diff --git a/Makefile b/Makefile
index ea8cd283e2..6e9ab57999 100644
--- a/Makefile
+++ b/Makefile
@@ -159,7 +159,7 @@ PROGRAMS = \
 	git-show-index$X git-ssh-fetch$X \
 	git-ssh-upload$X git-unpack-file$X \
 	git-unpack-objects$X git-update-index$X git-update-server-info$X \
-	git-upload-pack$X git-verify-pack$X git-write-tree$X \
+	git-upload-pack$X git-verify-pack$X \
 	git-update-ref$X git-symbolic-ref$X \
 	git-name-rev$X git-pack-redundant$X git-repo-config$X git-var$X \
 	git-describe$X git-merge-tree$X git-blame$X git-imap-send$X
@@ -170,7 +170,7 @@ BUILT_INS = git-log$X git-whatchanged$X git-show$X \
 	git-check-ref-format$X git-rev-parse$X \
 	git-init-db$X git-tar-tree$X git-upload-tar$X git-format-patch$X \
 	git-ls-files$X git-ls-tree$X git-get-tar-commit-id$X \
-	git-read-tree$X git-commit-tree$X \
+	git-read-tree$X git-commit-tree$X git-write-tree$X \
 	git-apply$X git-show-branch$X git-diff-files$X \
 	git-diff-index$X git-diff-stages$X git-diff-tree$X git-cat-file$X
 
@@ -223,7 +223,7 @@ BUILTIN_OBJS = \
 	builtin-grep.o builtin-add.o builtin-rev-list.o builtin-check-ref-format.o \
 	builtin-rm.o builtin-init-db.o builtin-rev-parse.o \
 	builtin-tar-tree.o builtin-upload-tar.o \
-	builtin-ls-files.o builtin-ls-tree.o \
+	builtin-ls-files.o builtin-ls-tree.o builtin-write-tree.o \
 	builtin-read-tree.o builtin-commit-tree.o \
 	builtin-apply.o builtin-show-branch.o builtin-diff-files.o \
 	builtin-diff-index.o builtin-diff-stages.o builtin-diff-tree.o \
diff --git a/write-tree.c b/builtin-write-tree.c
similarity index 66%
rename from write-tree.c
rename to builtin-write-tree.c
index bd07da6183..c3aac36024 100644
--- a/write-tree.c
+++ b/builtin-write-tree.c
@@ -3,29 +3,72 @@
  *
  * Copyright (C) Linus Torvalds, 2005
  */
+#include "builtin.h"
 #include "cache.h"
 #include "tree.h"
 #include "cache-tree.h"
 
-static int missing_ok = 0;
-static char *prefix = NULL;
-
 static const char write_tree_usage[] =
 "git-write-tree [--missing-ok] [--prefix=<prefix>/]";
 
-static struct lock_file lock_file;
-
-int main(int argc, char **argv)
+int write_tree(unsigned char *sha1, int missing_ok, const char *prefix)
 {
 	int entries, was_valid, newfd;
 
+	/* We can't free this memory, it becomes part of a linked list parsed atexit() */
+	struct lock_file *lock_file = xmalloc(sizeof(struct lock_file));
+
+	newfd = hold_lock_file_for_update(lock_file, get_index_file());
+
+	entries = read_cache();
+	if (entries < 0)
+		die("git-write-tree: error reading cache");
+
+	if (!active_cache_tree)
+		active_cache_tree = cache_tree();
+
+	was_valid = cache_tree_fully_valid(active_cache_tree);
+
+	if (!was_valid) {
+		if (cache_tree_update(active_cache_tree,
+				      active_cache, active_nr,
+				      missing_ok, 0) < 0)
+			die("git-write-tree: error building trees");
+		if (0 <= newfd) {
+			if (!write_cache(newfd, active_cache, active_nr))
+				commit_lock_file(lock_file);
+		}
+		/* Not being able to write is fine -- we are only interested
+		 * in updating the cache-tree part, and if the next caller
+		 * ends up using the old index with unupdated cache-tree part
+		 * it misses the work we did here, but that is just a
+		 * performance penalty and not a big deal.
+		 */
+	}
+
+	if (prefix) {
+		struct cache_tree *subtree =
+			cache_tree_find(active_cache_tree, prefix);
+		memcpy(sha1, subtree->sha1, 20);
+	}
+	else
+		memcpy(sha1, active_cache_tree->sha1, 20);
+
+	rollback_lock_file(lock_file);
+
+	return 0;
+}
+
+int cmd_write_tree(int argc, const char **argv, char **envp)
+{
+	int missing_ok = 0, ret;
+	const char *prefix = NULL;
+	unsigned char sha1[20];
+
 	setup_git_directory();
 
-	newfd = hold_lock_file_for_update(&lock_file, get_index_file());
-	entries = read_cache();
-
 	while (1 < argc) {
-		char *arg = argv[1];
+		const char *arg = argv[1];
 		if (!strcmp(arg, "--missing-ok"))
 			missing_ok = 1;
 		else if (!strncmp(arg, "--prefix=", 9))
@@ -38,35 +81,8 @@ int main(int argc, char **argv)
 	if (argc > 2)
 		die("too many options");
 
-	if (entries < 0)
-		die("git-write-tree: error reading cache");
+	ret = write_tree(sha1, missing_ok, prefix);
+	printf("%s\n", sha1_to_hex(sha1));
 
-	if (!active_cache_tree)
-		active_cache_tree = cache_tree();
-
-	was_valid = cache_tree_fully_valid(active_cache_tree);
-	if (!was_valid) {
-		if (cache_tree_update(active_cache_tree,
-				      active_cache, active_nr,
-				      missing_ok, 0) < 0)
-			die("git-write-tree: error building trees");
-		if (0 <= newfd) {
-			if (!write_cache(newfd, active_cache, active_nr))
-				commit_lock_file(&lock_file);
-		}
-		/* Not being able to write is fine -- we are only interested
-		 * in updating the cache-tree part, and if the next caller
-		 * ends up using the old index with unupdated cache-tree part
-		 * it misses the work we did here, but that is just a
-		 * performance penalty and not a big deal.
-		 */
-	}
-	if (prefix) {
-		struct cache_tree *subtree =
-			cache_tree_find(active_cache_tree, prefix);
-		printf("%s\n", sha1_to_hex(subtree->sha1));
-	}
-	else
-		printf("%s\n", sha1_to_hex(active_cache_tree->sha1));
-	return 0;
+	return ret;
 }
diff --git a/builtin.h b/builtin.h
index b9f36beb66..885422e9f8 100644
--- a/builtin.h
+++ b/builtin.h
@@ -46,4 +46,7 @@ extern int cmd_diff_tree(int argc, const char **argv, char **envp);
 extern int cmd_cat_file(int argc, const char **argv, char **envp);
 extern int cmd_rev_parse(int argc, const char **argv, char **envp);
 
+extern int cmd_write_tree(int argc, const char **argv, char **envp);
+extern int write_tree(unsigned char *sha1, int missing_ok, const char *prefix);
+
 #endif
diff --git a/git.c b/git.c
index 329ebec78c..b4b01326a9 100644
--- a/git.c
+++ b/git.c
@@ -178,7 +178,8 @@ static void handle_internal_command(int argc, const char **argv, char **envp)
 		{ "diff-stages", cmd_diff_stages },
 		{ "diff-tree", cmd_diff_tree },
 		{ "cat-file", cmd_cat_file },
-		{ "rev-parse", cmd_rev_parse }
+		{ "rev-parse", cmd_rev_parse },
+		{ "write-tree", cmd_write_tree }
 	};
 	int i;