No bash e possivelmente em outros shells, a guia -completar envolvendo um símbolo @ :
Attempt to perform completion on the text before point. The actual completion performed is application-specific. Bash attempts completion treating the text as a variable (if the text begins with ‘$’), username (if the text begins with ‘~’), hostname (if the text begins with ‘@’), or command (including aliases and functions) in turn. If none of these produces a match, filename completion is attempted.
O código do bash está em /usr/share/bash-completion/bash_completion , iniciando na função _user_at_host .