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I know that on a local non-networked machine, it's possible to use the local apt database to search for packages to install using apt-cache search and apt-cache show. These commands print a package's short description summary, but not the package's extended description.

I know also that once a package's *.deb file has been downloaded onto a local machine it's possible to get that package's very useful extended description using dpkg --info /path/to/foo.deb, which reads the contents of a deb package's /info or /Debian/control file. This extended description is also available on-line, and is the text we all see when viewing a package on http://packages.debian.org

How can I get a local copy of all packages' extended descriptions so that I can better decide which packages to download?

As a follow-up, in response to @Gilles request for an example:

# apt-cache search "^apt$"
apt - commandline package manager

# apt-cache show apt | sed -n '/^Desc/,/^Desc/p'
Description: commandline package manager
Description-md5: 9fb97a88cb7383934ef963352b53b4a7
Description: commandline package manager
Description-md5: 9fb97a88cb7383934ef963352b53b4a7

#dpkg --info /var/cache/apt/archives/apt_1.4_amd64.deb | sed -n '/^ Description/,${p}'
 Description: commandline package manager
  This package provides commandline tools for searching and
  managing as well as querying information about packages
  as a low-level access to all features of the libapt-pkg library.
  .
  These include:
   * apt-get for retrieval of packages and information about them
     from authenticated sources and for installation, upgrade and
     removal of packages together with their dependencies
   * apt-cache for querying available information about installed
     as well as installable packages
   * apt-cdrom to use removable media as a source for packages
   * apt-config as an interface to the configuration settings
   * apt-key as an interface to manage authentication keys
  • What “extended description” are you talking about? apt-cache show shows the package's Description: field, which is what appears on the website between the package name and the tags. I can't see what someone might call “extended description”. – Gilles Jun 16 '17 at 23:32
  • It seems you didn't read past the first paragraph of the question's introduction. – user1404316 Jun 16 '17 at 23:36
  • I have read it. The command apt-cache show shows the same description as dpkg --info. But you say that something is present in dpkg --info and not in apt-cache show. So what are you refering to? Give an example. – Gilles Jun 16 '17 at 23:38
  • Presuming you have the 'apt' package installed on your system, and you haven't cleaned the archive, you can compare the description output of 'apt-cache show apt' to the extended description of 'dpkg --info /var/cache/apt/archives/apt_<tab><tab>_<tab><tab>'. If you have cleaned the archive, you'll need to download the package first. Would you like me copy/paste the outputs for you? The former is a single line; the latter is 14 lines. – user1404316 Jun 16 '17 at 23:43
  • The comments feature limits comments' sizes, so I can't post an example here. – user1404316 Jun 16 '17 at 23:48
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Some research shows that this has been an on-again off-again bug in apt. The work-around, when it happens, is to rename folder '/var/apt/lib'. Then, the extended descriptions will re-appear, at least until the next time you perform 'apt-get update' or until debian fixes the bug once and for all.

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