Re: using a variable from global scope or checking whether it is local to some scope

[Christoph Anton Mitterer]

Hey.

On Fri, 2023-10-20 at 09:40 +0900, Koichi Murase wrote:
> To read a global variable, you can repeat `unset -v' until you reach
> the global variable in a command substitution and return the result
> through stdout. Since it is performed in a subshell, it is not
> destructive in the main-shell environment (If you don't like fork, I
> don't know the other non-destructive way). There is a case that this
> approach doesn't work: If any local variable of the same name in the
> function call stack has a readonly attribute, it cannot unset the
> local variable, so it is impossible to access the global variable
> anyway.
>  If we assume that all the local variables of the same name are
> not readonly, one can detect the global variable by setting `declare
> -g -r var' in advance. When `unset -v' fails, it is expected to be
> the
> global variable.

Neat. And one could check before hand whether it is readonly already
and fail then.

> This is implemented in the following function:
> 
> https://github.com/akinomyoga/ble.sh/blob/742cf2cce6792a9460d38e92f715beb9870a6ff3/src/util.sh#L3031-L3151
> You can use `ble/util/print-global-definitions varname' from inside a
> function to get the declaration of the variable. If you want to copy
> it to the current scope, you may simply run eval
> "$(ble/util/print-global-definitions varname)".

Won't this fail if the value would end in newlines?



> > 2) Or at least find out, whether in a function, the variable is
> >    actually from the global scope or not.
> 
> In a subshell, you can set the readonly attribute to the currently
> visible variable and try to override it in a child function call. It
> fails if the variable is a global variable because a global readonly
> variable is not allowed to be hidden even by a local variable.

So that again requires, that the variable isn't already readonly?!

> It
> succeeds if the variable is local because a local readonly variable
> is
> allowed to be hidden by another local variable declared in a child
> function call.

Ah I see. That I didn't know.


Thanks :-)

Chris.