diff --git a/Makefile b/Makefile
index 372139f1f2..19574f5133 100644
--- a/Makefile
+++ b/Makefile
@@ -596,7 +596,6 @@ SCRIPT_SH += git-bisect.sh
 SCRIPT_SH += git-difftool--helper.sh
 SCRIPT_SH += git-filter-branch.sh
 SCRIPT_SH += git-merge-octopus.sh
-SCRIPT_SH += git-merge-one-file.sh
 SCRIPT_SH += git-merge-resolve.sh
 SCRIPT_SH += git-mergetool.sh
 SCRIPT_SH += git-quiltimport.sh
@@ -1089,6 +1088,7 @@ BUILTIN_OBJS += builtin/mailsplit.o
 BUILTIN_OBJS += builtin/merge-base.o
 BUILTIN_OBJS += builtin/merge-file.o
 BUILTIN_OBJS += builtin/merge-index.o
+BUILTIN_OBJS += builtin/merge-one-file.o
 BUILTIN_OBJS += builtin/merge-ours.o
 BUILTIN_OBJS += builtin/merge-recursive.o
 BUILTIN_OBJS += builtin/merge-tree.o
diff --git a/builtin.h b/builtin.h
index a5ae15bfe5..9205d5ecdc 100644
--- a/builtin.h
+++ b/builtin.h
@@ -172,6 +172,7 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix);
 int cmd_merge_index(int argc, const char **argv, const char *prefix);
 int cmd_merge_ours(int argc, const char **argv, const char *prefix);
 int cmd_merge_file(int argc, const char **argv, const char *prefix);
+int cmd_merge_one_file(int argc, const char **argv, const char *prefix);
 int cmd_merge_recursive(int argc, const char **argv, const char *prefix);
 int cmd_merge_tree(int argc, const char **argv, const char *prefix);
 int cmd_mktag(int argc, const char **argv, const char *prefix);
diff --git a/builtin/merge-one-file.c b/builtin/merge-one-file.c
new file mode 100644
index 0000000000..4992a6cd30
--- /dev/null
+++ b/builtin/merge-one-file.c
@@ -0,0 +1,275 @@
+/*
+ * Builtin "git merge-one-file"
+ *
+ * Copyright (c) 2020 Alban Gruin
+ *
+ * Based on git-merge-one-file.sh, written by Linus Torvalds.
+ *
+ * This is the git per-file merge script, called with
+ *
+ *   $1 - original file SHA1 (or empty)
+ *   $2 - file in branch1 SHA1 (or empty)
+ *   $3 - file in branch2 SHA1 (or empty)
+ *   $4 - pathname in repository
+ *   $5 - original file mode (or empty)
+ *   $6 - file in branch1 mode (or empty)
+ *   $7 - file in branch2 mode (or empty)
+ *
+ * Handle some trivial cases.. The _really_ trivial cases have
+ * been handled already by git read-tree, but that one doesn't
+ * do any merges that might change the tree layout.
+ */
+
+#define USE_THE_INDEX_COMPATIBILITY_MACROS
+#include "cache.h"
+#include "builtin.h"
+#include "commit.h"
+#include "dir.h"
+#include "lockfile.h"
+#include "object-store.h"
+#include "run-command.h"
+#include "xdiff-interface.h"
+
+static int create_temp_file(const struct object_id *oid, struct strbuf *path)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+	struct strbuf err = STRBUF_INIT;
+	int ret;
+
+	cp.git_cmd = 1;
+	argv_array_pushl(&cp.args, "unpack-file", oid_to_hex(oid), NULL);
+	ret = pipe_command(&cp, NULL, 0, path, 0, &err, 0);
+	if (!ret && path->len > 0)
+		strbuf_trim_trailing_newline(path);
+
+	fprintf(stderr, "%.*s", (int) err.len, err.buf);
+	strbuf_release(&err);
+
+	return ret;
+}
+
+static int add_to_index_cacheinfo(unsigned int mode,
+				  const struct object_id *oid, const char *path)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	cp.git_cmd = 1;
+	argv_array_pushl(&cp.args, "update-index", "--add", "--cacheinfo", NULL);
+	argv_array_pushf(&cp.args, "%o,%s,%s", mode, oid_to_hex(oid), path);
+	return run_command(&cp);
+}
+
+static int remove_from_index(const char *path)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	cp.git_cmd = 1;
+	argv_array_pushl(&cp.args, "update-index", "--remove", "--", path, NULL);
+	return run_command(&cp);
+}
+
+static int checkout_from_index(const char *path)
+{
+	struct child_process cp = CHILD_PROCESS_INIT;
+
+	cp.git_cmd = 1;
+	argv_array_pushl(&cp.args, "checkout-index", "-u", "-f", "--", path, NULL);
