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.

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

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
...

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 $Statusn' | 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.

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.

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 want to reinstall the packages now there on 12.04, right?

If so, it's very easy. You'll need an "Ubuntu Single Sign On account." (Create it before reinstalling so that your system is synced.)

1. Go to the Software Center and look for the "Sync Between Computers..." option under the File menu.




2. When you click on it you will see your computer registered and a list of all apps on your computer.




3. When you will install fresh, that computer will be considered a new computer.




4. You just have to sign in to your Ubuntu account and your previous computer will be shown.




5. Click on it; you'll get a list of all apps. Select "install" on the app you want to install.

There's also a tool called Aptik (both command line and GUI) which can help you view a list of all installed packages, with an option to select/unselect some of them, make a backup list, and then restore the same set of packages in another system.

To install:

sudo add-apt-repository -y ppa:teejee2008/ppa
sudo apt-get update
sudo apt-get install aptik

Further info: http://www.teejeetech.in/2014/04/aptik-v14.html

As can be seen in the screenshot, Aptik lets you also backup and restore PPAs, which will certainly be necessary to install some of the packages installed.

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)
   

   
   

It make 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

There's a great explanation on Unix StackExchange that describes how to use aptitude to list packages not installed as dependencies, and how to compare that list with the list of default packages for your Ubuntu release.

To obtain the manifest file for desktop versions of 12.04 and newer, visit this site, choose your release, and scroll down below the CD images to the files section. You'll find something like "ubuntu-12.04.4-desktop-amd64+mac.manifest" that matches your architecture.

For server versions you'll need to obtain the manifest file from the ISO that was used to install the original system. For a VPS or cloud server, your provider may make the images available or you might need to contact them.

Here's an example using the code from the referenced post, along with modifications to install on the new server.

Old server (code from other post, output saved to file):

aptitude search '~i !~M' -F '%p' --disable-columns | sort -u > currently-installed.list
wget -qO - http://mirror.pnl.gov/releases/precise/ubuntu-12.04.3-desktop-amd64.manifest 
 | cut -f1 | sort -u > default-installed.list
comm -23 currently-installed.list default-installed.list > user-installed.list

On the new server, copy the file using scp, then use sed to append 'install' to every line (-i performs an inline-replace). Then you can use the list as input to 'dpkg --set-selections' and install the packages with apt-get:

scp user@oldserver:user-installed.list .
sed -i 's/$/tinstall/' user-installed.list
sudo dpkg --set-selections < user-installed.list
sudo apt-get dselect-upgrade

Before starting this task, I recommend reading and understanding all parts of the post mentioned in the beginning, and then consult the aptitude reference guide for details on search patterns.

You can look at the apt log under /var/log/apt/ and the dpkg log under /var/log/

and you can get the list of the installed packages with just a command:

dpkg -l | grep '^ii '

I'm surprised the apt-cache command designed exactly for this purpose hasn't been mentioned above...

apt-cache pkgnames

For more info, run apt-cache --help:

**apt-cache is a low-level tool used to query information
from APT's binary cache files

Commands:
 gencaches - Build both the package and source cache
 showpkg - Show some general information for a single package
 showsrc - Show source records
 stats - Show some basic statistics
 dump - Show the entire file in a terse form
 dumpavail - Print an available file to stdout
 unmet - Show unmet dependencies
 search - Search the package list for a regex pattern
 show - Show a readable record for the package
 depends - Show raw dependency information for a package
 rdepends - Show reverse dependency information for a package
 pkgnames - List the names of all packages in the system
 dotty - Generate package graphs for GraphViz
 xvcg - Generate package graphs for xvcg
 policy - Show policy settings

Options:
 -h This help text.
 -p=? The package cache.
 -s=? The source cache.
 -q Disable progress indicator.
 -i Show only important deps for the unmet command.
 -c=? Read this configuration file
 -o=? Set an arbitrary configuration option, eg -o dir::cache=/tmp
See the apt-cache(8) and apt.conf(5) manual pages for more information.
**

For the complete rundown see:

https://help.ubuntu.com/community/SwitchingToUbuntu/FromLinux/RedHatEnterpriseLinuxAndFedora#Command_Line_Tools

half way across the page:

dpkg --list

The below command will also lists all the installed packages,

grep ' installed ' /var/log/dpkg.log /var/log/dpkg.log.1 | awk 'print $5' | sort -u

To save a list of installed packages to a file named installed_packages.txt, just run:

dpkg-query --list >> installed_packages.txt

Help out this community wiki - Add up-to-date solutions.

dpkg, xargs, & apt-get

This command should accomplish the creation of a text file containing installed packages:

dpkg -l | awk 'print $2' > package_list.txt

To accomplish the bulk installation of the listed packages you'll need to edit 'package_list.txt'. Remove the weird lines at the top of the file using a text editor. You can then use this command to install packages from the created file using:

xargs < package_list.txt apt-get install -y

apt-cache, xargs, & apt-get

Only use this method if you want all current packages to be installed using the list (which includes automatically installed, etc).

Output the response of 'apt-cache pkgnames' to a file we'll simply name "package_list.txt". You can accomplish this with:

apt-cache pkgnames > package_list.txt

Then when you want to install packages from "package_list.txt" you would use this command:

xargs < package_list.txt apt-get install -y

apt-mark, xargs, & apt-get

We can use the command apt-mark showmanual to give a list of packages that were manually or initially installed with Ubuntu. We'll want to output that to a file we'll just call "package-list.txt". Use this command to accomplish that:

apt-mark showmanual > package-list.txt

The command we would use to install packages from the file "package_list.txt" is below.

xargs < package_list.txt apt-get install -y

Aptik Migration Utility

Utility to simplify re-installation of software packages after upgrading/re-installing Ubuntu-based distributions.

