Para descobrir a qual tipo de repositório (main, contrib ou non-free) um determinado pacote pertence, é possível usar a saída do comando "apt-cache show", como no exemplo a seguir:

apt-cache show trn

Package: trn
Priority: optional
Section: non-free/news
Installed-Size: 528
Maintainer: Matthew Vernon <matthew@debian.org>
Architecture: i386
Version: 3.6-20
Provides: news-reader
Depends: libc6 (>= 2.7-1), libncurses5 (>= 5.6+20071006-3), inews | inewsinn
Recommends: mail-transport-agent
Suggests: ispell
Filename: pool/non-free/t/trn/trn_3.6-20_i386.deb
Size: 217534
MD5sum: bc63eea395dab95a441cd0d350d46f63
SHA1: 63497f598d8bd186eeccc51eb7e5c2fb32d1cba1
SHA256: 17de6d86cab3ee9f09d5f9efa2c55a404d2f1ba037d0e3990ca63a717bdd999f
Description: Threaded USENET news reader, based on rn
trn is the most widely-used newsreader on USENET - even to the
point of being a de-facto standard. It has a powerful full screen
text mode user interface, where most actions are available at the
press of a single key. It is highly configurable.
.
This version of trn reads news from a news server via NNTP (the
Network News Transfer Protocol). You must have a news server
available - large sites usually provide a site-wide server.
.
If you install a news server on your system (either containing
just local newsgroups or with a partial or full USENET feed) you
can configure trn to access it.
Tag: network::client, protocol::nntp, role::program, uitoolkit::ncurses

Observem que a linha "Filename" contém a indicação "non-free":

Filename: pool/non-free/t/trn/trn_3.6-20_i386.deb

Para obter o resultado de forma mais sucinta, é possível rodar a seguinte sequência de comandos:

apt-cache show <pacote> | grep "Filename" | cut -d "/" -f 2

Exemplo:

apt-cache show trn | grep "Filename" | cut -d "/" -f 2

non-free

Ou, caso prefiram, usem o script que desenvolvi exclusivamente para este propósito:

• Descobrindo a que tipo de repositório um pacote pertence [Shell Script]

Para mais informações a respeito dos tipos de repositórios Debian, consulte:

• Debian Policy Manual - The Debian Archive