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 09:10:19 -0600
> Peng Yu <pengyu.ut@gmail.com> wrote:
>
>> 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`?
>
> I would consider it highly unlikely but would also suggest that you conduct
> your own benchmarks.
>
> time for ((i=0; i < 1000; i++)); do X=x f | cat >/dev/null; done
> time for ((i=0; i < 1000; i++)); do { X=x; f; } | cat >/dev/null; done
>
> Obviously, you may need to tune the number of iterations in order to incur
> an acceptably substantive wall time.

I see a slight difference (the long XX... name is to make the
assignment takes more time). It seems that `{ X=x; f; } | true` is
faster. Is it always true? What is done under the hood that leads to
the timing difference?

time for ((i=0; i < 1000; i++)); do
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX=x
f | true; done

real    0m0.952s
user    0m0.572s
sys     0m0.951s
time for ((i=0; i < 1000; i++)); do {
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX=x;
f; } | true; done

real    0m0.932s
user    0m0.540s
sys     0m0.912s

-- 
Regards,
Peng