diff --git a/t/t1006-cat-file.sh b/t/t1006-cat-file.sh
index 2ac3b94..13dd510 100755
--- a/t/t1006-cat-file.sh
+++ b/t/t1006-cat-file.sh
@@ -236,8 +236,8 @@
 '
 
 test_expect_success 'empty --batch-check notices missing object' '
-	echo "$_z40 missing" >expect &&
-	echo "$_z40" | git cat-file --batch-check="" >actual &&
+	echo "$ZERO_OID missing" >expect &&
+	echo "$ZERO_OID" | git cat-file --batch-check="" >actual &&
 	test_cmp expect actual
 '
 
@@ -294,8 +294,8 @@
 
 test_expect_success 'confirm that neither loose blob is a delta' '
 	cat >expect <<-EOF &&
-	$_z40
-	$_z40
+	$ZERO_OID
+	$ZERO_OID
 	EOF
 	git cat-file --batch-check="%(deltabase)" <blobs >actual &&
 	test_cmp expect actual
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index 664a3a4..f6dc056 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -6,7 +6,7 @@
 test_description='Test git update-ref and basic ref logging'
 . ./test-lib.sh
 
-Z=$_z40
+Z=$ZERO_OID
 
 m=refs/heads/master
 n_dir=refs/heads/gu
diff --git a/t/t1407-worktree-ref-store.sh b/t/t1407-worktree-ref-store.sh
index 2211f98..4623ae1 100755
--- a/t/t1407-worktree-ref-store.sh
+++ b/t/t1407-worktree-ref-store.sh
@@ -50,13 +50,13 @@
 '
 
 test_expect_success 'for_each_reflog()' '
-	echo $_z40 > .git/logs/PSEUDO-MAIN &&
+	echo $ZERO_OID > .git/logs/PSEUDO-MAIN &&
 	mkdir -p     .git/logs/refs/bisect &&
-	echo $_z40 > .git/logs/refs/bisect/random &&
+	echo $ZERO_OID > .git/logs/refs/bisect/random &&
 
-	echo $_z40 > .git/worktrees/wt/logs/PSEUDO-WT &&
+	echo $ZERO_OID > .git/worktrees/wt/logs/PSEUDO-WT &&
 	mkdir -p     .git/worktrees/wt/logs/refs/bisect &&
-	echo $_z40 > .git/worktrees/wt/logs/refs/bisect/wt-random &&
+	echo $ZERO_OID > .git/worktrees/wt/logs/refs/bisect/wt-random &&
 
 	$RWT for-each-reflog | cut -c 42- | sort >actual &&
 	cat >expected <<-\EOF &&
diff --git a/t/t1450-fsck.sh b/t/t1450-fsck.sh
index cb4b66e..91fd714 100755
--- a/t/t1450-fsck.sh
+++ b/t/t1450-fsck.sh
@@ -713,7 +713,7 @@
 
 test_expect_success 'fsck $name notices bogus $name' '
 	test_must_fail git fsck bogus &&
-	test_must_fail git fsck $_z40
+	test_must_fail git fsck $ZERO_OID
 '
 
 test_expect_success 'bogus head does not fallback to all heads' '
@@ -723,7 +723,7 @@
 	blob=$(git rev-parse :foo) &&
 	test_when_finished "git rm --cached foo" &&
 	remove_object $blob &&
-	test_must_fail git fsck $_z40 >out 2>&1 &&
+	test_must_fail git fsck $ZERO_OID >out 2>&1 &&
 	! grep $blob out
 '
 
diff --git a/t/t1501-work-tree.sh b/t/t1501-work-tree.sh
index 9c0bc65..afcdfaf 100755
--- a/t/t1501-work-tree.sh
+++ b/t/t1501-work-tree.sh
@@ -238,10 +238,10 @@
 
 test_expect_success 'diff-index respects work tree under .git dir' '
 	cat >diff-index-cached.expected <<-EOF &&
-	:000000 100644 $_z40 $EMPTY_BLOB A	sub/dir/tracked
+	:000000 100644 $ZERO_OID $EMPTY_BLOB A	sub/dir/tracked
 	EOF
 	cat >diff-index.expected <<-EOF &&
-	:000000 100644 $_z40 $_z40 A	sub/dir/tracked
+	:000000 100644 $ZERO_OID $ZERO_OID A	sub/dir/tracked
 	EOF
 
 	(
@@ -257,7 +257,7 @@
 
 test_expect_success 'diff-files respects work tree under .git dir' '
 	cat >diff-files.expected <<-EOF &&
-	:100644 100644 $EMPTY_BLOB $_z40 M	sub/dir/tracked
+	:100644 100644 $EMPTY_BLOB $ZERO_OID M	sub/dir/tracked
 	EOF
 
