Re: snippet of configure script - explain please
Malcolm Kay wrote:
On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set"; };
then
I find this line somewhat strange as I've not been able
to find documentation for the expansion of ${parameter+set} under the
Bourne shell. (nor bash, nor ksh)
*
Presumably someone out there knows where to find it?
*
It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise you
get "set":
$ echo ${foo+set}
$ echo ${HOME+set}
set
So it appears; but is it stated anywhere that this shorthand is legitimate?
I find it quite frequently arising from the GNU configuring tools but
haven't found it elsewhere.
Is it a deliberate shorthand or just a consequence of the way sh and bash
happen to have been programmed? In other words is it a safe shorthand?
Anyway thanks for the clarification,
This syntax is certainly legitimate, and it is covered in the sh(1)
man page. However, the relevant section talks mostly about the related
form:
${parameter:+word}
which means 'evaluate to null unless ${parameter} is unset or null, otherwise evaluate to
"word"'. Then there's a very small and easily
missed note to the effect:
"In the parameter expansions shown previously, use of the colon in the
format results in a test for a parameter that is unset or null; omission
of the colon results in a test for a parameter that is only unset."
So ${parameter+word} means 'evaluate to null unless ${parameter} is unset,
otherwise evaluate to "word"'
Cheers,
Matthew
--
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Re: snippet of configure script - explain please
On Fri, 11 Jul 2008 12:34 am, Mel wrote:
> On Thursday 10 July 2008 16:48:42 Malcolm Kay wrote:
> > On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
> > > On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
> > > > >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" =
> > > > > set"; }; then
> > > >
> > > > I find this line somewhat strange as I've not been able
> > > > to find documentation for the expansion of ${parameter+set} under the
> > > > Bourne shell. (nor bash, nor ksh)
> > > > *
> > > > Presumably someone out there knows where to find it?
> > > > *
> > >
> > > It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise
> > > you get "set":
> > > $ echo ${foo+set}
> > >
> > > $ echo ${HOME+set}
> > > set
> >
> > So it appears; but is it stated anywhere that this shorthand is legitimate?
> > I find it quite frequently arising from the GNU configuring tools but
> > haven't found it elsewhere.
> >
> > Is it a deliberate shorthand or just a consequence of the way sh and bash
> > happen to have been programmed? In other words is it a safe shorthand?
> >
> > Anyway thanks for the clarification,
>
> Hmm, I'm not sure if the colon syntax came first.
Looks like your supposition is correct. I dug out an old DEC Ultrix manual
which ducuments an 'sh' shell and a 'sh5' shell the 'sh' being the "normal"
bsd version of the Bourne shell and 'sh5' being a compatibility version
for system V scripts. The former (bsd version) does not use the ':' in
parameter substitutions. But the system V version does.
> Autotools claims to create
> portable shell code, though they
> also claim to make software developer's
> lives easier.
Malcolm
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Re: snippet of configure script - explain please
On Fri, 11 Jul 2008 00:18:42 +0930, Malcolm Kay <[EMAIL PROTECTED]> wrote:
> On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
>> On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
>>
>> > >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set"; };
>> > > then
>> >
>> > I find this line somewhat strange as I've not been able
>> > to find documentation for the expansion of ${parameter+set} under the
>> > Bourne shell. (nor bash, nor ksh)
>> > *
>> > Presumably someone out there knows where to find it?
>> > *
>>
>> It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise you
>> get "set":
>> $ echo ${foo+set}
>>
>> $ echo ${HOME+set}
>> set
>
> So it appears; but is it stated anywhere that this shorthand is
> legitimate? I find it quite frequently arising from the GNU
> configuring tools but haven't found it elsewhere.
It's legitimate.
> Is it a deliberate shorthand or just a consequence of the way sh and
> bash happen to have been programmed? In other words is it a safe
> shorthand?
The shorthand version would work too:
if "${foo+set}" = 'set' ; then
bar
fi
The interesting bits in the Autoconf generated code are, however, the
side-effects of the expression:
* ${ac_var} is set to the name of the variable to check.
