In simpler words, a package manager is a tool that allows users to install, remove, upgrade, configure and manage software packages on an operating system. The package manager can be a graphical application like a software center or a command line tool like apt-get or pacman.
You’ll often find me using the term ‘package’ in tutorials and articles, To understand package manager, you must understand what a package is.
A package is usually referred to an application but it could be a GUI application, command line tool or a software library (required by other software programs). A package is essentially an archive file containing the binary executable, configuration file and sometimes information about the dependencies.
Package Managers differ based on packaging system but same packaging system may have more than one package manager.
For example, RPM has Yum and DNF package managers. For DEB, you have apt-get, aptitude command line based package managers.
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You have to install docker and jenkins in your system from your terminal using package managers
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Write a small blog or article to install these tools using package managers on Ubuntu and CentOS
systemctl is used to examine and control the state of “systemd” system and service manager. systemd is system and service manager for Unix like operating systems(most of the distributions, not all).
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check the status of docker service in your system (make sure you completed above tasks, else docker won't be installed)
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stop the service jenkins and post before and after screenshots
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read about the commands systemctl vs service
eg. systemctl status docker vs service docker status
For Reference, read this