Re: [PATCH v2 0/2] env: setenv add resolve value option

[Wolfgang Denk <wd@denx.de>]

Dear Artem,

In message <CAKaHn9+yL3naiDqbsQJWQ0Po=g=TmjjcPSKxkx5C4ay1M1AsUA@mail.gmail.com> you wrote:
> > >
> > > next examples just demonstrate how its works for already defined env
> > > variables which contain other variables (like storred env variables)
> >
> > Which next examples?
>
> Usage examples (from commit message):
>
> => setenv a hello; setenv b world; setenv c '${a} ${b}'
> => setenv -r d '${c}! ${a}...'
> => printenv d
> d=hello world! hello...

This is a very simple example, and I showed you how you can solve
this one by just omitting the apostrophes:

=> setenv a hello; setenv b world; setenv c ${a} ${b}
=> setenv d ${c}! ${a}...
=> printenv d
d=hello world! hello...


I _think_ what you actually have in mind is something like this:

=> setenv a hello
=> setenv b world
=> setenv c '${a} ${b}'
=> setenv a goodbye
=> setenv b sunshine

something to set d to: '${c}! ${a}...'

=> printenv d

Here my simple approach does not show what you want to have:

=> setenv a hello
=> setenv b world
=> setenv c ${a} ${b}
=> setenv a goodbye
=> setenv b sunshine
=> setenv d ${c}! ${a}...
=> printenv d
d=hello world! goodbye...

That's because here evaluation takes place at assignment, but you
want it when used - but being recursive here is neither a good idea
nor standard.

How would you do it in a standard posix shell?  You would have to
use "eval", like that:

$ a=hello
$ b=world
$ c='${a} ${b}'
$ a=goodbye
$ b=sunshine
$ d=$(eval echo $c)
$ echo $d
goodbye sunshine

But please note that "eval" is _not_ recursive!!

$ a='$b'
$ eval echo $c
$b sunshine

And this is why I object against this patch.

Oh, and in U-Boot you could write this as:

=> setenv a hello
=> setenv b world
=> setenv c '${a} ${b}'
=> setenv a goodbye
=> setenv b sunshine
=> setenv foo "setenv d ${c}! ${a}..."
=> run foo
=> printenv d                         
d=goodbye sunshine! goodbye...

And yes, here you have to be careful about using ' or " as there is
no recursion like you might expect.

So yes, it would be nice if we had "eval" (which will ocme with the
hush update), and no, "eval" does not recurse either.



Best regards,

Wolfgang Denk

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