Re: The difference between `X=x f | cat` and `{ X=x; f; } | cat`

[Peng Yu]

On 1/19/23, Kerin Millar <kfm@plushkava.net> wrote:
> On Thu, 19 Jan 2023 08:02:07 -0600
> Peng Yu <pengyu.ut@gmail.com> wrote:
>
>> Hi,
>>
>> f is a function that uses a variable called X, which is not declared in
>> f.
>>
>> As far as I can tell, the following two ways produce the same results.
>>
>> - `X=x f | cat`
>> - `{ X=x; f; } | cat`
>
> They are not equivalent.
>
> $ bash -c 'f() { declare -p X; }; X=x f; declare -p X'
> declare -x X="x"
> bash: line 1: declare: X: not found
>
> $ bash -c 'f() { declare -p X; }; X=x; f; declare -p X'
> declare -- X="x"
> declare -- X="x"
>
> The first version defines X in such a way that it affects only the operating
> environment in which f is run. Note, also, that the variable is marked as
> being exportable, meaning that it would be present in the environment of any
> subprocesses that might be launched by f. For a more rigorous explanation,
> refer to the SIMPLE COMMAND EXPANSION section of the manual.
>
> The second version just defines X as an ordinary shell variable then
> proceeds to run f. The use of a { list; } ensures that both of those things
> happen within the same subshell - the one implied by the left hand side of
> |.

In my question, X is never accessed after calling f.

Let's put my question this way to make it clearer.

Suppose that in `X=x f | cat`, f is a function, that uses X read-only,
no external commands called by f directly or indirectly use X. And I
know that `{ X=x; f; } | cat` produces the same results that I care
about (for example, X=x is available even after f is something I don't
care).

In this case, is `{ X=x; f; } | cat` faster than `X=x f | cat`?

-- 
Regards,
Peng