[Launchpad | Aptik]

For information on Aptik, try visiting its official page, and for a screenshot click here or view the end of this section.

Installing Aptik is simple. Follow these steps:

1. Add the PPA with:
   sudo add-apt-repository -y ppa:teejee2008/ppa




2. Update apt with the below command.
   sudo apt-get update




3. Install Aptik using:
   sudo apt-get install aptik

1. List the installed software packages on Ubuntu

To list the installed software packages on your machine you can use the following command:

sudo apt list --installed

The output of the command will be very similar to the following one, depending on which packages are currently installed:

Listing...
acl/xenial,now 2.2.52-3 amd64 [installed]
adduser/xenial,xenial,now 3.113+nmu3ubuntu4 all [installed]
apache2/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed]
apache2-bin/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed,automatic]
apache2-data/xenial-updates,xenial-updates,xenial-security,xenial-security,now 2.4.18-2ubuntu3.1 all [installed,automatic]
apache2-doc/xenial-updates,xenial-updates,xenial-security,xenial-security,now 2.4.18-2ubuntu3.1 all [installed]
apache2-utils/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed]
apparmor/xenial-updates,now 2.10.95-0ubuntu2.5 amd64 [installed,automatic]
apt/xenial-updates,now 1.2.19 amd64 [installed]
apt-utils/xenial-updates,now 1.2.19 amd64 [installed]
...

2. Use the LESS program

To easily read the entire output you can use the less program.

sudo apt list --installed | less

3. Use the GREP Command

You can look for a specific package through the output using the grep program.

sudo apt list --installed | grep -i apache

4. List all packages that include Apache

The output from the above command will list all packages that include apache in their names.

apache2/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed]
apache2-bin/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed,automatic]
apache2-data/xenial-updates,xenial-updates,xenial-security,xenial-security,now 2.4.18-2ubuntu3.1 all [installed,automatic]
apache2-doc/xenial-updates,xenial-updates,xenial-security,xenial-security,now 2.4.18-2ubuntu3.1 all [installed]
apache2-utils/xenial-updates,xenial-security,now 2.4.18-2ubuntu3.1 amd64 [installed]
libapache2-mod-php/xenial,xenial,now 1:7.0+35ubuntu6 all [installed,automatic]
libapache2-mod-php7.0/xenial-updates,now 7.0.13-0ubuntu0.16.04.1 amd64 [installed,automatic]
libapache2-mod-security2/xenial,now 2.9.0-1 amd64 [installed]
libapache2-modsecurity/xenial,xenial,now 2.9.0-1 all [installed]

Apt supports patterns to match package names and options to list installed (--installed) packages, upgradeable (--upgradeable) packages or all available (--all-versions) package versions.

5. Use the DPKG program

Another alternative that you can use to list the installed software packages on your Ubuntu VPS is the dpkg command.

sudo dpkg -l

The output of the command will provide you with information such as the name of the package, version, architecture and short description about the package. Of course, you can use the grep program again to search for a specific package.

sudo dpkg -l | grep -i apache

The output should look like the one below:

ii apache2 2.4.18-2ubuntu3.1 amd64 Apache HTTP Server
ii apache2-bin 2.4.18-2ubuntu3.1 amd64 Apache HTTP Server (modules and other binary files)
ii apache2-data 2.4.18-2ubuntu3.1 all Apache HTTP Server (common files)
ii apache2-doc 2.4.18-2ubuntu3.1 all Apache HTTP Server (on-site documentation)
ii apache2-utils 2.4.18-2ubuntu3.1 amd64 Apache HTTP Server (utility programs for web servers)
rc apache2.2-common 2.2.22-6ubuntu5.1 amd64 Apache HTTP Server common files
ii libapache2-mod-php 1:7.0+35ubuntu6 all server-side, HTML-embedded scripting language (Apache 2 module) (default)
rc libapache2-mod-php5 5.5.9+dfsg-1ubuntu4.16 amd64 server-side, HTML-embedded scripting language (Apache 2 module)
ii libapache2-mod-php7.0 7.0.13-0ubuntu0.16.04.1 amd64 server-side, HTML-embedded scripting language (Apache 2 module)
ii libapache2-mod-security2 2.9.0-1 amd64 Tighten web applications security for Apache
ii libapache2-modsecurity 2.9.0-1 all Dummy transitional package
ii libapr1:amd64 1.5.2-3 amd64 Apache Portable Runtime Library
ii libaprutil1:amd64 1.5.4-1build1 amd64 Apache Portable Runtime Utility Library
ii libaprutil1-dbd-sqlite3:amd64 1.5.4-1build1 amd64 Apache Portable Runtime Utility Library - SQLite3 Driver
ii libaprutil1-ldap:amd64 1.5.4-1build1 amd64 Apache Portable Runtime Utility Library - LDAP Driver
.

With the competition of this tutorial, you have successfully learned how to list installed packages in Ubuntu.

I think it is interesting to note apt list --installed or dpkg-query --list actually use the file called /var/lib/dpkg/status in behind where all the info about the packages is beard.

So if you would like to deal with the super extended list of packages just cat /var/lib/dpkg/status.

Note: Do not alter /var/lib/dpkg/status file.

The package dctrl-tools provide the grep-status tool to get the list of the packages marked as installed on your system:

sudo apt install dctrl-tools

Usage:

grep-status -FStatus -sPackage -n "install ok installed"

see : man dctrl-tools

There are many ways to do that. In case you are using CentOS like me you can use these:
1. yum list installed
2. rpm -qa