Why does 'apt-get install' suggest packages that I already have?

Question

On Ubuntu Desktop 10.04.1, I have just run apt-get install to install a package. Amongst the list of suggested packages is ssh.

I'm confused, because I am sure that I already have ssh installed:

# ssh --version
usage: ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
           [-D [bind_address:]port] [-e escape_char] [-F configfile]
           [-i identity_file] [-L [bind_address:]port:host:hostport]
           [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
           [-R [bind_address:]port:host:hostport] [-S ctl_path]
           [-w local_tun[:remote_tun]] [user@]hostname [command]

So...

When apt-get install suggests packages, does it take my existing set-up into consideration before making the suggestion? If so, why does it suggest something that I already have?

Answer 1

You have the ssh program. You don't have the package called ssh.

Ubuntu splits ssh into two packages: openssh-server and openssh-client. The reason for the split is that many people just need the client, not the server. Having the server installed and running when you don't want it isn't just (tiny) a waste of resources, it's a security risk if you have weak passwords.

There's also a package called ssh. It's intended as a way to say “I just want ssh, all of it, don't bother me with the details”.

APT suggests a package P if one of the packages it's installing suggests P and P isn't installed yet. A suggestion means that

the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.

(in the words of the Debian Policy Manual, which defines the packaging format introduced by Debian and also used by Ubuntu).