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 apt-get 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.

ALTERNATIVE axi-cache

apt-cache search is a very rudimentary tool, basically implementing grep on package's descriptions. It often returns too many results or none at all when you include too many keywords.

axi-cache search term, on the other hand, provides better results, sorted by relevancy. It uses the Xapian search engine and is part of the apt-xapian-index package whichs indexes all package information (and more, like the .desktop files from all Debian packages). It knows about tags (see sidebar GOING FURTHER The Tag field) and returns results in a matter of milliseconds.

$ axi-cache search package use::searching
105 results found.
Results 1-20:
100% packagesearch - GUI for searching packages and viewing package information
98% debtags - Enables support for package tags
94% debian-goodies - Small toolbox-style utilities 
93% dpkg-awk - Gawk script to parse /var/lib/dpkg/{status,available} and Packages
93% goplay - games (and more) package browser using DebTags
[...]
87% apt-xapian-index - maintenance and search tools for a Xapian index of Debian packages
[...]
More terms: search debian searching strigi debtags bsearch libbsearch
More tags: suite::debian works-with::software:package role::program interface::commandline implemented-in::c++ admin::package-management use::analysing
`axi-cache more' will give more results

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.