 	(
diff --git a/t/t1601-index-bogus.sh b/t/t1601-index-bogus.sh
index 73cc932..4171f1e 100755
--- a/t/t1601-index-bogus.sh
+++ b/t/t1601-index-bogus.sh
@@ -4,7 +4,7 @@
 . ./test-lib.sh
 
 test_expect_success 'create tree with null sha1' '
-	tree=$(printf "160000 commit $_z40\\tbroken\\n" | git mktree)
+	tree=$(printf "160000 commit $ZERO_OID\\tbroken\\n" | git mktree)
 '
 
 test_expect_success 'read-tree refuses to read null sha1' '
diff --git a/t/t1700-split-index.sh b/t/t1700-split-index.sh
index e4f4c4d..1e81b33 100755
--- a/t/t1700-split-index.sh
+++ b/t/t1700-split-index.sh
@@ -426,7 +426,7 @@
 	git commit -m "commit" &&
 	{
 		git ls-tree HEAD &&
-		printf "160000 commit $_z40\\tbroken\\n"
+		printf "160000 commit $ZERO_OID\\tbroken\\n"
 	} >broken-tree &&
 	echo "add broken entry" >msg &&
 
diff --git a/t/t2011-checkout-invalid-head.sh b/t/t2011-checkout-invalid-head.sh
index c5501b0..0e8d56a 100755
--- a/t/t2011-checkout-invalid-head.sh
+++ b/t/t2011-checkout-invalid-head.sh
@@ -15,7 +15,7 @@
 '
 
 test_expect_success 'checkout master from invalid HEAD' '
-	echo $_z40 >.git/HEAD &&
+	echo $ZERO_OID >.git/HEAD &&
 	git checkout master --
 '
 
diff --git a/t/t2025-worktree-add.sh b/t/t2025-worktree-add.sh
index d0d2e4f..beaed5e 100755
--- a/t/t2025-worktree-add.sh
+++ b/t/t2025-worktree-add.sh
@@ -465,7 +465,7 @@
 test_expect_success '"add" invokes post-checkout hook (branch)' '
 	post_checkout_hook &&
 	{
-		echo $_z40 $(git rev-parse HEAD) 1 &&
+		echo $ZERO_OID $(git rev-parse HEAD) 1 &&
 		echo $(pwd)/.git/worktrees/gumby &&
 		echo $(pwd)/gumby
 	} >hook.expect &&
@@ -476,7 +476,7 @@
 test_expect_success '"add" invokes post-checkout hook (detached)' '
 	post_checkout_hook &&
 	{
-		echo $_z40 $(git rev-parse HEAD) 1 &&
+		echo $ZERO_OID $(git rev-parse HEAD) 1 &&
 		echo $(pwd)/.git/worktrees/grumpy &&
 		echo $(pwd)/grumpy
 	} >hook.expect &&
@@ -494,7 +494,7 @@
 test_expect_success '"add" in other worktree invokes post-checkout hook' '
 	post_checkout_hook &&
 	{
-		echo $_z40 $(git rev-parse HEAD) 1 &&
+		echo $ZERO_OID $(git rev-parse HEAD) 1 &&
 		echo $(pwd)/.git/worktrees/guppy &&
 		echo $(pwd)/guppy
 	} >hook.expect &&
@@ -506,7 +506,7 @@
 	rm -rf bare &&
 	git clone --bare . bare &&
 	{
-		echo $_z40 $(git --git-dir=bare rev-parse HEAD) 1 &&
+		echo $ZERO_OID $(git --git-dir=bare rev-parse HEAD) 1 &&
 		echo $(pwd)/bare/worktrees/goozy &&
 		echo $(pwd)/goozy
 	} >hook.expect &&
diff --git a/t/t2027-worktree-list.sh b/t/t2027-worktree-list.sh
index 720063b..bb6fb9b 100755
--- a/t/t2027-worktree-list.sh
+++ b/t/t2027-worktree-list.sh
@@ -116,7 +116,7 @@
 		git worktree add linked &&
 		cat >expected <<-EOF &&
 		worktree $(pwd)
-		HEAD $_z40
+		HEAD $ZERO_OID
 
