add variant sample

variant_path/.gitignore

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+/build-default
+!/build-default/tup.config

variant_path/Tupfile

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+: |> ./generate_header.sh > %o |> header.txt ./<header_group>

variant_path/Tuprules.tup

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+&root = .
+
+# This doesn't work, cannot find the generated header.txt
+# I want to access the variant path for . but $(TUP_VARIANTDIR) will follow the
+# current Tupfile and there is no variant "anchor" like &-variables.
+!add_header = | &(root)/<header_group> |> cat &(root)/header.txt %f &(root)/footer.txt > %o |> %B.gen.txt
+
+# This works
+#!add_header = | &(root)/<header_group> |> cat %f &(root)/footer.txt > %o |> %B.gen.txt

variant_path/footer.txt

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+-- 
+the end

variant_path/generate_header.sh

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+#!/usr/bin/env bash
+
+set -euo pipefail
+
+cat << EOF
+YEAR $(date +%Y)
+====
+EOF

variant_path/src/Tupfile

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+include_rules
+
+: a.txt |> !add_header |>

variant_path/src/a.txt

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+letter a

variant_path/src/sub/Tupfile

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+include_rules
+
+: b.txt |> !add_header |>

variant_path/src/sub/b.txt

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+letter b