+	return run_command(&cp);
+}
+
+static int merge_one_file_deleted(const struct object_id *orig_blob,
+				  const struct object_id *our_blob,
+				  const struct object_id *their_blob, const char *path,
+				  unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	if ((our_blob && orig_mode != our_mode) ||
+	    (their_blob && orig_mode != their_mode)) {
+		fprintf(stderr, "ERROR: File %s deleted on one branch but had its\n", path);
+		fprintf(stderr, "ERROR: permissions changed on the other.\n");
+		return 1;
+	}
+
+	if (our_blob) {
+		printf("Removing %s\n", path);
+
+		if (file_exists(path))
+			remove_path(path);
+	}
+
+	return remove_from_index(path);
+}
+
+static int do_merge_one_file(const struct object_id *orig_blob,
+			     const struct object_id *our_blob,
+			     const struct object_id *their_blob, const char *path,
+			     unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	int ret, source, dest;
+	struct strbuf src1 = STRBUF_INIT, src2 = STRBUF_INIT, orig = STRBUF_INIT;
+	struct child_process cp_merge = CHILD_PROCESS_INIT,
+		cp_checkout = CHILD_PROCESS_INIT,
+		cp_update = CHILD_PROCESS_INIT;
+
+	if (our_mode == S_IFLNK || their_mode == S_IFLNK) {
+		fprintf(stderr, "ERROR: %s: Not merging symbolic link changes.\n", path);
+		return 1;
+	} else if (our_mode == S_IFGITLINK || their_mode == S_IFGITLINK) {
+		fprintf(stderr, "ERROR: %s: Not merging conflicting submodule changes.\n",
+			path);
+		return 1;
+	}
+
+	create_temp_file(our_blob, &src1);
+	create_temp_file(their_blob, &src2);
+
+	if (orig_blob) {
+		printf("Auto-merging %s\n", path);
+		create_temp_file(orig_blob, &orig);
+	} else {
+		printf("Added %s in both, but differently.\n", path);
+		create_temp_file(the_hash_algo->empty_blob, &orig);
+	}
+
+	cp_merge.git_cmd = 1;
+	argv_array_pushl(&cp_merge.args, "merge-file", src1.buf, orig.buf, src2.buf,
+			 NULL);
+	ret = run_command(&cp_merge);
+
+	if (ret != 0)
+		ret = 1;
+
+	cp_checkout.git_cmd = 1;
+	argv_array_pushl(&cp_checkout.args, "checkout-index", "-f", "--stage=2",
+			 "--", path, NULL);
+	if (run_command(&cp_checkout))
+		return 1;
+
+	source = open(src1.buf, O_RDONLY);
+	dest = open(path, O_WRONLY | O_TRUNC);
+
+	copy_fd(source, dest);
+
+	close(source);
+	close(dest);
+
+	unlink(orig.buf);
+	unlink(src1.buf);
+	unlink(src2.buf);
+
+	strbuf_release(&src1);
+	strbuf_release(&src2);
+	strbuf_release(&orig);
+
+	if (ret) {
+		fprintf(stderr, "ERROR: ");
+
+		if (!orig_blob) {
+			fprintf(stderr, "content conflict");
+			if (our_mode != their_mode)
+				fprintf(stderr, ", ");
+		}
+
+		if (our_mode != their_mode)
+			fprintf(stderr, "permissions conflict: %o->%o,%o",
+				orig_mode, our_mode, their_mode);
+
+		fprintf(stderr, " in %s\n", path);
+
+		return 1;
+	}
+
+	cp_update.git_cmd = 1;
+	argv_array_pushl(&cp_update.args, "update-index", "--", path, NULL);
+	return run_command(&cp_update);
+}
+
+static int merge_one_file(const struct object_id *orig_blob,
+			  const struct object_id *our_blob,
+			  const struct object_id *their_blob, const char *path,
+			  unsigned int orig_mode, unsigned int our_mode, unsigned int their_mode)
+{
+	if (orig_blob &&
+	    ((our_blob && oideq(orig_blob, our_blob)) ||
+	     (their_blob && oideq(orig_blob, their_blob))))
+		return merge_one_file_deleted(orig_blob, our_blob, their_blob, path,
+					      orig_mode, our_mode, their_mode);
+	else if (!orig_blob && our_blob && !their_blob) {
+		return add_to_index_cacheinfo(our_mode, our_blob, path);
+	} else if (!orig_blob && !our_blob && their_blob) {
+		printf("Adding %s\n", path);
+
+		if (file_exists(path)) {
+			fprintf(stderr, "ERROR: untracked %s is overwritten by the merge.\n", path);
+			return 1;
