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

[Kerin Millar]

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 |.

-- 
Kerin Millar