Re: env: can it be used to let only certain variables through (if they're set)?

[Pádraig Brady]

On 21/01/2022 00:28, Christoph Anton Mitterer wrote:
> Hey.
>
> What I'd like to do, is set up a shebang line e.g. for shell scripts,
> so that the environment (in terms of variables) it gets is kinda
> predictable.
>
> For certain tasks this is very easy, e.g. I can set some
> PATH=/usr/bin:/bin or so.
>
> It's also easy to filter out anything I don't set in specific, by using
> -i ... so I can get rid of things like POSIXLY_CORRECT which would
> alter behaviour of tools like GNU's sed & friends... and also
> everything in terms of such variable which I don't even think about.
>
>
>
> Now what I cannot get fully working, is to let certain variables
> through, only if they are set.
> E.g. it could make sense to block LC_CTYPE, but let LC_MESSAGES
> through.
>
> /usr/bin/env -iS 'LC_MESSAGES=${LC_MESSAGES} command args'
>
> get's close to it, but has the problem that LC_MESSAGES is set (though
> empty) if it wasn't set outside.
>
> And there are env vars, which have altering behaviour to certain
> programs, purely when they're set (regardless of whether empty or so).
>
>
>
> On the command line (i.e. without shebang) I can do:
>
> /usr/bin/env -i ${LC_MESSAGES+"LC_MESSAGES=${LC_MESSAGES}"} command args
>
> can this be done for -S, too? Or added as a feature?

--split-string doesn't support that currently.
The standard shell syntax to support
setting a variable iff it's not previously set is:
  ${var="value"}
Or relaxing a bit to set a variable if it's empty or unset use
  ${var:="value"}

So expanding your example this would be:
  ${LC_MESSAGES=${LC_MESSAGES}}

Arguments against this would be:
  Quite obtuse syntax
  Wouldn't be backwards compat with older or other env -S implementations
  Quite verbose as you would probably want to list many of these on the -S line

Another possibility to implement this functionality
while avoiding all of the above disadvantages
would be to augment the recently proposed --file option
to also support removing, or passing if set functionality, like:

  var1=explicit_val
  var2=${referenced_val}
  # explicitly remove var3
  -var3
  # pass var4 through if set (for use with -i)
  +var4
  # pass var5 if set, or else init to val
  +var5=val
  # pass var6 if non empty, or else init to val
  :var6=val
  # pass var7 if non empty
  :var7

Another possibility might be to support a POSIX shell compat syntax for this?

  var1=explicit_val
  var2=$'escaped$\nval'
  unset var3
  : ${var4=${var4}}
  : ${var5=val}
  : ${var6:=val}
  : ${var7:=${var7}}

I'm leaning towards the non shell compat syntax,
but need to think a bit more about this.

cheers,
Pádraig