 		EOF
 		cd linked &&
diff --git a/t/t2107-update-index-basic.sh b/t/t2107-update-index-basic.sh
index 1db7e6a..2242cd0 100755
--- a/t/t2107-update-index-basic.sh
+++ b/t/t2107-update-index-basic.sh
@@ -37,7 +37,7 @@
 	echo content >file &&
 	git add file &&
 	git rev-parse :file >expect &&
-	test_must_fail git update-index --cacheinfo 100644 $_z40 file &&
+	test_must_fail git update-index --cacheinfo 100644 $ZERO_OID file &&
 	git rev-parse :file >actual &&
 	test_cmp expect actual
 '
@@ -47,7 +47,7 @@
 	(cd submodule && test_commit foo) &&
 	git add submodule &&
 	git rev-parse :submodule >expect &&
-	test_must_fail git update-index --cacheinfo 160000 $_z40 submodule &&
+	test_must_fail git update-index --cacheinfo 160000 $ZERO_OID submodule &&
 	git rev-parse :submodule >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t2201-add-update-typechange.sh b/t/t2201-add-update-typechange.sh
index 954fc51..a4eec0a 100755
--- a/t/t2201-add-update-typechange.sh
+++ b/t/t2201-add-update-typechange.sh
@@ -75,35 +75,35 @@
 		git ls-tree -r HEAD |
 		sed -e "s/^/:/" -e "
 			/	caskly/{
-				s/	caskly/ $_z40 D&/
+				s/	caskly/ $ZERO_OID D&/
 				s/blob/000000/
 			}
 			/	nitfol/{
-				s/	nitfol/ $_z40 $T_letter&/
+				s/	nitfol/ $ZERO_OID $T_letter&/
 				s/blob/100644/
 			}
 			/	rezrov.bozbar/{
-				s/	rezrov.bozbar/ $_z40 D&/
+				s/	rezrov.bozbar/ $ZERO_OID D&/
 				s/blob/000000/
 			}
 			/	xyzzy/{
-				s/	xyzzy/ $_z40 D&/
+				s/	xyzzy/ $ZERO_OID D&/
 				s/blob/000000/
 			}
 			/	yomin/{
-			    s/	yomin/ $_z40 T&/
+			    s/	yomin/ $ZERO_OID T&/
 				s/blob/160000/
 			}
 		"
 	} >expect &&
 	{
 		cat expect
-		echo ":100644 160000 $_empty $_z40 T	yonk"
-		echo ":100644 000000 $_empty $_z40 D	zifmia"
+		echo ":100644 160000 $_empty $ZERO_OID T	yonk"
+		echo ":100644 000000 $_empty $ZERO_OID D	zifmia"
 	} >expect-files &&
 	{
 		cat expect
-		echo ":000000 160000 $_z40 $_z40 A	yonk"
+		echo ":000000 160000 $ZERO_OID $ZERO_OID A	yonk"
 	} >expect-index &&
 	{
 		echo "100644 $_empty 0	nitfol"
diff --git a/t/t2203-add-intent.sh b/t/t2203-add-intent.sh
index 78236dc..1797f94 100755
--- a/t/t2203-add-intent.sh
+++ b/t/t2203-add-intent.sh
@@ -30,9 +30,9 @@
 	nam1=d00491fd7e5bb6fa28c517a0bb32b8b506539d4d &&
 	nam2=ce013625030ba8dba906f756967f9e9ca394464a &&
 	cat >expect <<-EOF &&
-	1 DA N... 100644 000000 100644 $nam1 $_z40 1.t
-	1 A. N... 000000 100644 100644 $_z40 $nam2 elif
-	1 .A N... 000000 000000 100644 $_z40 $_z40 file
+	1 DA N... 100644 000000 100644 $nam1 $ZERO_OID 1.t
+	1 A. N... 000000 100644 100644 $ZERO_OID $nam2 elif
+	1 .A N... 000000 000000 100644 $ZERO_OID $ZERO_OID file
 	EOF
 	test_cmp expect actual
 '
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index c0ef946..b52c655 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -47,7 +47,7 @@
 '
 
 cat >expect <<EOF
-$_z40 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	branch: Created from master
+$ZERO_OID $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	branch: Created from master
 EOF
 test_expect_success 'git branch -l d/e/f should create a branch and a log' '
 	GIT_COMMITTER_DATE="2005-05-26 23:30" \
@@ -901,7 +901,7 @@
 
 # Keep this test last, as it changes the current branch
 cat >expect <<EOF
-$_z40 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	branch: Created from master
+$ZERO_OID $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	branch: Created from master
 EOF
 test_expect_success 'git checkout -b g/h/i -l should create a branch and a log' '
 	GIT_COMMITTER_DATE="2005-05-26 23:30" \
diff --git a/t/t4002-diff-basic.sh b/t/t4002-diff-basic.sh
index a5e8b83..5af773d 100755
--- a/t/t4002-diff-basic.sh
+++ b/t/t4002-diff-basic.sh
@@ -131,7 +131,7 @@
     # object ID for the changed files because it wants you to look at the
     # filesystem.
     sed <"$2" >.test-tmp \
-	-e '/^:000000 /d;s/'$_x40'\( [MCRNDU][0-9]*\)	/'$_z40'\1	/' &&
+	-e '/^:000000 /d;s/'$_x40'\( [MCRNDU][0-9]*\)	/'$ZERO_OID'\1	/' &&
     test_cmp "$1" .test-tmp
 }
 
diff --git a/t/t4014-format-patch.sh b/t/t4014-format-patch.sh
index 482112c..0dbf7d5 100755
--- a/t/t4014-format-patch.sh
+++ b/t/t4014-format-patch.sh
@@ -1523,7 +1523,7 @@
 test_expect_success 'format-patch --zero-commit' '
 	git format-patch --zero-commit --stdout v2..v1 >patch2 &&
 	grep "^From " patch2 | sort | uniq >actual &&
-	echo "From $_z40 Mon Sep 17 00:00:00 2001" >expect &&
+	echo "From $ZERO_OID Mon Sep 17 00:00:00 2001" >expect &&
 	test_cmp expect actual
 '
 
diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index 0446201..49d3f54 100755
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -26,7 +26,7 @@
 		read path oldfile oldhex oldmode newfile newhex newmode &&
 		test "z$path" = zfile &&
 		test "z$oldmode" = z100644 &&
-		test "z$newhex" = "z$_z40" &&
+		test "z$newhex" = "z$ZERO_OID" &&
 		test "z$newmode" = z100644 &&
 		oh=$(git rev-parse --verify HEAD:file) &&
 		test "z$oh" = "z$oldhex"
@@ -55,7 +55,7 @@
 		read path oldfile oldhex oldmode newfile newhex newmode &&
 		test "z$path" = zfile &&
 		test "z$oldmode" = z100644 &&
-		test "z$newhex" = "z$_z40" &&
+		test "z$newhex" = "z$ZERO_OID" &&
 		test "z$newmode" = z120000 &&
 		oh=$(git rev-parse --verify HEAD:file) &&
 		test "z$oh" = "z$oldhex"
@@ -73,7 +73,7 @@
 		read path oldfile oldhex oldmode newfile newhex newmode &&
 		test "z$path" = zfile &&
 		test "z$oldmode" = z100644 &&
-		test "z$newhex" = "z$_z40" &&
+		test "z$newhex" = "z$ZERO_OID" &&
 		test "z$newmode" = z100644 &&
 		oh=$(git rev-parse --verify HEAD:file) &&
 		test "z$oh" = "z$oldhex"
@@ -104,7 +104,7 @@
 		read path oldfile oldhex oldmode newfile newhex newmode &&
 		test "z$path" = zfile &&
 		test "z$oldmode" = z100644 &&
-		test "z$newhex" = "z$_z40" &&
+		test "z$newhex" = "z$ZERO_OID" &&
 		test "z$newmode" = z100644 &&
 		oh=$(git rev-parse --verify HEAD:file) &&
 		test "z$oh" = "z$oldhex"
@@ -137,7 +137,7 @@
 		read path oldfile oldhex oldmode newfile newhex newmode &&
 		test "z$path" = zfile &&
 		test "z$oldmode" = z100644 &&
-		test "z$newhex" = "z$_z40" &&
+		test "z$newhex" = "z$ZERO_OID" &&
 		test "z$newmode" = z100644 &&
 		oh=$(git rev-parse --verify HEAD:file) &&
 		test "z$oh" = "z$oldhex"
diff --git a/t/t4027-diff-submodule.sh b/t/t4027-diff-submodule.sh
index 2ffd11a..6304130 100755
--- a/t/t4027-diff-submodule.sh
+++ b/t/t4027-diff-submodule.sh
@@ -31,7 +31,7 @@
 		cd sub &&
 		git rev-list HEAD
 	) &&
-	echo ":160000 160000 $3 $_z40 M	sub" >expect &&
+	echo ":160000 160000 $3 $ZERO_OID M	sub" >expect &&
 	subtip=$3 subprev=$2
 '
 
@@ -250,7 +250,7 @@
 	# 39 efs
 	c=fffffffffffffffffffffffffffffffffffffff &&
 	(
-		echo "000000 $_z40 0	sub" &&
+		echo "000000 $ZERO_OID 0	sub" &&
 		echo "160000 1$c 1	sub" &&
 		echo "160000 2$c 2	sub" &&
 		echo "160000 3$c 3	sub"
@@ -265,7 +265,7 @@
 ++Subproject commit 0000000000000000000000000000000000000000'\'' &&
 
 	hh=$(git rev-parse HEAD) &&
-	sed -e "s/$_z40/$hh/" expect.nosub >expect.withsub
+	sed -e "s/$ZERO_OID/$hh/" expect.nosub >expect.withsub
 
 '
 
diff --git a/t/t4046-diff-unmerged.sh b/t/t4046-diff-unmerged.sh
index d0f1447..ff7cfd8 100755
--- a/t/t4046-diff-unmerged.sh
+++ b/t/t4046-diff-unmerged.sh
@@ -37,7 +37,7 @@
 	for path in $paths
 	do
 		>"$path" &&
-		echo ":000000 100644 $_z40 $_z40 U	$path"
+		echo ":000000 100644 $ZERO_OID $ZERO_OID U	$path"
 	done >diff-files-0.expect &&
 	git diff-files -0 >diff-files-0.actual &&
 	test_cmp diff-files-0.expect diff-files-0.actual
@@ -47,9 +47,9 @@
 	for path in $paths
 	do
 		>"$path" &&
-		echo ":000000 100644 $_z40 $_z40 U	$path" &&
+		echo ":000000 100644 $ZERO_OID $ZERO_OID U	$path" &&
 		case "$path" in
-		x??) echo ":100644 100644 $blob1 $_z40 M	$path"
+		x??) echo ":100644 100644 $blob1 $ZERO_OID M	$path"
 		esac
 	done >diff-files-1.expect &&
 	git diff-files -1 >diff-files-1.actual &&
@@ -60,9 +60,9 @@
 	for path in $paths
 	do
 		>"$path" &&
-		echo ":000000 100644 $_z40 $_z40 U	$path" &&
+		echo ":000000 100644 $ZERO_OID $ZERO_OID U	$path" &&
 		case "$path" in
-		?x?) echo ":100644 100644 $blob2 $_z40 M	$path"
+		?x?) echo ":100644 100644 $blob2 $ZERO_OID M	$path"
 		esac
 	done >diff-files-2.expect &&
 	git diff-files -2 >diff-files-2.actual &&
@@ -75,9 +75,9 @@
 	for path in $paths
 	do
 		>"$path" &&
-		echo ":000000 100644 $_z40 $_z40 U	$path" &&
+		echo ":000000 100644 $ZERO_OID $ZERO_OID U	$path" &&
 		case "$path" in
-		??x) echo ":100644 100644 $blob3 $_z40 M	$path"
+		??x) echo ":100644 100644 $blob3 $ZERO_OID M	$path"
 		esac
 	done >diff-files-3.expect &&
 	git diff-files -3 >diff-files-3.actual &&
diff --git a/t/t4054-diff-bogus-tree.sh b/t/t4054-diff-bogus-tree.sh
index 18f42c5..fcae82f 100755
--- a/t/t4054-diff-bogus-tree.sh
+++ b/t/t4054-diff-bogus-tree.sh
@@ -19,37 +19,37 @@
 '
 
