How to check that if a package is installed?

[Mihamina Rakotomandimby]

> dreadpiratejeff at gmail.com :
>or if you know the command from a given package, just try running it:
>  bladernr at klaatu:/media/TOSHIBA EXT$ fop
>  The program 'fop' is currently not installed.  You can install it by
>  typing: sudo apt-get install fop

That will work only for package of software meant to be launched.

For example, libraries aren't:
  mihamina at packard:~$ libc
  libc: command not found

The clean way is to use dpkg + some filtering (awk or grep)

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