Re: nameref and referenced variable scope, setting other attributes (was "local -g" declaration references local var in enclosing scope)

[Chet Ramey]

On 3/10/24 7:29 PM, Zachary Santer wrote:

> Bash Version: 5.2
> Patch Level: 26
> Release Status: release
>
> Description:
>
> On Sun, Mar 10, 2024 at 3:55 PM Zachary Santer <zsanter@gmail.com> wrote:
> > Relatedly, how would one set attributes on a variable declared in a
> > calling function? 'readonly' and 'export' can do it for their
> > respective attributes, but otherwise, I think you just can't.
>
> Second-guessed myself.
>
> The manual says about 'declare -n':
> -n  Give each name the nameref attribute, making it a name reference to
> another variable. That other variable is defined by the value of name. All
> references, assignments, and attribute modifications to name,  except those
> using or changing the -n attribute itself, are performed on the variable
> referenced by name's value. The nameref attribute cannot be applied to
> array variables.
>
> local, when called on a nameref variable referencing a variable declared in
> a calling function, creates a new local variable named the same as the
> value of the nameref variable. Given the above, I would expect it to
> instead allow the setting of attributes and value of a variable declared in
> a calling function.

`local' always creates variables at the current scope, or at the global
scope if `-g' is supplied. If it's supplied the name of a nameref, it first
resolves the nameref to find the name of the variable it's supposed to act
on, failing if it can't. Once it has the name it needs, it creates or
modifies the variable at the current scope. It doesn't try to create or
modify the variable at the nameref's scope. This is one consequence of
dynamic  scoping that affects the implementation: a nameref's value is just
a name, not a pointer to a specific instance of a variable. Once you have
that name, the normal scoping rules apply.

If you want to look at it from a filesystem perspective, a nameref is a
symlink, rather than a hard link.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/