4.1Package Management with apt

Installing software on Linux is not "download and run"—you fetch packages from repositories and manage them with their dependencies. On Debian/Ubuntu the front door is apt, whose biggest value is automatic dependency resolution.

4.1.1Core apt operations

• Install with apt install nginx; remove with apt remove nginx (apt purge also deletes config); refresh the index with apt update (fetch latest repo metadata); upgrade packages with apt upgrade.
• apt-get and apt-cache are the traditional lower-level tools (apt consolidates their common features for interactive use). Search/info: apt-cache search keyword, apt-cache show pkg (or apt search, apt show).
• Find which package provides a file with apt-file (apt-file search /usr/bin/convert—covers packages not yet installed; needs apt-file update).
• Repositories are defined in /etc/apt/sources.list (plus /etc/apt/sources.list.d/); indexes and packages come from the URLs listed there.