_
At shell startup, set to the absolute file name of the shell or
shell script being executed as passed in the argument list.
Subsequently, expands to the last argument to the previous
command, after expansion. Also set to the full file name of
each command executed and placed in the environment exported to
that command. When checking mail, this parameter holds the name
of the mail file currently being checked.
RIGHTS
see the Bash copyrights.