* The `ac_var' variable is expanded *twice* in the check. It is
expanded once before eval runs, to get the _name_ of the variable to
check, and then eval sees something like "${foo+set}".
* The assignment to `ac_var' is done inside a { ... } pair of
brackets; not in parentheses. This means that the rest of the
script can keep using ${ac_var} to find the name of the `autoconf
variable' this part of the script has set (the assignment to ac_var
happens in the current shell, and not in a sub-process).
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Re: snippet of configure script - explain please
On Thursday 10 July 2008 16:48:42 Malcolm Kay wrote:
> On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
> > On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
> > > >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" =
> > > > set"; }; then
> > >
> > > I find this line somewhat strange as I've not been able
> > > to find documentation for the expansion of ${parameter+set} under the
> > > Bourne shell. (nor bash, nor ksh)
> > > *
> > > Presumably someone out there knows where to find it?
> > > *
> >
> > It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise
> > you get "set":
> > $ echo ${foo+set}
> >
> > $ echo ${HOME+set}
> > set
>
> So it appears; but is it stated anywhere that this shorthand is legitimate?
> I find it quite frequently arising from the GNU configuring tools but
> haven't found it elsewhere.
>
> Is it a deliberate shorthand or just a consequence of the way sh and bash
> happen to have been programmed? In other words is it a safe shorthand?
>
> Anyway thanks for the clarification,
Hmm, I'm not sure if the colon syntax came first. Autotools claims to create
portable shell code, though they also claim to make software developer's
lives easier.
The person to ask would probably be Doug Barton, since he's capable of writing
things in shell, "normal" people would jump to a "real" language ;)
--
Mel
Problem with today's modular software: they start with the modules
and never get to the software part.
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Re: snippet of configure script - explain please
On Fri, Jul 11, 2008 at 12:18:42AM +0930, Malcolm Kay wrote:
> On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
> > On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
> >
> > > >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set";
> > > > };
> > > > then
it's interesting, but config.log would probably show the actual check
that was made to fill in the shell variable...
> > > I find this line somewhat strange as I've not been able
> > > to find documentation for the expansion of ${parameter+set} under the
> > > Bourne shell. (nor bash, nor ksh)
> > > *
> > > Presumably someone out there knows where to find it?
> > > *
> >
> > It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise you
> > get "set":
> > $ echo ${foo+set}
> >
> > $ echo ${HOME+set}
> > set
>
> So it appears; but is it stated anywhere that this shorthand is legitimate?
> I find it quite frequently arising from the GNU configuring tools but
> haven't found it elsewhere.
>
> Is it a deliberate shorthand or just a consequence of the way sh and bash
> happen to have been programmed? In other words is it a safe shorthand?
man sh on Solaris for instance:
${parameter:+word}
If parameter is set and is non-null, substitute word;
otherwise substitute nothing.
In the above, word is not evaluated unless it is to be used
as the substituted string, so that, in the following exam-
ple, pwd is executed only if d is not set or is null:
echo ${d:-`pwd`}
SunOS 5.8Last change: 9 May 19974
The same feature is on OpenBSD - I don't have FreeBSD at hand, but
think it's likely to be found in the manpage - I just looked for
/+.*}
--
Thomas E. Dickey
http://invisible-island.net
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Re: snippet of configure script - explain please
On Thu, 10 Jul 2008 09:45 pm, Mel wrote:
> On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
>
> > >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set"; };
> > > then
> >
> > I find this line somewhat strange as I've not been able
> > to find documentation for the expansion of ${parameter+set} under the
> > Bourne shell. (nor bash, nor ksh)
> > *
> > Presumably someone out there knows where to find it?
> > *
>
> It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise you
> get "set":
> $ echo ${foo+set}
>
> $ echo ${HOME+set}
> set
So it appears; but is it stated anywhere that this shorthand is legitimate?
I find it quite frequently arising from the GNU configuring tools but
haven't found it elsewhere.
Is it a deliberate shorthand or just a consequence of the way sh and bash
happen to have been programmed? In other words is it a safe shorthand?
