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From:	Sam Habiel <sam DOT habiel AT gmail DOT com>
Date:	Wed, 30 May 2018 22:22:10 -0400
Message-ID:	<CABHT9632LBGQZmRQjyifmd2vWLu24XCP8oJEaMYWu8qHsvpP+A@mail.gmail.com>
Subject:	Re: Help with C clearenv and setenv
To:	cygwin AT cygwin DOT com
X-IsSubscribed:	yes

Thank you Keith for you reply; but I am actually looking for the C APIs to
manipulate the environment; esp. to clear it.

On Wed, May 30, 2018 at 6:05 PM, Keith Christian <keith1christian AT gmail DOT com>
wrote:

> Sam,
>
> Here is a short demonstration of how to detect unset (possibly null,
> too?) variables in BASH.  Not sure if this is exactly what you are
> looking for but presented for info.
>
> set -x;A_VAR="${RANDOM}";echo "1. ${A_VAR}";echo "2.
> ${A_VAR:?IS_NOT_SET}";unset A_VAR;set +x
> + set -x
> + A_VAR=28641
> + echo '1. 28641'
> 1. 28641
> + echo '2. 28641'
> 2. 28641
> + unset A_VAR
> -bash: A_VAR: IS_NOT_SET
>
>
> Documentation for is in the BASH texinfo docs, read it in a Cygwin
> terminal by typing "info bash" and go to this section:
>
>
> 3.5.3 Shell Parameter Expansion
> -------------------------------
> (( lines deleted ))  (( lines deleted ))  (( lines deleted ))
> '${PARAMETER:-WORD}'
>      If PARAMETER is unset or null, the expansion of WORD is
>      substituted.  Otherwise, the value of PARAMETER is substituted.
>
> '${PARAMETER:=WORD}'
>      If PARAMETER is unset or null, the expansion of WORD is assigned to
>      PARAMETER.  The value of PARAMETER is then substituted.  Positional
>      parameters and special parameters may not be assigned to in this
>      way.
>
> '${PARAMETER:?WORD}'
>      If PARAMETER is null or unset, the expansion of WORD (or a message
>      to that effect if WORD is not present) is written to the standard
>      error and the shell, if it is not interactive, exits.  Otherwise,
>      the value of PARAMETER is substituted.
>
> '${PARAMETER:+WORD}'
>      If PARAMETER is null or unset, nothing is substituted, otherwise
>      the expansion of WORD is substituted.
> (( lines deleted ))  (( lines deleted ))  (( lines deleted ))
>
> Keith
>
> On Wed, May 30, 2018 at 8:48 AM, Sam Habiel <sam DOT habiel AT gmail DOT com> wrote:
> > I have code for a database I am porting to Cygwin.
> >
> > Part of that code is a clearenv() then a couple of setenvs. There is
> > an ifdef for Cygwin, as it doesn't implement clearenv. It just sets
> > environ = NULL. Well--that really breaks setenv! It returns a "Bad
> > Poniter" error (-1).
> >
> > What is the correct way to clear environment variables in Cygwin?
> >
> > --Sam
> > (About me: http://smh101.com/)
> >
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