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Date:	Tue, 12 Jun 2001 12:45:37 -0500
From:	JT Williams <jeffw AT darwin DOT sfbr DOT org>
To:	djgpp-workers AT delorie DOT com
Subject:	documentation addition for djgpp `update'
Message-ID:	<20010612124537.A24961@kendall.sfbr.org>
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(If you think this is silly, I'll drop it.)
--
jtw


--- utils.tex.old	Sat Apr 21 13:28:46 2001
+++ utils.tex	Tue Jun 12 12:43:18 2001
@@ -665,6 +665,54 @@
         update y_tab.c foo.c
 @end example
 
+Another use of @code{update} might be to validate (and replace if
+necessary) a local copy of some master data file:
+
+@example
+foo.dat:        /master/data/foo.dat
+                [ -f $@@ ] && cmp -s $< $@@ || cp -f $< $@@ && touch $@@
+@end example
+
+@code{update} is similar to the GNU-standard shell script
+@code{move-if-change} that comes with some GNU packages
+(e.g., GCC, binutils).
+The two are @emph{not} functionally equivalent, however;
+in particular, @code{move-if-change} always @strong{removes}
+the source file in the process of updating the target,
+whereas @code{update} leaves the source file intact.
+
+If you need a portable replacement for @code{update},
+there are several options:
+
+@itemize @bullet
+
+@item Modify the GNU @code{move-if-change} script
+to perform a `copy-if-change' operation.
+
+@item Unconditionally copy source to target:
+
+@example
+cp -f @var{new_file} @var{copied_to} && touch @var{copied_to}
+@end example
+
+@item Copy only if target differs from source:
+
+@example
+[ -f @var{copied_to} ] \
+  && cmp -s @var{new_file} @var{copied_to} \
+  || cp -f @var{new_file} @var{copied_to} \
+  && touch @var{copied_to}
+@end example
+
+@end itemize
+
+An important feature of @code{update} is that @var{copied_to}
+is created with the current timestamp.
+If you are certain that @code{cp} on your system is not aliased
+to @code{cp -p} or @code{cp --preserve} (to preserve timestamps),
+then you can omit the @samp{&& touch @var{copied_to}} bit in
+the above command strings.
+
 @c -----------------------------------------------------------------------------
 @node djasm, , update, Top
 @chapter djasm

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