Editing APT (software) (section)

==Sources==
APT relies on the concept of [[Software repository|repositories]] in order to find software and resolve dependencies. For APT, a repository is a directory containing packages along with an index file. This can be specified as a networked or [[CD-ROM]] location. {{As of|2021|08|14|post=,}} the Debian project keeps a central repository of over 50,000 software packages ready for download and installation.<ref>{{Cite web |title=Debian 11 "bullseye" released |url=https://www.debian.org/News/2021/20210814.en.html |access-date=2022-05-11 |website=www.debian.org}}</ref>

Any number of additional repositories can be added to APT's ''sources.list'' configuration file (<code>/etc/apt/sources.list</code>) and then be queried by APT. Graphical front ends often allow modifying <code>sources.list</code> more simply (<code>apt-setup</code>). Once a package repository has been specified (like during the system installation), packages in that repository can be installed without specifying a source and will be kept up-to-date automatically.

In addition to network repositories, [[compact disc]]s and other storage media (USB keydrive, hard disks...) can be used as well, using <code>apt-cdrom</code><ref>{{cite web|url=https://manpages.debian.org/unstable/apt/apt-cdrom.8.en.html|title=apt-cdrom(8)|website=Debian Manpages|access-date=27 December 2021}}</ref> or adding <code>file:/</code> URI<ref>{{citation|title=''e.g. deb file:/mnt/install stable main contrib non-free''|mode=cs1|postscript=.}}</ref> to the source list file. <code>apt-cdrom</code> can specify a folder other than a CD-ROM, using the <code>-d</code> option (i.e. a hard disk or a USB keydrive). The Debian CDs available for download contain Debian repositories. This allows non-networked machines to be upgraded. One can also use <code>apt-zip</code>.

Problems may appear when several sources offer the same package(s). Systems that have such possibly conflicting sources can use APT pinning to control which sources should be preferred.