What's changed: Initial version (topic 1.04, subtopics 1.04.1–1.04.4)
4.1Package Management with apt
Learn repository-based package management on Debian/Ubuntu: installing, updating, and removing with apt (apt-get, apt-cache, apt-file), finding which package contains a given file, and repository configuration in /etc/apt/sources.list.
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 withapt remove nginx(apt purgealso deletes config); refresh the index withapt update(fetch latest repo metadata); upgrade packages withapt 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(orapt search,apt show). - Find which package provides a file with apt-file (
apt-file search /usr/bin/convert—covers packages not yet installed; needsapt-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.
The top distinction: apt update = refresh the index (does not upgrade packages) / apt upgrade = upgrade packages. Pair it with file→package lookup including uninstalled = apt-file and repo definitions = /etc/apt/sources.list.
The classic "nginx stays old even though I upgraded" comes from misunderstanding apt update: apt works from the local index (a catalog of available packages), so the correct two-step is apt update to refresh the catalog, then apt upgrade (or a targeted apt install nginx). The remove/purge difference matters operationally: remove deletes the program but keeps config files (reinstall restores settings); purge deletes config too. For reverse lookups—"which package ships ImageMagick's convert?"—choose by scope: dpkg -S (next section) for installed packages, apt-file to include uninstalled ones. When adding a corporate mirror or extra repo to /etc/apt/sources.list, remember the follow-up apt update, or the new repo is never seen.
| Goal | Command | Key point |
|---|---|---|
| Refresh the index | apt update | Upgrades nothing |
| Upgrade packages | apt upgrade | Run after update |
| Remove incl. config | apt purge | remove keeps config |
| File → package lookup | apt-file search | Covers uninstalled packages |
Trap: "apt update brings installed packages up to date" is wrong—update refreshes only the repository index; upgrading packages is apt upgrade. And "apt remove deletes config files too" is wrong—that is purge.
4.1.2Section summary
- update = index, upgrade = packages—in that order; install/remove/purge; search via apt-cache (apt search)
- File lookup = apt-file (incl. uninstalled); repos defined in /etc/apt/sources.list
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Quick check
(just a quick review)Q1. On Ubuntu, you want to fetch the latest package info and then bring installed packages up to date. Correct sequence?
Q2. You want to find which package provides /usr/bin/convert, including packages not yet installed. Which command?
Q3. Which file defines the repository URLs apt uses?
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