Re: "local -g" declaration references local var in enclosing scope

[Kerin Millar]

On Sun, 10 Mar 2024 23:09:37 +0800
Adrian Ho <lexfiend@gmail.com> wrote:

> [Apologies, an earlier edition of this bug report was sent from the address
> aho@lex.03s.net, which can only be replied to from my internal network.
> Please ignore that report. Thanks much!]
> 
> Configuration Information [Automatically generated, do not change]:
> Machine: x86_64
> OS: linux-gnu
> Compiler: gcc-11
> Compilation CFLAGS: -DSSH_SOURCE_BASHRC
> uname output: Linux lex 6.5.0-25-generic #25-Ubuntu SMP PREEMPT_DYNAMIC Wed
> Feb  7 14:58:39 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux
> Machine Type: x86_64-pc-linux-gnu
> 
> Bash Version: 5.2
> Patch Level: 26
> Release Status: release
> 
> Description:
>         "local -g var" incorrectly references a local variable in an
> enclosing scope, while "local -g var=val" correctly references and modifies
> the global variable.
> 
> Repeat-By:
>         Test script:
> 
> #!/usr/bin/env bash
> echo "Bash version: $BASH_VERSION"
> 
> b() {
>   local -g a=2
>   c
> }
> c() {
>   local -g a
>   a=3
> }
> 
> y() {
>   local z=1
>   x
> }
> x() {
>   local -g z=2
>   w
> }
> w() {
>   local -g z
>   echo "w: z=$z"
>   z=3
> }
> 
> b; y
> echo "a=$a z=$z"
> 
>         Expected output:
> 
> Bash version: 5.2.26(1)-release
> w: z=2
> a=3 z=3
> 
>         Actual output:
> 
> Bash version: 5.2.26(1)-release
> w: z=1
> a=3 z=2

Dynamic scoping can be tremendously confusing. The following examples should 
help to clarify the present state of affairs.

$ x() { local a; y; echo "outer: $a"; }
$ y() { local a; a=123; echo "inner: $a"; }
$ x; echo "outermost: $a"
inner: 123
outer:
outermost:

This is likely as you would expect.

$ y() { local -g a; a=123; echo "inner: $a"; }
$ x; echo "outermost: $a"
inner: 123
outer: 123
outermost:

This may not be. There, the effect of the -g option effectively ends at the 
outermost scope in which the variable, a, was declared. Namely, that of the x 
function.

$ y() { local -g a=123; echo "inner: $a"; }
$ x; echo "outermost: $a"
inner:
outer:
outermost: 123

There, by combining the -g option with assignment syntax, the outermost scope - 
that which most would consider as being implied by the term, global - is duly 
reached.

The manual states that the "-g option forces variables to be created or 
modified at the global scope, even when declare is executed in a shell 
function" and that "it is ignored in all other cases". I would consider this 
wording insufficient for a user to be able to effectively reason with the 
difference between the second case and the other two cases presented.

-- 
Kerin Millar