+		}
+
+		if (add_to_index_cacheinfo(their_mode, their_blob, path))
+			return 1;
+		return checkout_from_index(path);
+	} else if (!orig_blob && our_blob && their_blob &&
+		   oideq(our_blob, their_blob)) {
+		if (our_mode != their_mode) {
+			fprintf(stderr, "ERROR: File %s added identically in both branches,", path);
+			fprintf(stderr, "ERROR: but permissions conflict %o->%o.\n",
+				our_mode, their_mode);
+			return 1;
+		}
+
+		printf("Adding %s\n", path);
+
+		if (add_to_index_cacheinfo(our_mode, our_blob, path))
+			return 1;
+		return checkout_from_index(path);
+	} else if (our_blob && their_blob)
+		return do_merge_one_file(orig_blob, our_blob, their_blob, path,
+					 orig_mode, our_mode, their_mode);
+	else {
+		char *orig_hex = "", *our_hex = "", *their_hex = "";
+
+		if (orig_blob)
+			orig_hex = oid_to_hex(orig_blob);
+		if (our_blob)
+			our_hex = oid_to_hex(our_blob);
+		if (their_blob)
+			their_hex = oid_to_hex(their_blob);
+
+		fprintf(stderr, "ERROR: %s: Not handling case %s -> %s -> %s\n",
+			path, orig_hex, our_hex, their_hex);
+		return 1;
+	}
+
+	return 0;
+}
+
+static const char builtin_merge_one_file_usage[] =
+	"git merge-one-file <orig blob> <our blob> <their blob> <path> "
+	"<orig mode> <our mode> <their mode>\n\n"
+	"Blob ids and modes should be empty for missing files.";
+
+int cmd_merge_one_file(int argc, const char **argv, const char *prefix)
+{
+	struct object_id orig_blob, our_blob, their_blob,
+		*p_orig_blob = NULL, *p_our_blob = NULL, *p_their_blob = NULL;
+	unsigned int orig_mode = 0, our_mode = 0, their_mode = 0;
+
+	if (argc != 8)
+		usage(builtin_merge_one_file_usage);
+
+	if (!get_oid(argv[1], &orig_blob)) {
+		p_orig_blob = &orig_blob;
+		orig_mode = strtol(argv[5], NULL, 8);
+	}
+
+	if (!get_oid(argv[2], &our_blob)) {
+		p_our_blob = &our_blob;
+		our_mode = strtol(argv[6], NULL, 8);
+	}
+
+	if (!get_oid(argv[3], &their_blob)) {
+		p_their_blob = &their_blob;
+		their_mode = strtol(argv[7], NULL, 8);
+	}
+
+	return merge_one_file(p_orig_blob, p_our_blob, p_their_blob, argv[4],
+			      orig_mode, our_mode, their_mode);
+}
diff --git a/git-merge-one-file.sh b/git-merge-one-file.sh
deleted file mode 100755
index f6d9852d2f..0000000000
--- a/git-merge-one-file.sh
+++ /dev/null
@@ -1,167 +0,0 @@
-#!/bin/sh
-#
-# Copyright (c) Linus Torvalds, 2005
-#
-# This is the git per-file merge script, called with
-#
-#   $1 - original file SHA1 (or empty)
-#   $2 - file in branch1 SHA1 (or empty)
-#   $3 - file in branch2 SHA1 (or empty)
-#   $4 - pathname in repository
-#   $5 - original file mode (or empty)
-#   $6 - file in branch1 mode (or empty)
-#   $7 - file in branch2 mode (or empty)
-#
-# Handle some trivial cases.. The _really_ trivial cases have
-# been handled already by git read-tree, but that one doesn't
-# do any merges that might change the tree layout.
-
-USAGE='<orig blob> <our blob> <their blob> <path>'
-USAGE="$USAGE <orig mode> <our mode> <their mode>"
-LONG_USAGE="usage: git merge-one-file $USAGE
-
-Blob ids and modes should be empty for missing files."
-
-SUBDIRECTORY_OK=Yes
-. git-sh-setup
-cd_to_toplevel
-require_work_tree
-
-if test $# != 7
-then
-	echo "$LONG_USAGE"
-	exit 1
-fi
-
-case "${1:-.}${2:-.}${3:-.}" in
-#
-# Deleted in both or deleted in one and unchanged in the other
-#
-"$1.." | "$1.$1" | "$1$1.")
-	if { test -z "$6" && test "$5" != "$7"; } ||
-	   { test -z "$7" && test "$5" != "$6"; }
-	then
-		echo "ERROR: File $4 deleted on one branch but had its" >&2
-		echo "ERROR: permissions changed on the other." >&2
-		exit 1
-	fi
-
-	if test -n "$2"
-	then
-		echo "Removing $4"
-	else
-		# read-tree checked that index matches HEAD already,
-		# so we know we do not have this path tracked.
