Re: variables set on command line

[Eric Blake]

On 08/24/2011 10:07 AM, Sam Steingold wrote:
> > * Eric Blake<roynxr@erqung.pbz>  [2011-08-24 09:31:45 -0600]:
> > > f(){ echo a=$a b=$b c=$c ; }
> > > f
> > > a= b= c=
> > > a=a b=b f
> > > a=a b=b c=
> > > f
> > > a=a b=b c=
> >
> > Which is indeed correct under the rules for POSIX
>
> This sucks big time.

Such is life when dealing with shell portability.

> So if I want to bind a variable for an eval invocation and do this:
>
>    eval "`./libtool --tag=CC --config | grep '^archive_cmds='`"
>    CC='${CC}' libobjs='$libs' deplibs='${CLFLAGS}' compiler_flags='${CFLAGS}' \
>      soname='$dll' lib='$lib' output_objdir='$dyndir' \
>      eval XCC_CREATESHARED=\"${archive_cmds}\"
>
> and I want CC to have an old value after the second eval, I need to save
> it and restore it by hand, like this:
>
>    CC_save=$CC
>    CC='${CC}' libobjs='$libs' deplibs='${CLFLAGS}' compiler_flags='${CFLAGS}' \
>      soname='$dll' lib='$lib' output_objdir='$dyndir' \
>      eval XCC_CREATESHARED=\"${archive_cmds}\"
>    CC=$CC_save
>
> however, this does not distinguish between unset CC and CC=''.
> (is there a way to distinguish these two situations?)

Yes - autoconf does this all the time, using an idiom roughly like this:

CC_set=${CC+set}
CC_save=$CC
do stuff that modifies $CC
if test "$CC_set" = set; then
  CC=$CC_save
else
  unset CC
fi

Also, you can use command to suppress the ability of built-ins like eval 
(but not function calls) to affect the current environment:

$ unset foo
$ foo=bar eval :
$ echo $foo
bar
$ unset foo
$ foo=bar command eval :
$ echo $foo
$

--
Eric Blake   eblake@redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org