on Sat, May 05, 2001 at 04:08:26PM -0700, Eric G. Miller (egm2@jps.net) wrote:
> On Sat, May 05, 2001 at 02:34:47PM -0700, Karsten M. Self wrote:
> > This came up on another list. The problem involves testing existence of
> > a Unix shell variable from another program with limited system
> > interaction features.
> >
> > I usually write this in bash as:
> >
> > if [ x${MYVAR} = x ]; then
> > echo 'MYVAR doesn't exist (or isn't set)'
> > else echo "MYVAR exists, value: $MYVAR"
> > fi
> >
> > ...which essentially checks whether or not the variable has a non-null
> > value. But would report that $MYVAR doesn't exist if in fact it was set
> > equal to "".
> >
> > In contrast, csh and derivatives have:
> >
> > $?MYVAR
> >
> > ...which allows testing of presence of a variable.
> >
> > ...but I'm not aware of a similar bash/korn/bourne feature. Anyone?
>
> #! /bin/sh
>
> ...
>
> if test -z ${MYVAR} ; then
> ...
> fi
>
> The [...] shorthand isn't guaranteed AFAIK. Also, be sure variables are
> always "declared" first, at least like:
>
> MYVAR = ""
>
> Some bourne shells don't like testing variables that don't exist yet.
That's precisely what we're trying to test.
--
Karsten M. Self <kmself@ix.netcom.com> http://kmself.home.netcom.com/
What part of "Gestalt" don't you understand? There is no K5 cabal
http://gestalt-system.sourceforge.net/ http://www.kuro5hin.org
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