Re: replacement of variable placeholders in files

[Benoit SIGOURE]

On Aug 22, 2007, at 12:23 PM, Jason Curl wrote:

> Dizzy wrote:
> > Hi
> >
> > I have an input file named project.conf.in in which I have some  
> > variable placeholders of the form ${VARIABLE}. I am trying to use  
> > sed to replace them with some actual contents so I have added a  
> > make rule in Makefile.am such as:
> >
> > project.conf:project.conf.in $(top_builddir)/config.status
> > $(SED) -e 'address@hidden/@'"${localstatedir}/@g" $ 
> > (top_srcdir)/conf/project.conf.in > $@
> Instead of having the Makefile do this for you, why not during  
> 'configure' time with the AC_CONFIG_FILES? And then define the  
> variable substitutions as part of the configure script? Even define  
> a macro if you want to logically separate everything.
>
> e.g.
>
> AC_CONFIG_FILES([project.conf
>     Makefile
>     subdir/Makefile])

That doesn't always work for several reasons.  First off, people  
happen to this frequently to pass path information around.  Paths are  
mostly always defined in term of $prefix (at least).  At configure  
time, prefix is set to NONE so you can't do it here [1].  Moreover,  
several paths are defined in terms of others, eg:

bindir='${exec_prefix}/bin'
exec_prefix='${prefix}'
prefix='/usr/local'

So you end up doing several calls to `eval' to simulate the variable  
expansion performed by make.  Needless to say this is fragile (but  
doable).

So Dizzy's idea wasn't bad, IMO it's the most reliable thing you can  
do: rely on make to expand the variables.

[1] AFAIR this is mostly historical because RMS initially wanted to  
make it possible to change the prefix at the last minute, e.g. by  
doing make install prefix=/some/path (which happens to be useful if  
you use stow)

> > Notice that I first have a string with single quotes so I tell  
> > bash to not deal with ${LOCALSTATEDIR} then I have a string with  
> > double quotes so bash can replace the ${localstatedir} variable  
> > contents and no space between which seems to work well if I run  
> > that command in shell.

Back to Dizzy's question:
I think the line should be:

$(SED) -e 's,$${LOCALSTATEDIR},${localstatedir},g' $(top_srcdir)/conf/ 
project.conf.in >$@

You don't need to worry about the double-quotes thing because it's  
not your shell that expands the value of ${localstatedir} but make.   
Actually you could write $(localstatedir) instead and it wouldn't  
change anything.

> > However I think that configure replaces the ${LOCALSTATEDIR} from  
> > the generated Makefile.in with "" (because there is no such  
> > variable so it puts an empty string) and ignores my single quotes.

This sounds unlikely, configure does not deal with unknown variables  
(moreover configure, well actually config.status, substitutes  
variables of the form @FOO@ and not ${FOO} and it ignores variables  
it doesn't know).

Actually what happens is that ${LOCALSTATEDIR} is recognized by make  
as a non-existant make variable and is (as such) expanded to an empty  
string.  What you want to do is to escape this variable so that make  
ignores it (by using `$$' instead of a single `$').

> > Any idea how to implement such a make rule that should replace  
> > variable placeholders with contents of actual variables?
> >
> > Thanks!

I hope this helps.

$ cat >configure.ac
AC_INIT([foo], [0.1])
AM_INIT_AUTOMAKE([foreign])
AC_CONFIG_FILES([Makefile])
AC_OUTPUT
$ cat >Makefile.am
foo: foo.in
^Ised -e 's,$${LOCALSTATEDIR},${localstatedir},g' $(top_srcdir)/ 
foo.in >$@
$ cat >foo.in
echo '${LOCALSTATEDIR}'
$ autoreconf -i
configure.ac:2: installing `./missing'
configure.ac:2: installing `./install-sh'
$ ./configure
checking for a BSD-compatible install... /opt/local/bin/ginstall -c
checking whether build environment is sane... yes
checking for a thread-safe mkdir -p... /opt/local/bin/mkdir -p
checking for gawk... gawk
checking whether make sets $(MAKE)... yes
configure: creating ./config.status
config.status: creating Makefile
$ make foo
sed -e 's,${LOCALSTATEDIR},/usr/local/var,g' ./foo.in >foo
$ sh foo
/usr/local/var

--
Benoit Sigoure aka Tsuna
EPITA Research and Development Laboratory

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