6.3. The apt-cache Command

6.3. The `apt-cache` Command

The apt-cache command can display much of the information stored in APT's internal database. This information is a sort of cache since it is gathered from the different sources listed in the sources.list file. This happens during the aptitude update operation.

VOCABULARY Cache

A cache is a temporary storage system used to speed up frequent data access when the usual access method is expensive (performance-wise). This concept can be applied in numerous situations and at different scales, from the core of microprocessors up to high-end storage systems.

In the case of APT, the reference Packages files are those located on Debian mirrors. That said, it would be very ineffective to go through the network for every search that we might want to do in the database of available packages. That is why APT stores a copy of those files (in /var/lib/apt/lists/) and searches are done within those local files. Similarly, /var/cache/apt/archives/ contains a cache of already downloaded packages to avoid downloading them again if you need to reinstall them after a removal.

The apt-cache command can do keyword-based package searches with apt-cache search keyword. It can also display the headers of the package's available versions with apt-cache show package. This command provides the package's description, its dependencies, the name of its maintainer, etc. Note that aptitude search and aptitude show work in the same way.

Some features are more rarely used. For instance, apt-cache policy displays the priorities of package sources as well as those of individual packages. Another example is apt-cache dumpavail which displays the headers of all available versions of all packages. apt-cache pkgnames displays the list of all the packages which appear at least once in the cache.