How to list all installed packages

Ubuntu 14.04 and above

The apt tool on Ubuntu 14.04 and above makes this very easy.

apt list --installed

Older Versions

To get a list of packages installed locally do this in your terminal:

dpkg --get-selections | grep -v deinstall

^{(The -v tag "inverts" grep to return non-matching lines)}

To get a list of a specific package installed:

dpkg --get-selections | grep postgres

To save that list to a text file called packages on your desktop do this in your terminal:

dpkg --get-selections | grep -v deinstall > ~/Desktop/packages

Alternatively, simply use

dpkg -l

(you don't need to run any of these commands as the superuser, so no sudo or any other variants necessary here)

To get just the packages which were expressly installed (not just installed as dependencies), you can run

aptitude search '~i!~M'

This will also include a brief description, which you may want. If not, also add the option -F '%p', as mentioned by karthick87.

Yet another option seems to be to copy the file /var/lib/apt/extended_states, which is a text file database in this format:

Package: grub-common
Architecture: amd64
Auto-Installed: 0

Package: linux-headers-2.6.35-22-generic
Architecture: amd64
Auto-Installed: 1

Auto-Installed: 0 indicates that the package was expressly installed and is not just a dependency.

To list all packages intentionally installed (not as dependencies) by apt commands, run the following :

(zcat $(ls -tr /var/log/apt/history.log*.gz); cat /var/log/apt/history.log) 2>/dev/null |
  egrep '^(Start-Date:|Commandline:)' |
  grep -v aptdaemon |
  egrep '^Commandline:'

This provides a reverse time based view, with older commands listed first:

Commandline: apt-get install k3b
Commandline: apt-get install jhead
...

Installation data also showing synaptic usage, but without details (the same with installation date) :

(zcat $(ls -tr /var/log/apt/history.log*.gz); cat /var/log/apt/history.log) 2>/dev/null |
  egrep '^(Start-Date:|Commandline:)' |
  grep -v aptdaemon |
  egrep -B1 '^Commandline:'

providing the following:

Start-Date: 2012-09-23  14:02:14
Commandline: apt-get install gparted
Start-Date: 2012-09-23  15:02:51
Commandline: apt-get install sysstat
...