (4.09-2), gnome-control-center (1:2)
Filename: pool/main/x/xscreensaver/xscreensaver_4.21-5_i386.deb
Size: 3850212
MD5sum: 4c4d20ef312359cf836f3164a6eac293

Description: Automatic screensaver for X
The purpose of xscreensaver is to display pretty pictures on your screen when it is not in use, in keeping with the philosophy that unattended monitors should always be doing something interesting, just like they do in the movies.

The benefit that this program has over the combination of the xlock and xautolock programs is the ease with
 which new graphics hacks can be installed: you don't need to recompile this program to add a new display mode,
 you just change some resource settings. Any program which can be invoked in such a way that it draws on the root
 window of the screen can now be used as a screensaver without modification. The programs that are being run as
 screensavers don't need to have any special knowledge about what it means to be a screensaver.

Tag: interface::x11, uitoolkit::gtk, x11::screensaver
Task: gnome-desktop

A complete description of the APT-System can be found at APT-HOWTO