The command we need to use is dpkg –get-selections, which will give us a list of all the currently installed packages.
$ dpkg --get-selections adduser install alsa-base install alsa-utils install apache2 install apache2-mpm-prefork install apache2-utils install apache2.2-common install apt install apt-utils install
The full list can be long and unwieldy, so it’s much easier to filter through grep to get results for the exact package you need. For instance, I wanted to see which php packages I had already installed through apt-get:
dpkg --get-selections | grep phplibapache2-mod-php5 install php-db install php-pear install php-sqlite3 install php5 install php5-cli install php5-common install php5-gd install php5-memcache install php5-mysql install php5-sqlite install php5-sqlite3 install php5-xsl install
For extra credit, you can find the locations of the files within a package from the list by using the dpkg -L command, such as:
dpkg -L php5-gd /. /usr /usr/lib /usr/lib/php5 /usr/lib/php5/20060613 /usr/lib/php5/20060613/gd.so /usr/share /usr/share/doc /etc /etc/php5 /etc/php5/conf.d /etc/php5/conf.d/gd.ini /usr/share/doc/php5-gd
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