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
...

Create a backup of what packages are currently installed:

dpkg --get-selections > list.txt

Then (on another system) restore installations from that list:

dpkg --clear-selections
sudo dpkg --set-selections < list.txt

To get rid of stale packages:

sudo apt-get autoremove

To get installed like at backup time (i.e. to install packages set by dpkg --set-selections):

sudo apt-get dselect-upgrade

apt-mark showmanual

man pages state:

will print a list of manually installed packages

So, it should just give a list of explicitly installed packages (though this includes packages that were part of the default initial install) without all of the dependencies included due to these packages being installed.

To output the result into a text file:

apt-mark showmanual > list-manually-installed.txt

dpkg-query (instead of dpkg --get-selections, which lists some packages that are not installed) as follows:

dpkg-query -W -f='${PackageSpec} ${Status}\n' | grep installed |  sort -u | cut -f1 -d \ > installed-pkgs

Or:

dpkg -l | grep ^ii | sed 's_  _\t_g' | cut -f 2 > installed-pkgs

To list all installed packages,

dpkg -l |awk '/^[hi]i/{print $2}' > 1.txt

or

aptitude search -F '%p' '~i' > 1.txt

or

dpkg --get-selections > 1.txt

Note:
You will get the result 1.txt file in your home folder or you can specify your own path.

APT-Clone

This package can be used to clone/restore the packages on a apt based system.

• It will save/restore the packages, sources.list, keyring and automatic-installed states.
• It can also save/restore no longer downloadable packages using dpkg-repack.

source: man apt-clone

APT-Clone is used by ubiquity (Ubuntu installer) for upgrade process. It is much better than the dpkg --get-selections solution because:

1. It preserves all repositories information.
2. It keeps track of what packages were automatically installed.
3. It allows to repack locally installed DEB files.

How to Use

1. Install

    sudo apt-get install apt-clone
   

2. Make backup

    sudo apt-clone clone path-to/apt-clone-state-ubuntu-$(lsb_release -sr)-$(date +%F).tar.gz
   

3. Restore backup

    sudo apt-clone restore path-to/apt-clone-state-ubuntu.tar.gz
   

   Restore to newer release:

    sudo apt-clone restore-new-distro path-to/apt-clone-state-ubuntu.tar.gz $(lsb_release -sc)
   

Result structure

It makes a simple gzipped tar file which can be easily edited and reviewed before restoring on the other machines. Here an example of its structure:

/
├── etc
│   └── apt
│       ├── preferences.d
│       ├── sources.list
│       ├── sources.list.d
│       │   ├── anton_-ubuntu-dnscrypt-vivid.list
│       │   ├── maarten-baert-ubuntu-simplescreenrecorder-vivid.list
│       │   └── megasync.list
│       ├── trusted.gpg
│       └── trusted.gpg.d
│           ├── anton__ubuntu_dnscrypt.gpg
│           ├── anton__ubuntu_dnscrypt.gpg~
│           ├── maarten-baert_ubuntu_simplescreenrecorder.gpg
│           └── maarten-baert_ubuntu_simplescreenrecorder.gpg~
└── var
    └── lib
        └── apt-clone
            ├── extended_states
            ├── installed.pkgs
            └── uname

I recommend using blueprint. Even though it is designed for servers, it can be also used from desktops as well. It will create a shell script/chef/puppet that you ca use to re-install all you packages.

You can use Synaptic to save the current state of your installed packaged. In Synaptic, select "file/save markings", Enter the name of the file to save the state to, and make sure to check the "Save full state, not only changes" box.

The file saved from this can be loaded into a new machine using "file/read markings" in Synaptic.