Re: [PATCH v2 01/10] t: add helper to convert object IDs to paths

[Johannes Schindelin <Johannes.Schindelin@gmx.de>]

Hi Peff,

On Tue, 18 Jun 2019, Jeff King wrote:

> On Tue, Jun 18, 2019 at 06:15:46PM +0200, Johannes Schindelin wrote:
>
> > > And looking through this patch series, I see a gazillion of *new*
> > > process substitutions $(test_something...) and $(basename $whatever).
> > > Can't we do something about it?
> >
> > I wish there was. Unix shell scripting has not evolved much in the past,
> > what, 3 decades? So I don't really see a way to "pass variables by
> > reference" to shell functions, short of calling `eval` (which buys
> > preciously little as it _also_ has to spawn a new process [*1*]).
>
> Really? An eval can impact the caller's state, so it _can't_ happen in a
> sub-process in most cases.
>
> E.g., if I run this:
>
> -- >8 --
> #!/bin/sh
>
> # usage: test_oid_to_path <var> <oid>
> # to set the variable <var> in the caller's environment to the path of <oid>
> test_oid_to_path() {
> 	path="${2%${2#??}}/${2#??}"
> 	eval "$1=\$path"
> }
>
> test_oid_to_path foo 1234abcd
> echo foo: $foo
> -- >8 --
>
> it all happens in a single process, under both bash and dash.

Oops. I think I may have read too much into the name `eval.c` in Dash's
source code, I assumed that it was all about the `eval` command. But now
that I look at this again, I see:

/*
 * Execute a command inside back quotes.  If it's a builtin command, we
 * want to save its output in a block obtained from malloc.  Otherwise
 * we fork off a subprocess and get the output of the command via a pipe.
 * Should be called with interrupts off.
 */

void
evalbackcmd(union node *n, struct backcmd *result)
{
	[...]

I saw this at
https://git.kernel.org/pub/scm/utils/dash/dash.git/tree/src/eval.c#n608

So this is actually about `$(...)` (or the old, non-nestable backtick
version of it) and not about `eval`.

And of course I was also making another incorrect connection to the Perl
command `eval` which allows you to catch code that `die()`s (which *must*
run in a subprocess).

So yeah, I guess `eval` would work here to avoid the `$(...)` constructs.

Sorry for the noise,
Dscho