This feature is only available with the Bourne Again Shell (bash). It is not a feature of any other shell.
To complete a command name press the TAB key.
$ tel<TAB>net telnet>
If the shell is unable to identify a unique command from the characters already entered it will ring the terminal bell.
Pressing the TAB key again displays all the commands on your path that begin with those letters. For example:
$ma<TAB><TAB> mach mail makecookie man maze macptopbm make makedepend mapname
This produces a list of commands that start with the letters ma.
Command name completion also works in the middle of a command line. For example the command name
$ teln<TAB> orion.geo.aber
$ telnet orion.geo.aber