on Sun, May 06, 2001 at 02:22:40AM +0300, Tommi Komulainen ([EMAIL PROTECTED])
wrote:
> On Sat, May 05, 2001 at 02:34:47PM -0700, Karsten M. Self wrote:
> >
> > In contrast, csh and derivatives have:
> >
> > $?MYVAR
> >
> > ...which allows testing of presence of a variable.
> >
> > ...but I'm not aware of a similar bash/korn/bourne feature. Anyone?
>
> Coming soon to ./configure scripts near you:
>
> if test "${LANG+set}" = set; then LANG=C; export LANG; fi
Close, but not quite. It's probably the nearest thing to a winner I've
seen posted though. And you just edged out Alan's suggestion of an
equivalent but opposite test.
Note that this is a specific application of the Use Alternate Value bash
parameter expansion. The value "set" isn't special, though it's a good
mnemonic.
> or in your case:
>
> if test "${MYVAR+set}" = set; then
> echo "MYVAR doesn't exist"
> else
> echo "MYVAR exists, value: $MYVAR"
> fi
>
>
> also available in bash(1):
>
> In each of the cases below, word is subject to tilde
> expansion, parameter expansion, command substitution, and
> arithmetic expansion. When not performing substring
> expansion, bash tests for a parameter that is unset or
> null; omitting the colon results in a test only for a
> parameter that is unset.
>
> ${parameter:+word}
> Use Alternate Value. If parameter is null or
> unset, nothing is substituted, otherwise the expanÂ
> sion of word is substituted.
The possible problem" "*null or* unset". An existing, but null,
environment variable will be reported as unset.
It's possible to create a null environment variable, e.g.:
$ foo=
$ echo $foo
$ echo ${foo:-unset}
unset
$ typeset -p foo
declare -- foo=""
$ unset foo
$ typeset -p foo
bash: typeset: foo: not found
$ echo $?
1
...which suggests
if ! typeset -p myvar 2>/dev/null
...as a test under bash which accurately reports whether or not an
environment variable exists, evaluated or otherwise, rather than merely
has a non-null value.
Note that yet another option would be to access /proc/self/environ to
check for the existence of an environment variable. Which works under
GNU/Linux but not other Unices.
> Since it is used in configure scripts, I'd guess it's pretty portable.
It does the job. It may not be optimal. And, yes, portability is an
issue here.
--
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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