J. An Introduction to Programmable Completion (Revision #1)

The programmable completion feature in Bash permits typing a partial command, then pressing the [Tab] key to auto-complete the command sequence. [1] If multiple completions are possible, then [Tab] lists them all. Let's see how it works.

bash$ xtra[Tab]
xtraceroute       xtrapin           xtrapproto
 xtraceroute.real  xtrapinfo         xtrapreset
 xtrapchar         xtrapout          xtrapstats

bash$ xtrac[Tab]
xtraceroute       xtraceroute.real

bash$ xtraceroute.r[Tab]
xtraceroute.real

Tab completion also works for variables and path names.

bash$ echo $BASH[Tab]
$BASH                 $BASH_COMPLETION      $BASH_SUBSHELL
 $BASH_ARGC            $BASH_COMPLETION_DIR  $BASH_VERSINFO
 $BASH_ARGV            $BASH_LINENO          $BASH_VERSION
 $BASH_COMMAND         $BASH_SOURCE

bash$ echo /usr/local/[Tab]
bin/     etc/     include/ libexec/ sbin/    src/    
 doc/     games/   lib/     man/     share/