Cookbook: add an example of using default.do for text processing.

Documentation/cookbook/defaults/.gitignore

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+date
+version
+include/version.h
+version.py
+test.txt

Documentation/cookbook/defaults/all.do

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+redo-ifchange test.txt version.py \
+    test.py include/version.h

Documentation/cookbook/defaults/clean.do

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+rm -f date version include/version.h \
+    version.py test.txt *~ .*~

Documentation/cookbook/defaults/date.do

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+date +'%Y-%m-%d' >$3
+redo-always
+redo-stamp <$3

Documentation/cookbook/defaults/default.do

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+# $1 is the target name, eg. test.txt
+# $2 in the same as $1.  We'll talk about
+#    that in a later example.
+# $3 is the temporary output file we should
+#    create.  If this script is successful,
+#    redo will atomically replace $1 with $3.
+
+if [ -e "$1.in" ]; then
+    # if a .in file exists, then do some
+    # text substitution.
+    #
+    # Remember, the user asks redo to build
+    # a particular *target* name.  It's the .do
+    # file's job to figure out what source file(s)
+    # to use to generate the target.
+    redo-ifchange "$1.in" version date
+    read VERSION <version
+    read DATE <date
+    sed -e "s/%VERSION%/$VERSION/g" \
+        -e "s/%DATE%/$DATE/g" \
+        <$1.in >$3
+else
+    echo "$0: Fatal: don't know how to build '$1'" >&2
+    exit 99
+fi

Documentation/cookbook/defaults/include/version.h.in

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+// C/C++ header file identifying the current version
+#ifndef __VERSION_H
+#define __VERSION_H
+
+#define VERSION "%VERSION%"
+#define DATE "%DATE%"
+
+#endif // __VERSION_H

Documentation/cookbook/defaults/test.py

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+#!/usr/bin/env python
+"""Test program for auto-generated version.py"""
+import version
+
+print('Version %r has build date %r'
+      % (version.VERSION, version.DATE))

Documentation/cookbook/defaults/test.txt.in

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+This is the documentation for MyProgram version
+%VERSION%.  It was generated on %DATE%.

Documentation/cookbook/defaults/version.do

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+# Try to get a version number from git, if possible.
+if ! git describe >$3; then
+    echo "$0: Falling back to static version." >&2
+    echo 'UNKNOWN' >$3
+fi
+redo-always
+redo-stamp <$3

Documentation/cookbook/defaults/version.py.in

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+# python module identifying the current version
+VERSION='%VERSION%'
+DATE='%DATE%'