Notes on ppa and APT...

Personal Package Archive

http://en.wikipedia.org/wiki/Personal_Package_Archive

A Personal Package Archive (PPA) is a special software repository for uploading source packages to be built and published as an APT repository by Launchpad or a similar application.^[1] While the term is used exclusively within Ubuntu, Launchpad host Canonical envisions adoption beyond the Ubuntu community.^[2]

Advanced Packaging Tool

The Advanced Packaging Tool, or APT, is a free user interface that works with core libraries to handle the installation and removal of software on the Debian GNU/Linux distribution and its variants.^[3] APT simplifies the process of managing software on Unix-like computer systems by automating the retrieval, configuration and installation of software packages, either from binary files or by compiling source code.^[3]


APT was originally designed as a front-end for dpkg to work with Debian's .deb packages, but it has since been modified to also work with the RPM Package Manager system via apt-rpm.^[4]

The Fink project has ported APT to Mac OS X for some of its own package management tasks, and APT is also available in OpenSolaris (included in the Nexenta OS distribution).^[5]

 

Usage

There is no single "apt" program; apt is itself the package name containing the set of tools (and requiring the libraries) that support its functionality. A significant part of apt is a C++ library of functions (another package known as libapt) which are used by these related front-end programs for dealing with packages, such as apt-get and apt-cache. They are commonly used in examples due to their simplicity and ubiquity; apt-get and apt-cache are of "important" priority in all current Debian releases, and are therefore installed in a default Debian installation. Apt can be functionally considered to be a front-end to dpkg, and a friendlier front end to this than dselect. While dpkg performs actions on individual packages, apt tools manage relations (especially dependencies) between them, as well as sourcing and management of higher-level versioning decisions (release tracking and version pinning).

APT is often hailed as one of Debian's best features.^[6][7][8][9] It is remarked that this quality comes from the strict quality controls of Debian policy.^[10][11]
A major feature in APT is the way it calls dpkg — it does topological sorting of the list of packages to be installed or removed and calls dpkg in the best possible sequence. In some cases, it utilizes the --force options in dpkg. However, it only does this when it is unable to calculate how to avoid the reason dpkg requires the action to be forced.

Installation of software

An install directive is followed by the name of one or more packages desired for installation.

Each package name is phrased as just the name portion of the package, not a fully qualified filename (for instance, in a Debian GNU/Linux system, libc6 would be the argument provided, not libc6_1.9.6-2.deb).

Notably, all packages containing dependencies required by the package(s) specified for installation will also be automatically retrieved and installed. This was an original distinguishing characteristic of apt-based package management systems whereby software installation failure due to missing dependencies, a type of dependency hell, was specifically avoided.

Another such distinction is remote repository retrieval of packages. A location configuration file (/etc/apt/sources.list) is used to locate the desired packages and retrieve them, and also obtain information about available (but not installed) packages.

Other command option features (switches) may be used to override decisions made by apt-get's conflict resolution system. If a hyphen is appended to the package name (with no intervening space), the identified package will be removed if it is installed. 

Similarly, a plus sign can be used to designate a package to install. A specific version of a package can be selected for installation by following the package name with an equals and the version of the package to select. This will cause that version to be located and selected for install. Alternatively, a specific distribution can be selected by following the package name with a slash and the version of the distribution or the archive name (stable, testing, unstable).
Both of the version selection mechanisms can downgrade packages and must be used with care.


Finally, the apt_preferences mechanism allows creating an alternative installation policy for individual packages.

If no package matches the given expression and the expression contains one of '.', '?' or '*', it is assumed to be a POSIX regular expression and it is applied to all package names in the database. Any matches are then installed (or removed). Note that matching is done by substring, so "lo.*" matches "how-lo" and "lowest". If this is undesired, the regular expression can be anchored with a '^' or '$' character, or a more specific regular expression can be created.

Update, upgrade and dist-upgrade

• update is used to resynchronize the package index files from their sources. The lists of available packages are fetched from the location(s) specified in /etc/apt/sources.list. For example, when using a Debian archive, this command retrieves and scans the Packages.gz files, so that information about new and updated packages is available. An update should always be performed before a safe-upgrade or dist-upgrade. Be aware that the overall progress meter will not always be correct as the size of the package files cannot be known in advance.

