Check if a Package is installed on Linux or not

https://www.cyberciti.biz/faq/apt-get-list-packages-are-installed-on-ubuntu-linux/

https://www.2daygeek.com/find-out-if-package-is-installed-or-not-in-linux-unix/

#RHEL 6, RHEL7

yum list installed openssh

#RHEL 8

dnf list installed httpd

#Ubuntu

apt list -a pkgNameHere

apt list --installed

apt -qq list nano