3 - Package Managers

What is a package?

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.
In older days, software used to installed from its source code. You would refer to a file (usually named readme) and see what software components it needs, location of binaries. A configure script or makefile is often included. You will have to compile the software or on your own along with handling all the dependencies (some software require installation of other software) on your own.
To get rid of this complexity, Linux distributions created their own packaging format to provide the end users ready-to-use binary files (precompiled software) for installing software along with some metadata (version number, description) and dependencies.

Package manager

A package manager or package-management system is a collection of software tools that automates the process of installing, upgrading, configuring, and removing computer programs for a computer's operating system in a consistent manner.
A package manager deals with packages, distributions of software and data in archive files. Packages contain metadata, such as the software's name, description of its purpose, version number, vendor, checksum (preferably a cryptographic hash function), and a list of dependencies necessary for the software to run properly. Upon installation, metadata is stored in a local package database.
Package managers typically maintain a database of software dependencies and version information to prevent software mismatches and missing prerequisites. They work closely with software repositories, binary repository managers, and app stores.
Package managers are designed to eliminate the need for manual installs and updates. This can be particularly useful for large enterprises whose operating systems are typically consisting of hundreds or even tens of thousands of distinct software packages.

Functions of a Package Manager

Package managers are charged with the task of finding, installing, maintaining or uninstalling software packages upon the user's command. Typical functions of a package management system include:

Working with file archivers to extract package archives
Ensuring the integrity and authenticity of the package by verifying their checksums and digital certificates, respectively
Looking up, downloading, installing, or updating existing software from a software repository or app store
Grouping packages by function to reduce user confusion
Managing dependencies to ensure a package is installed with all packages it requires, thus avoiding "dependency hell"

How does the package manager work?

Almost all Linux distributions have software repositories which is basically collection of software packages. Yes, there could be more than one repository. The repositories contain software packages of different kind.
Repositories also have metadata files that contain information about the packages such as the name of the package, version number, description of package and the repository name etc. This is what you see if you use the apt show command in Ubuntu/Debian.
Your system’s package manager first interacts with the metadata. The package manager creates a local cache of metadata on your system. When you run the update option of the package manager (for example apt update), it updates this local cache of metadata by referring to metadata from the repository.
When you run the installation command of your package manager (for example apt install package_name), the package manager refers to this cache. If it finds the package information in the cache, it uses the internet connection to connect to the appropriate repository and downloads the package first before installing on your system.
A package may have dependencies. Meaning that it may require other packages to be installed. The package manager often takes care of the dependencies and installs it automatically along with the package you are installing. Similarly, when you remove a package using the package manager, it either automatically removes or informs you that your system has unused packages that can be cleaned.