-		# there may be an unrelated working tree file here,
-		# which we should just leave unmolested.  Make sure
-		# we do not have it in the index, though.
-		exec git update-index --remove -- "$4"
-	fi
-	if test -f "$4"
-	then
-		rm -f -- "$4" &&
-		rmdir -p "$(expr "z$4" : 'z\(.*\)/')" 2>/dev/null || :
-	fi &&
-		exec git update-index --remove -- "$4"
-	;;
-
-#
-# Added in one.
-#
-".$2.")
-	# the other side did not add and we added so there is nothing
-	# to be done, except making the path merged.
-	exec git update-index --add --cacheinfo "$6" "$2" "$4"
-	;;
-"..$3")
-	echo "Adding $4"
-	if test -f "$4"
-	then
-		echo "ERROR: untracked $4 is overwritten by the merge." >&2
-		exit 1
-	fi
-	git update-index --add --cacheinfo "$7" "$3" "$4" &&
-		exec git checkout-index -u -f -- "$4"
-	;;
-
-#
-# Added in both, identically (check for same permissions).
-#
-".$3$2")
-	if test "$6" != "$7"
-	then
-		echo "ERROR: File $4 added identically in both branches," >&2
-		echo "ERROR: but permissions conflict $6->$7." >&2
-		exit 1
-	fi
-	echo "Adding $4"
-	git update-index --add --cacheinfo "$6" "$2" "$4" &&
-		exec git checkout-index -u -f -- "$4"
-	;;
-
-#
-# Modified in both, but differently.
-#
-"$1$2$3" | ".$2$3")
-
-	case ",$6,$7," in
-	*,120000,*)
-		echo "ERROR: $4: Not merging symbolic link changes." >&2
-		exit 1
-		;;
-	*,160000,*)
-		echo "ERROR: $4: Not merging conflicting submodule changes." >&2
-		exit 1
-		;;
-	esac
-
-	src1=$(git unpack-file $2)
-	src2=$(git unpack-file $3)
-	case "$1" in
-	'')
-		echo "Added $4 in both, but differently."
-		orig=$(git unpack-file $(git hash-object /dev/null))
-		;;
-	*)
-		echo "Auto-merging $4"
-		orig=$(git unpack-file $1)
-		;;
-	esac
-
-	git merge-file "$src1" "$orig" "$src2"
-	ret=$?
-	msg=
-	if test $ret != 0 || test -z "$1"
-	then
-		msg='content conflict'
-		ret=1
-	fi
-
-	# Create the working tree file, using "our tree" version from the
-	# index, and then store the result of the merge.
-	git checkout-index -f --stage=2 -- "$4" && cat "$src1" >"$4" || exit 1
-	rm -f -- "$orig" "$src1" "$src2"
-
-	if test "$6" != "$7"
-	then
-		if test -n "$msg"
-		then
-			msg="$msg, "
-		fi
-		msg="${msg}permissions conflict: $5->$6,$7"
-		ret=1
-	fi
-
-	if test $ret != 0
-	then
-		echo "ERROR: $msg in $4" >&2
-		exit 1
-	fi
-	exec git update-index -- "$4"
-	;;
-
-*)
-	echo "ERROR: $4: Not handling case $1 -> $2 -> $3" >&2
-	;;
-esac
-exit 1
diff --git a/git.c b/git.c
index a2d337eed7..058d91a2a5 100644
--- a/git.c
+++ b/git.c
@@ -532,6 +532,7 @@ static struct cmd_struct commands[] = {
 	{ "merge-file", cmd_merge_file, RUN_SETUP_GENTLY },
 	{ "merge-index", cmd_merge_index, RUN_SETUP | NO_PARSEOPT },
 	{ "merge-ours", cmd_merge_ours, RUN_SETUP | NO_PARSEOPT },
+	{ "merge-one-file", cmd_merge_one_file, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive-ours", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "merge-recursive-theirs", cmd_merge_recursive, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
diff --git a/t/t6035-merge-dir-to-symlink.sh b/t/t6035-merge-dir-to-symlink.sh
index 2eddcc7664..5fb74e39a0 100755
--- a/t/t6035-merge-dir-to-symlink.sh
+++ b/t/t6035-merge-dir-to-symlink.sh
@@ -94,7 +94,7 @@ test_expect_success SYMLINKS 'a/b was resolved as symlink' '
 	test -h a/b
 '
 
-test_expect_failure 'do not lose untracked in merge (resolve)' '
+test_expect_success 'do not lose untracked in merge (resolve)' '
 	git reset --hard &&
 	git checkout baseline^0 &&
 	>a/b/c/e &&