 test_expect_success 'raw diff shows null sha1 (addition)' '
-	echo ":000000 100644 $_z40 $_z40 A	foo" >expect &&
+	echo ":000000 100644 $ZERO_OID $ZERO_OID A	foo" >expect &&
 	git diff-tree $EMPTY_TREE $bogus_tree >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'raw diff shows null sha1 (removal)' '
-	echo ":100644 000000 $_z40 $_z40 D	foo" >expect &&
+	echo ":100644 000000 $ZERO_OID $ZERO_OID D	foo" >expect &&
 	git diff-tree $bogus_tree $EMPTY_TREE >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'raw diff shows null sha1 (modification)' '
-	echo ":100644 100644 $blob $_z40 M	foo" >expect &&
+	echo ":100644 100644 $blob $ZERO_OID M	foo" >expect &&
 	git diff-tree $good_tree $bogus_tree >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'raw diff shows null sha1 (other direction)' '
-	echo ":100644 100644 $_z40 $blob M	foo" >expect &&
+	echo ":100644 100644 $ZERO_OID $blob M	foo" >expect &&
 	git diff-tree $bogus_tree $good_tree >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'raw diff shows null sha1 (reverse)' '
-	echo ":100644 100644 $_z40 $blob M	foo" >expect &&
+	echo ":100644 100644 $ZERO_OID $blob M	foo" >expect &&
 	git diff-tree -R $good_tree $bogus_tree >actual &&
 	test_cmp expect actual
 '
 
 test_expect_success 'raw diff shows null sha1 (index)' '
-	echo ":100644 100644 $_z40 $blob M	foo" >expect &&
+	echo ":100644 100644 $ZERO_OID $blob M	foo" >expect &&
 	git diff-index $bogus_tree >actual &&
 	test_cmp expect actual
 '
diff --git a/t/t4058-diff-duplicates.sh b/t/t4058-diff-duplicates.sh
index 0a23242..c24ee17 100755
--- a/t/t4058-diff-duplicates.sh
+++ b/t/t4058-diff-duplicates.sh
@@ -59,12 +59,12 @@
 
 test_expect_success 'diff-tree between trees' '
 	{
-		printf ":000000 100644 $_z40 $blob_two A\touter/inner\n" &&
-		printf ":000000 100644 $_z40 $blob_two A\touter/inner\n" &&
-		printf ":000000 100644 $_z40 $blob_two A\touter/inner\n" &&
-		printf ":100644 000000 $blob_two $_z40 D\touter/inner\n" &&
-		printf ":100644 000000 $blob_two $_z40 D\touter/inner\n" &&
-		printf ":100644 000000 $blob_two $_z40 D\touter/inner\n"
+		printf ":000000 100644 $ZERO_OID $blob_two A\touter/inner\n" &&
+		printf ":000000 100644 $ZERO_OID $blob_two A\touter/inner\n" &&
+		printf ":000000 100644 $ZERO_OID $blob_two A\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n" &&
+		printf ":100644 000000 $blob_two $ZERO_OID D\touter/inner\n"
 	} >expect &&
 	git diff-tree -r --no-abbrev one two >actual &&
 	test_cmp expect actual
diff --git a/t/t4150-am.sh b/t/t4150-am.sh
index 1eccfb7..1ebc587 100755
--- a/t/t4150-am.sh
+++ b/t/t4150-am.sh
@@ -140,8 +140,8 @@
 		echo "# User $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL>" &&
 		echo "# Date $test_tick 25200" &&
 		echo "#      $(git show --pretty="%aD" -s second)" &&
-		echo "# Node ID $_z40" &&
-		echo "# Parent  $_z40" &&
+		echo "# Node ID $ZERO_OID" &&
+		echo "# Parent  $ZERO_OID" &&
 		cat msg &&
 		echo &&
 		git diff-tree --no-commit-id -p second
diff --git a/t/t4200-rerere.sh b/t/t4200-rerere.sh
index eaf18c8..8417e5a 100755
--- a/t/t4200-rerere.sh
+++ b/t/t4200-rerere.sh
@@ -243,7 +243,7 @@
 	five_days="$1" right_now="$2"
 	test_expect_success "rerere gc with custom expiry ($five_days, $right_now)" '
 		rm -fr .git/rr-cache &&
-		rr=.git/rr-cache/$_z40 &&
+		rr=.git/rr-cache/$ZERO_OID &&
 		mkdir -p "$rr" &&
 		>"$rr/preimage" &&
 		>"$rr/postimage" &&
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index 8223913..d9c90f2 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -634,7 +634,7 @@
 	orgmaster=$(cd testrepo && git show-ref -s --verify refs/heads/master) &&
 	newmaster=$(git show-ref -s --verify refs/heads/master) &&
 	orgnext=$(cd testrepo && git show-ref -s --verify refs/heads/next) &&
-	newnext=$_z40 &&
+	newnext=$ZERO_OID &&
 	git push testrepo refs/heads/master:refs/heads/master :refs/heads/next &&
 	(
 		cd testrepo/.git &&
@@ -672,15 +672,15 @@
 	(
 		cd testrepo/.git &&
 		cat >pre-receive.expect <<-EOF &&
-		$_z40 $_z40 refs/heads/master
+		$ZERO_OID $ZERO_OID refs/heads/master
 		EOF
 
