_ At shell startup, set to the absolute pathname used to invoke
the shell or shell script being executed as passed in the envi‐
ronment or argument list. Subsequently, expands to the last ar‐
gument to the previous simple command executed in the fore‐
ground, after expansion. Also set to the full pathname used to
invoke each command executed and placed in the environment ex‐
ported to that command. When checking mail, this parameter