Q: When I type apt-cache show, where does debian check for the packages?
A: When you run the command apt-cache show, the system checks the package cache to retrieve and display information about a package.
The package cache is a local repository of metadata about available packages that is stored on your system.
It's used by the package manager (apt in this case) to quickly retrieve information about packages without having to download the entire package files.
The package cache is typically located in the /var/cache/apt directory.
Within this directory, you'll find subdirectories for each repository you have configured on your system.
These subdirectories contain the metadata and index files necessary for apt to provide information about packages,
dependencies, and other related data.
When you run apt-cache show, the command accesses the appropriate cache files to display detailed information about the specified package,
including its description, version, dependencies, and other relevant data.
Q: what's inside /var/cache/apt/archives?
A: The /var/cache/apt/archives directory contains cached copies of the downloaded .deb package files.
These files are the actual binary packages that apt retrieves from repositories when you install or upgrade software on your system.
By storing these package files locally, apt can avoid re-downloading them if they are already present in the cache.
This can save both time and bandwidth when installing or upgrading packages.
The files in the /var/cache/apt/archives directory have filenames corresponding to the package names and versions.
For example, if you've downloaded the package "example-package" version 1.2.3,
you might find a file named example-package_1.2.3.deb in this directory.
If you were to install the same package again or upgrade it to a newer version,
apt would check the local cache first before attempting to download the package again from the repository.
If the required package file is found in the cache, it will be used for the installation or upgrade.
If not, apt will download the package from the repository and store it in the cache for future use.
This local package cache helps improve the efficiency of package management operations on your system
by reducing the need to repeatedly download the same package files.
Q: What about the content of /var/cache/apt/pkgcache.bin?
A: The /var/cache/apt/pkgcache.bin file is a binary cache file that contains a serialized representation of the package cache metadata.
This file is used by apt to quickly access and search package information without having to parse and process the individual package
metadata files from the repositories every time a package-related operation is performed.
The pkgcache.bin file is generated by apt to speed up package management operations,
such as package searches, dependency calculations, and generating package lists.
By storing the metadata in a binary format, apt can read and access this data more efficiently
than parsing the plain text metadata files stored in the repository.
It's important to note that while the pkgcache.bin file accelerates package operations,
it doesn't store the actual package files (.deb files) themselves.
Those are stored in the /var/cache/apt/archives directory, as mentioned earlier.
The pkgcache.bin file specifically stores metadata about available packages,
their dependencies, versions, and other related information.
It's generally safe to leave this file alone, as apt manages it automatically.
However, if you encounter issues with package information or if you suspect corruption,
you might consider refreshing the cache using commands like apt update or apt-get update,
which will regenerate both the binary cache and the individual package metadata files.
Q: Where are the individual package metadata files?
A: The individual package metadata files in a Debian-based system are typically stored in the /var/lib/apt/lists directory.
This directory contains plain text files that list the available packages and their associated information from the repositories
configured on your system.
These files are generated during the apt update process, which fetches information from the repositories and
updates the package cache.
Inside the /var/lib/apt/lists directory, you'll find subdirectories corresponding to the repositories you have configured.
Within these subdirectories, there are files with names that represent the repository they belong to.
These files contain the metadata for the packages available in that repository, including package names, versions,
dependencies, and other package-related details.
For example, you might find files like archive.ubuntu.com_ubuntu_dists_focal_main_binary-amd64_Packages or
security.debian.org_debian-security_dists_buster_updates_main_i18n_Translation-en.
These files are human-readable but are not meant to be directly edited. They are used by apt to
quickly access information about packages without having to query the remote repository servers every time.
The apt update command fetches the latest information from the repositories and updates these files.
The apt-cache and apt commands then use the data in these files to provide package information,
perform dependency calculations, and manage package installations and upgrades.
