Use iwlist . A página man (na verdade, link ) diz:
wlist.8.gz
Provided by: wireless-tools_30~pre9-8ubuntu1_i386
NAME
iwlist - Get more detailed wireless information from a wireless
interface
SYNOPSIS
iwlist [interface] scanning
snip ...
PARAMETERS
scan[ning]
Give the list of Access Points and Ad-Hoc cells in range, and
optionally a whole bunch of information about them (ESSID,
Quality, Frequency, Mode...). The type of information returned
depends on what the card supports.
Triggering scanning is a privileged operation (root only) and
normal users can only read left-over scan results. By default,
the way scanning is done (the scope of the scan) is dependant on
the card and card settings.
This command takes optional arguments, however most drivers will
ignore those. The option essid is used to specify a scan on a
specific ESSID. With some card/driver, this enables to see
hidden networks. The option last does not trigger a scan and
read left-over scan results.
"Como eu me conecto a uma conexão sem fio ..." é uma segunda pergunta que deve ser feita sozinha, em uma postagem diferente.