[Toybox] Would someone please explain what bash is doing here?

[Rob Landley]

On 5/4/20 8:39 AM, Chet Ramey wrote:
> On 5/2/20 4:01 PM, Rob Landley wrote:
> 
>> Why does  the export flag on $_ toggle?
> 
> It does three different things. It's initially exported if bash inherits it
> from the environment at startup; after that it does its thing.
> 
> "_      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
>         holds the name of the mail file currently being checked."
> 
> We picked that up from ksh.

My question was why "declare -p _" was showing -x sometimes and not other times,
but I guess that's just an implementation detail showing through? (Unlike other
magic variables, if you unset _ it doesn't stop being magic, it still gets reset
every command. And if you "readonly _" bash gets REALLY chatty. :)

Still trying to work out what the "bash spec" would be, vs implementation details...

Rob