 		cat >update.expect <<-EOF &&
-		refs/heads/master $_z40 $_z40
+		refs/heads/master $ZERO_OID $ZERO_OID
 		EOF
 
 		cat >post-receive.expect <<-EOF &&
-		$_z40 $_z40 refs/heads/master
+		$ZERO_OID $ZERO_OID refs/heads/master
 		EOF
 
 		cat >post-update.expect <<-EOF &&
@@ -703,12 +703,12 @@
 		cd testrepo/.git &&
 		cat >pre-receive.expect <<-EOF &&
 		$orgmaster $newmaster refs/heads/master
-		$_z40 $_z40 refs/heads/nonexistent
+		$ZERO_OID $ZERO_OID refs/heads/nonexistent
 		EOF
 
 		cat >update.expect <<-EOF &&
 		refs/heads/master $orgmaster $newmaster
-		refs/heads/nonexistent $_z40 $_z40
+		refs/heads/nonexistent $ZERO_OID $ZERO_OID
 		EOF
 
 		cat >post-receive.expect <<-EOF &&
@@ -732,11 +732,11 @@
 	(
 		cd testrepo/.git &&
 		cat >pre-receive.expect <<-EOF &&
-		$_z40 $_z40 refs/heads/nonexistent
+		$ZERO_OID $ZERO_OID refs/heads/nonexistent
 		EOF
 
 		cat >update.expect <<-EOF &&
-		refs/heads/nonexistent $_z40 $_z40
+		refs/heads/nonexistent $ZERO_OID $ZERO_OID
 		EOF
 
 		test_cmp pre-receive.expect pre-receive.actual &&
@@ -751,7 +751,7 @@
 	orgmaster=$(cd testrepo && git show-ref -s --verify refs/heads/master) &&
 	newmaster=$(git show-ref -s --verify refs/heads/master) &&
 	orgnext=$(cd testrepo && git show-ref -s --verify refs/heads/next) &&
-	newnext=$_z40 &&
+	newnext=$ZERO_OID &&
 	orgpu=$(cd testrepo && git show-ref -s --verify refs/heads/pu) &&
 	newpu=$(git show-ref -s --verify refs/heads/master) &&
 	git push testrepo refs/heads/master:refs/heads/master \
@@ -763,14 +763,14 @@
 		$orgmaster $newmaster refs/heads/master
 		$orgnext $newnext refs/heads/next
 		$orgpu $newpu refs/heads/pu
-		$_z40 $_z40 refs/heads/nonexistent
+		$ZERO_OID $ZERO_OID refs/heads/nonexistent
 		EOF
 
 		cat >update.expect <<-EOF &&
 		refs/heads/master $orgmaster $newmaster
 		refs/heads/next $orgnext $newnext
 		refs/heads/pu $orgpu $newpu
-		refs/heads/nonexistent $_z40 $_z40
+		refs/heads/nonexistent $ZERO_OID $ZERO_OID
 		EOF
 
 		cat >post-receive.expect <<-EOF &&
diff --git a/t/t5527-fetch-odd-refs.sh b/t/t5527-fetch-odd-refs.sh
index 207899a..3b0cb98 100755
--- a/t/t5527-fetch-odd-refs.sh
+++ b/t/t5527-fetch-odd-refs.sh
@@ -27,7 +27,7 @@
 '
 
