Packages currently installed with new versions available are retrieved and upgraded; under no circumstances are currently installed packages removed, nor are packages that are 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.These latter features may be used to override decisions made by apt-get's conflict resolution system.following the package name with an equals (=) and the version of the package to select.An update must be performed first so that apt-get knows that new versions of packages are available.In addition to performing the function of upgrade, this option 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.This will cause that version to be located and selected for install. " or "*" then it is assumed to be a POSIX regular expression, and it is applied to all package names in the database. Note that matching is done by substring so "lo.*" matches "how-lo" and "lowest".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 (i.e. If this is undesired, anchor the regular expression with a "^" or "$" character, or create a more specific regular expression.
For boolean options you can override the config file by using something like -f-, --no-f, -f=no or several other variations. Attempt to correct a system with broken dependencies in place.
This allows a cache to be maintained over a long period of time without it growing out of control.
The configuration option APT:: Clean-Installed will prevent installed packages from being erased if it is set to off.
This probably will mean that you will not get the same source as the package you have installed, or could install.
If the --compile options is specified then the package will be compiled to a binary using rpmbuild, if --download-only is specified then the source package will not be unpacked.