List installed packages on Linux or FreeBSD / OpenBSD system

Q. How do I list all installed packages on Linux or FreeBSD system?

A. Most Linux distributions are based upon rpm or apt-get / dpkg packaging system.

RedHat, Fedora Core, Suse Linux, Cento os all are RPM based distribution

Use rpm command as follows to list all installed packages
rpm -qa
rpm -qa | grep 'package-name'
rpm -qa | grep -i '*httpd*'

Debian or Ubuntu Linux distribution

You can use dpkg command to list all installed packages:
dpkg --list
dpkg --list | grep 'package-name'
dpkg --list | grep -i '*php*'

Task: OpenBSD list installed package

Use pkg_info command for displaying information on all installed software packages
$ pkg_info
$ pkg_info | grep 'package-name'

Output:

bash-3.1.10_1       The GNU Project's Bourne Again SHell
expat-2.0.0_1       XML 1.0 parser written in C
gettext-0.14.5_2    GNU gettext package
lftp-3.4.0_3        Shell-like command line ftp client
libiconv-1.9.2_2    A character set conversion library
libstatgrab-0.12_1  Provides a useful interface to system statistics
nmap-4.01           Port scanning utility for large networks
pcre-6.6_1          Perl Compatible Regular Expressions library
pkgconfig-0.20      A utility to retrieve information about installed libraries
vim-lite-6.4.6_1    Vi "workalike", with many additional features (Lite package