[CMake] Why are blank-delimited strings in commands escaped on Linux?

[Bill Lorensen]

I think if you specify each arg as a string it should work as expected.
 "-L/usr/lib" "-lcairo"

On Thu, Oct 1, 2009 at 6:47 PM, Alan W. Irwin <irwin at beluga.phys.uvic.ca> wrote:
> First, to give some background for the question in the subject line, we have
> implemented OCaml language support for PLplot using custom commands. One
> issue with that support however, is that if a CMake variable is a
> blank-delimited string and used as part of a custom COMMAND, then in the
> command itself (as revealed by "make VERBOSE=1") the blanks are escaped with
> a backslash on Linux platforms.
>
> To take one example, the ocamlmklib command needs arguments in the -L -l
> form.  So we have a variable called CAIRO_LINK_FLAGS that is filled with the
> blank-delimited string
>
> "-L/usr/lib -lcairo"
>
> Then in the COMMAND itself ocamlmklib <options> ${CAIRO_LINK_FLAGS} is
> changed (as shown by "make VERBOSE=1") to
>
> ocamlmklib <options> -L/usr/lib\ -lcairo
>
> if VERBATIM is not used or
>
> ocamlmklib <options> "-L/usr/lib\ -lcairo"
>
> when VERBATIM is used.
>
> Both forms work on my platform (Debian stable, bash shell, ocaml version
> 3.10.2-3), but the first form (without any quoting) definitely causes
> trouble on another PLplot developer's Linux platform. (I am still
> discovering
> the specs for that platform, and I don't yet know where the double-quoted
> version will work on that platform or not).
>
> I am not exactly sure why either form worked at all on my platform. For the
> first case, my bash shell interprets unquoted escape-blank as a literal
> blank, and in the second case my shell interprets double-quoted escape-blank
> as a literal backslash followed by a literal blank.  I guess on my platform,
> ocamlmklib somehow translates either of those two combinations to a
> delimiting blank.  But on the other developer's platform, ocamlmklib
> produces errors for the first form and may well produce errors for the
> second form as well.
>
> Of course, for highest reliability what is really needed is the
>
> ocamlmklib <options> -L/usr/lib -lcairo
>
> form of command. Is there any easy way to get that form of command with
> quotes and escaped blanks dropped?
>
> This whole experience brings up the question of why the blank should be
> escaped at all for general Linux commands (ocamlmklib in this case) where
> CMake has no special knowledge of how to interpret the command.
>
> I assume there must be some motivation for why CMake escapes blanks for
> Linux commands. One I can think of is if someone were to use blanks in file
> names on Linux, then you would need those blanks to be escaped.  My guess
> though is that is a really rare occurrence; blanks in file names _always_
> cause trouble on Linux so developers quickly learn to avoid them on that
> platform.  For example, I am not aware of any Linux library name that has
> blanks in it.  On the other hand, blank-delimited options are quite common
> in Linux.  Thus, the present policy of escaping blanks in commands appears
> to cause problems for the majority Linux (and probably Unix) case just to
> fix an extremely rare problem.
>
> It's quite possible I am missing a much better motivation for escaping
> blanks for Linux commands, but in case it is just the above motivation, I
> would argue that blank escaping of commands should be dropped for the Linux
> platform and probably all Unix platforms.
>
> Alan
> __________________________
> Alan W. Irwin
>
> Astronomical research affiliation with Department of Physics and Astronomy,
> University of Victoria (astrowww.phys.uvic.ca).
>
> Programming affiliations with the FreeEOS equation-of-state implementation
> for stellar interiors (freeeos.sf.net); PLplot scientific plotting software
> package (plplot.org); the libLASi project (unifont.org/lasi); the Loads of
> Linux Links project (loll.sf.net); and the Linux Brochure Project
> (lbproject.sf.net).
> __________________________
>
> Linux-powered Science
> __________________________
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