variables set on command line

[Sam Steingold]

CYGWIN_NT-5.2-WOW64 sds 1.7.9(0.237/5/3) 2011-03-29 10:10 i686 Cygwin
BASH_VERSION='4.1.10(4)-release'

at the bash prompt I observe this:
$ f(){ echo a=$a b=$b c=$c ; }
$ unset a b c
$ a=a b=b f
a=a b=b c=
$ f
a= b= c=
which I believe is correct (i.e., variables set in "a=a b=b f" are unset
after f terminates).

alas, when I call /bin/sh on the same machine, I see this:

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=
exit

i.e., variables set in "a=a b=b f" survive to the next command.
$ /bin/sh --version
GNU bash, version 4.1.10(4)-release (i686-pc-cygwin)

is this the expected behavior?

thanks.

--
Sam Steingold (http://sds.podval.org/) on CentOS release 5.6 (Final) X 11.0.60900031
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He who laughs last thinks slowest.

[Eric Blake]

On 08/24/2011 09:24 AM, Sam Steingold wrote:
> CYGWIN_NT-5.2-WOW64 sds 1.7.9(0.237/5/3) 2011-03-29 10:10 i686 Cygwin
> BASH_VERSION='4.1.10(4)-release'
>
> at the bash prompt I observe this:
> $ f(){ echo a=$a b=$b c=$c ; }
> $ unset a b c
> $ a=a b=b f
> a=a b=b c=
> $ f
> a= b= c=
> which I believe is correct (i.e., variables set in "a=a b=b f" are unset
> after f terminates).

This is bash's default behavior, but it violates POSIX.

>
> alas, when I call /bin/sh on the same machine, I see this:

That tells bash to strictly obey POSIX, so you get the POSIX behavior.

>
> 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 (basically, POSIX
requires function calls to behave like special built-ins, such that
changes to the environment persist after the function call - the bash
developer thinks the posix rule is counterintuitive, which is why the
default bash behavior is opposite the posix behavior).

Your question is not cygwin-specific.

> is this the expected behavior?

Yes.

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

[Sam Steingold]

> * Eric Blake <roy...@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.
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?)

thanks!

--
Sam Steingold (http://sds.podval.org/) on CentOS release 5.6 (Final) X 11.0.60900031

http://ffii.org http://palestinefacts.org http://honestreporting.com
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Abandon all hope, all ye who press Enter.

[Eric Blake]

On 08/24/2011 10:07 AM, Sam Steingold wrote:
>> * Eric Blake<roy...@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
$