Temporary variable compatibility

Temporary variable compatibility

[Felipe Contreras]

Hi,

I found a discrepancy between different shells:

  f() { echo $var; }
  var=1 f
  test -z "${var-}"

Both zsh, bash, and dash set the variable temporarily, but ksh does not.

In zsh the emulation of ksh and sh both keep the variable assignment.

Is this the POSIX standard? If so, then how should one temporarily set
a variable? A subshell?

Cheers.

-- 
Felipe Contreras

Re: Temporary variable compatibility

[Lawrence Velázquez]

On Tue, Mar 21, 2023, at 8:21 PM, Felipe Contreras wrote:
> I found a discrepancy between different shells:
>
>   f() { echo $var; }
>   var=1 f
>   test -z "${var-}"
>
> Both zsh, bash, and dash set the variable temporarily, but ksh does not.
>
> In zsh the emulation of ksh and sh both keep the variable assignment.
>
> Is this the POSIX standard?

POSIX.1-2017 leaves it unspecified, so both behaviors are conformant.
As per section 2.9.1 "Simple Commands" (e.g., something like "var=val
cmd"):

	- If the command name is a function that is not a standard
	  utility implemented as a function, variable assignments
	  shall affect the current execution environment during the
	  execution of the function.  It is unspecified:

		- Whether or not the variable assignments persist
		  after the completion of the function
		- Whether or not the variables gain the export
		  attribute during the execution of the function
		- Whether or not export attributes gained as a
		  result of the variable assignments persist after
		  the completion of the function (if variable
		  assignments persist after the completion of the
		  function)

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01


> If so, then how should one temporarily set a variable? A subshell?

That's probably the most portable approach, yes.


-- 
vq