 test_expect_success 'try to create repo with absurdly long refname' '
-	ref240=$_z40/$_z40/$_z40/$_z40/$_z40/$_z40 &&
+	ref240=$ZERO_OID/$ZERO_OID/$ZERO_OID/$ZERO_OID/$ZERO_OID/$ZERO_OID &&
 	ref1440=$ref240/$ref240/$ref240/$ref240/$ref240/$ref240 &&
 	git init long &&
 	(
diff --git a/t/t5571-pre-push-hook.sh b/t/t5571-pre-push-hook.sh
index ba975bb..ac53d63 100755
--- a/t/t5571-pre-push-hook.sh
+++ b/t/t5571-pre-push-hook.sh
@@ -78,8 +78,8 @@
 cat >expected <<EOF
 parent1
 repo1
-refs/tags/one $COMMIT1 refs/tags/tag1 $_z40
-HEAD~ $COMMIT2 refs/heads/prev $_z40
+refs/tags/one $COMMIT1 refs/tags/tag1 $ZERO_OID
+HEAD~ $COMMIT2 refs/heads/prev $ZERO_OID
 EOF
 
 test_expect_success 'push non-branches' '
@@ -90,7 +90,7 @@
 cat >expected <<EOF
 parent1
 repo1
-(delete) $_z40 refs/heads/prev $COMMIT2
+(delete) $ZERO_OID refs/heads/prev $COMMIT2
 EOF
 
 test_expect_success 'push delete' '
@@ -101,7 +101,7 @@
 cat >expected <<EOF
 repo1
 repo1
-HEAD $COMMIT3 refs/heads/other $_z40
+HEAD $COMMIT3 refs/heads/other $ZERO_OID
 EOF
 
 test_expect_success 'push to URL' '
diff --git a/t/t6120-describe.sh b/t/t6120-describe.sh
index bae78c4..84dd1cb 100755
--- a/t/t6120-describe.sh
+++ b/t/t6120-describe.sh
@@ -383,7 +383,7 @@
 '
 
 test_expect_success 'describe complains about missing object' '
-	test_must_fail git describe $_z40
+	test_must_fail git describe $ZERO_OID
 '
 
 test_done
diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index 295d147..48379aa 100755
--- a/t/t6300-for-each-ref.sh
+++ b/t/t6300-for-each-ref.sh
@@ -310,7 +310,7 @@
 	echo >expected &&
 	git for-each-ref --format="%(authordate:format:)" refs/heads >actual &&
 	test_cmp expected actual &&
-	long="long format -- $_z40$_z40$_z40$_z40$_z40$_z40$_z40" &&
+	long="long format -- $ZERO_OID$ZERO_OID$ZERO_OID$ZERO_OID$ZERO_OID$ZERO_OID$ZERO_OID" &&
 	echo $long >expected &&
 	git for-each-ref --format="%(authordate:format:$long)" refs/heads >actual &&
 	test_cmp expected actual
diff --git a/t/t6301-for-each-ref-errors.sh b/t/t6301-for-each-ref-errors.sh
index c734ce2..49cc65b 100755
--- a/t/t6301-for-each-ref-errors.sh
+++ b/t/t6301-for-each-ref-errors.sh
@@ -4,7 +4,7 @@
 
 . ./test-lib.sh
 
-ZEROS=$_z40
+ZEROS=$ZERO_OID
 MISSING=abababababababababababababababababababab
 
 test_expect_success setup '
diff --git a/t/t7009-filter-branch-null-sha1.sh b/t/t7009-filter-branch-null-sha1.sh
index a8d9ec4..9ba9f24 100755
--- a/t/t7009-filter-branch-null-sha1.sh
+++ b/t/t7009-filter-branch-null-sha1.sh
@@ -12,7 +12,7 @@
 test_expect_success 'setup: a commit with a bogus null sha1 in the tree' '
 	{
 		git ls-tree HEAD &&
-		printf "160000 commit $_z40\\tbroken\\n"
+		printf "160000 commit $ZERO_OID\\tbroken\\n"
 	} >broken-tree &&
 	echo "add broken entry" >msg &&
 
diff --git a/t/t7011-skip-worktree-reading.sh b/t/t7011-skip-worktree-reading.sh
index 84f4145..37525ca 100755
--- a/t/t7011-skip-worktree-reading.sh
+++ b/t/t7011-skip-worktree-reading.sh
@@ -118,7 +118,7 @@
 	test "$(git grep --no-ext-grep test)" = "1:test"
 '
 