Anyway thanks for the clarification,
Malcolm
>
>
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Re: snippet of configure script - explain please
On Thursday 10 July 2008 06:24:46 Malcolm Kay wrote:
> >9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set"; };
> > then
>
> I find this line somewhat strange as I've not been able
> to find documentation for the expansion of ${parameter+set} under the
> Bourne shell. (nor bash, nor ksh)
> *
> Presumably someone out there knows where to find it?
> *
It's shorthand for ${paramter:+set}, so if unset, you get "", otherwise you
get "set":
$ echo ${foo+set}
$ echo ${HOME+set}
set
--
Mel
Problem with today's modular software: they start with the modules
and never get to the software part.
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Re: snippet of configure script - explain please
On Thu, 10 Jul 2008 02:55 am, Anton Shterenlikht wrote:
> I get errors building ports/graphviz on FBSD 6.3 on alpha.
> It seems the error is due to configure script erroneously
> deciding that there is no strncasecmp function on OS.
>
> Based on config.log:
>
> configure:9333: result: no
This probably the result for the previous check; probably strcasecmp
> configure:9253: checking for strncasecmp
>
> I think the following configure snippet is to blame.
> Could somebody explain, at least in general terms,
> what this piece of code does.
>
> many thanks
> anton
>
>9249
>9250 for ac_func in strcasecmp strncasecmp
>9251 do
>9252 as_ac_var=`echo "ac_cv_func_$ac_func" | $as_tr_sh`
This is eqivalent to:
as_ac_var=ac_cv_func_strcasecmp
or:
as_ac_var=ac_cv_func_strncasecmp
depending on which time through the loop.
>9253 { echo "$as_me:$LINENO: checking for $ac_func" >&5
>9254 echo $ECHO_N "checking for $ac_func... $ECHO_C" >&6; }
These lines simplly write reports of one type or another (including to
config.log ?)
The $ECHO_N and $ECHO_C are to cater for different versions of 'echo' wrt line
feeds.
>9255 if { as_var=$as_ac_var; eval "test \"\${$as_var+set}\" = set"; }; then
I find this line somewhat strange as I've not been able
to find documentation for the expansion of ${parameter+set} under the Bourne
shell.
(nor bash, nor ksh)
*
Presumably someone out there knows where to find it?
*
But it seems that it returns "set" if "parameter" has been assigned a value and
null otherwise.
Unless strcasecmp, strncasecmp have already been checked one expects these will
fail
so the 'else' section is executed.
>9256 echo $ECHO_N "(cached) $ECHO_C" >&6
>9257 else
>9258 cat >conftest.$ac_ext <<_ACEOF
>9259 /* confdefs.h. */
>9260 _ACEOF
>9261 cat confdefs.h >>conftest.$ac_ext
>9262 cat >>conftest.$ac_ext <<_ACEOF
>9263 /* end confdefs.h. */
This is the start of the build of a C program to test if the library function
exists.
But we've barely started -- the guts of the check is still to come.
One would expect that if it does not exist the configuration would implement
some alternative action so even then I would not expect the built to fail.
> where
>
> as_tr_sh :
>
> 549 # Sed expression to map a string onto a valid variable name.
> 550 as_tr_sh="eval sed 'y%*+%pp%;s%[^_$as_cr_alnum]%_%g'"
>
> as_cr_alnum :
>
> 40 as_cr_alnum=$as_cr_Letters$as_cr_digits
>
> as_me :
>
> 132 # Name of the executable.
> 133 as_me=`$as_basename -- "$0" ||
>
> ECHO_N and ECHO_C :
>
> 474 ECHO_C= ECHO_N= ECHO_T=
> 475 case `echo -n x` in
> 476 -n*)
> 477 case `echo 'x\c'` in
> 478 *c*) ECHO_T=' ';; # ECHO_T is single tab character.
> 479 *) ECHO_C='\c';;
> 480 esac;;
> 481 *)
> 482 ECHO_N='-n';;
> 483 esac
>
>
Best of luck,
Malcolm Kay
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