Re: [PLUG] Distinguishing between environment variable with null value and one that is not set

["Paul L. Snyder" <plsnyder@drexel.edu>]

On Tue, 13 Oct 2009, K.S. Bhaskar wrote:

> It appears that POSIX shell (and bash) have no built-in way to
> distinguish between the case where an environment variable exists with
> a null value and the case where it simply does not exist.  The
> simplest way I have been able to come up with is to grep the output of
> env and test for zero length, e.g.:
[...]
> Is there a better way?  Thanks in advance.

  % typeset -p foo
  typeset: no such variable: foo
  % echo $?
  1
  % foo=''
  % typeset -p foo
  typeset foo=''
  % echo $?
  0
  % if typeset -p foo 1>&2; then echo yup; else echo nope; fi 2> /dev/null
  yup
  % unset foo
  % if typeset -p foo 1>&2; then echo yup; else echo nope; fi 2> /dev/null
  nope

Doing a more normal '2>&1 >/dev/null' immediately after the typeset doesn't
work (I'm sure there's a good reason, but I'd have to figure it out), so we
hack it by redirecting stdout for the typeset (1) to stderr (2) and then
redirecting stderr for the whole compound command to /dev/null.

Paul
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