-echo ":000000 100644 $_z40 $EMPTY_BLOB A	1" > expected
+echo ":000000 100644 $ZERO_OID $EMPTY_BLOB A	1" > expected
 test_expect_success 'diff-index does not examine skip-worktree absent entries' '
 	setup_absent &&
 	git diff-index HEAD -- 1 > result &&
diff --git a/t/t7064-wtstatus-pv2.sh b/t/t7064-wtstatus-pv2.sh
index 8f79532..b9a86d3 100755
--- a/t/t7064-wtstatus-pv2.sh
+++ b/t/t7064-wtstatus-pv2.sh
@@ -46,11 +46,11 @@
 	cat >expect <<-EOF &&
 	# branch.oid (initial)
 	# branch.head master
-	1 A. N... 000000 100644 100644 $_z40 $OID_A dir1/file_a
-	1 A. N... 000000 100644 100644 $_z40 $OID_B dir1/file_b
-	1 A. N... 000000 100644 100644 $_z40 $OID_X file_x
-	1 A. N... 000000 100644 100644 $_z40 $OID_Y file_y
-	1 A. N... 000000 100644 100644 $_z40 $OID_Z file_z
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_A dir1/file_a
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_B dir1/file_b
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_X file_x
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_Y file_y
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_Z file_z
 	? actual
 	? expect
 	EOF
@@ -63,11 +63,11 @@
 	lf_to_nul >expect <<-EOF &&
 	# branch.oid (initial)
 	# branch.head master
-	1 A. N... 000000 100644 100644 $_z40 $OID_A dir1/file_a
-	1 A. N... 000000 100644 100644 $_z40 $OID_B dir1/file_b
-	1 A. N... 000000 100644 100644 $_z40 $OID_X file_x
-	1 A. N... 000000 100644 100644 $_z40 $OID_Y file_y
-	1 A. N... 000000 100644 100644 $_z40 $OID_Z file_z
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_A dir1/file_a
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_B dir1/file_b
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_X file_x
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_Y file_y
+	1 A. N... 000000 100644 100644 $ZERO_OID $OID_Z file_z
 	? actual
 	? expect
 	EOF
@@ -128,7 +128,7 @@
 	# branch.oid $H0
 	# branch.head master
 	1 M. N... 100644 100644 100644 $OID_X $OID_X1 file_x
-	1 D. N... 100644 000000 000000 $OID_Z $_z40 file_z
+	1 D. N... 100644 000000 000000 $OID_Z $ZERO_OID file_z
 	? actual
 	? expect
 	EOF
@@ -145,7 +145,7 @@
 	# branch.oid $H0
 	# branch.head master
 	1 M. N... 100644 100644 100644 $OID_X $OID_X1 file_x
-	1 D. N... 100644 000000 000000 $OID_Z $_z40 file_z
+	1 D. N... 100644 000000 000000 $OID_Z $ZERO_OID file_z
 	2 R. N... 100644 100644 100644 $OID_Y $OID_Y R100 renamed_yQfile_y
 	? actual
 	? expect
@@ -163,7 +163,7 @@
 	# branch.oid $H0
 	# branch.head master
 	1 M. N... 100644 100644 100644 $OID_X $OID_X1 file_x
-	1 D. N... 100644 000000 000000 $OID_Z $_z40 file_z
+	1 D. N... 100644 000000 000000 $OID_Z $ZERO_OID file_z
 	2 R. N... 100644 100644 100644 $OID_Y $OID_Y R100 renamed_yQfile_y
 	? actual
 	? expect
@@ -246,8 +246,8 @@
 	git add --intent-to-add intent1.add intent2.add &&
 
 	cat >expect <<-EOF &&
-	1 .A N... 000000 000000 100644 $_z40 $_z40 intent1.add
-	1 .A N... 000000 000000 100644 $_z40 $_z40 intent2.add
+	1 .A N... 000000 000000 100644 $ZERO_OID $ZERO_OID intent1.add
+	1 .A N... 000000 000000 100644 $ZERO_OID $ZERO_OID intent2.add
 	EOF
 
 	git status --porcelain=v2 >actual &&
@@ -280,7 +280,7 @@
 	cat >expect <<-EOF &&
 	# branch.oid $HM
 	# branch.head AA_M
-	u AA N... 000000 100644 100644 100644 $_z40 $OID_AA_B $OID_AA_A conflict.txt
+	u AA N... 000000 100644 100644 100644 $ZERO_OID $OID_AA_B $OID_AA_A conflict.txt
 	EOF
 
 	git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -373,7 +373,7 @@
 		## Test upstream-gone case. Fake this by pointing origin/master at
 		## a non-existing commit.
 		OLD=$(git rev-parse origin/master) &&
-		NEW=$_z40 &&
+		NEW=$ZERO_OID &&
 		mv .git/packed-refs .git/old-packed-refs &&
 		sed "s/$OLD/$NEW/g" <.git/old-packed-refs >.git/packed-refs &&
 
@@ -469,8 +469,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 A. S... 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 A. S... 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -494,8 +494,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 AM S..U 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 AM S..U 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -519,8 +519,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 AM S.M. 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 AM S.M. 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -546,8 +546,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 AM S.M. 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 AM S.M. 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -573,8 +573,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 AM S.MU 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 AM S.MU 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&
@@ -600,8 +600,8 @@
 		# branch.head master
 		# branch.upstream origin/master
 		# branch.ab +0 -0
-		1 A. N... 000000 100644 100644 $_z40 $HMOD .gitmodules
-		1 AM SC.. 000000 160000 160000 $_z40 $HSUB sub1
+		1 A. N... 000000 100644 100644 $ZERO_OID $HMOD .gitmodules
+		1 AM SC.. 000000 160000 160000 $ZERO_OID $HSUB sub1
 		EOF
 
 		git status --porcelain=v2 --branch --untracked-files=all >actual &&