• upgrade is used to install the newest versions of all packages currently installed on the system from the sources enumerated in /etc/apt/sources.list. Packages currently installed with new versions available are retrieved and upgraded; under no circumstances are currently installed packages removed, or packages not already installed retrieved and installed. New versions of currently installed packages that cannot be upgraded without changing the install status of another package will be left at their current version. An update must be performed first, so that apt-get knows that new versions of packages are available.

• dist-upgrade, in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages; apt-get has a "smart" conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones if necessary. The /etc/apt/sources.list file contains a list of locations from which to retrieve desired package files.^[12] aptitude has a smarter dist-upgrade feature called full-upgrade.^[13]

Configuration and files

/etc/apt has the apt configuration folders and files.
apt-config is the APT Configuration Query program.^[14] apt-config dump shows the configuration.^[15]

Files

• /etc/apt/sources.list: Locations to fetch packages from.
• /etc/apt/sources.list.d/: Additional source list fragments.
• /etc/apt/apt.conf: APT configuration file.
• /etc/apt/apt.conf.d/: APT configuration file fragments.
• /etc/apt/preferences: version preferences file. This is where you would specify "pinning", i.e. a preference to get certain packages from a separate source or from a different version of a distribution.
• /var/cache/apt/archives/: storage area for retrieved package files.
• /var/cache/apt/archives/partial/: storage area for package files in transit.
• /var/lib/apt/lists/: storage area for state information for each package resource specified in sources.list
• /var/lib/apt/lists/partial/: storage area for state information in transit.

Sources

APT relies on the concept of 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 CDROM location. The Debian project keeps a central repository of over 25,000 software packages ready for download and installation.
For extra packages, any number of additional repositories can be added to APT's sources.list configuration file (/etc/apt/sources.list) and then be queried by APT. Graphical front-ends often allow modifying sources.list more simply (apt-setup). Once a package repository has been specified (like during the system installation), packages in that repository can be installed without specifying a source.
In addition to network repositories, compact discs and other storage media (USB keydrive, hard disks...) can be used as well, using apt-cdrom ^[16] or adding file:/^[17] to the source list file. Apt-cdrom can specify a different folder than a cd-rom, using the -d 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. Also one can use apt-zip.
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.

APT pinning

The APT pinning feature allows administrators to force APT to choose particular versions of packages which may be available in different versions from different repositories. This allows administrators to ensure that packages are not upgraded to versions which may conflict with other packages on the system, or that have not been sufficiently tested for unwelcome changes.
In order to do this, the pins in APT's preferences file (/etc/apt/preferences) must be modified,^[18] although graphical front-ends often make pinning simpler.

Front-ends

Synaptic Package Manager is one of the frontends available for APT

Several other front-ends to APT exist, which provide more advanced installation functions and more intuitive interfaces. These include:

• Synaptic Package Manager, a GTK+ graphical user interface
• Ubuntu Software Center, a GTK+ graphical user interface replacement for Synaptic
• aptitude, a versatile alternative to dselect
• KPackage, part of KDE
• Adept Package Manager, a graphical user interface for KDE (deb, rpm, bsd)
• PackageKit, a freedesktop.org frontend.
• GDebi, a GTK-based tool sponsored for Ubuntu. (There is also a Qt version, available in the Ubuntu repositories a gdebi-kde.)
• apt-cdrom, a way to add a new CDROM to APT's list of available sources.lists (list of available repositories). It is necessary to use apt-cdrom to add CDs to the APT system, it cannot be done by hand.
• apt-zip, a way to use apt with removable media, specifically USB flash drives.
• apt:foo, an interface for downloading and installing software by clicking on a web-link (experimental) ^[19]
• Cydia, a package manager for jailbroken iOS based on APT (ported to iOS as part of the Telesphoreo project).^[20][21]
• gnome-apt, a gtk/GNOME-widget-based graphical front-end.
• Muon Software Center, a Qt based graphical user interface
• Hildon Application Manager (Maemo Application), a Maemo front-end
• APT Daemon, a front-end that runs as a service to allow standard users to install software through PolicyKit and is in turn the framework that the Ubuntu Software Center (along with the Linux Mint Software Manager) uses to not be root and still run.

APT front-ends can:

• Search for new packages.
• Upgrade packages.
• Install or remove packages.
• Upgrade the whole system to a new release.

APT front-ends can list the dependencies of packages being installed or upgraded, ask the administrator if packages recommended or suggested by newly installed packages should be installed too, automatically install dependencies and perform other operations on the system such as removing obsolete files and packages.