Sometimes, we only want to get the objects from output of `ls-tree`
and commands like `sed` or `cut` is usually used to intercept the
origin output to achieve this purpose in practical.

This commit supply an option names `--oid-only` to let `git ls-tree`
only print out the OID of the object. `--oid-only` and `--name-only`
are mutually exclusive in use.

Signed-off-by: Teng Long <redacted>
---
 Documentation/git-ls-tree.txt |  8 +++++--
 builtin/ls-tree.c             | 27 ++++++++++++++++-------
 t/t3104-ls-tree-oid.sh        | 40 +++++++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 10 deletions(-)
 create mode 100755 t/t3104-ls-tree-oid.sh

diff --git a/Documentation/git-ls-tree.txt b/Documentation/git-ls-tree.txt
index db02d6d79a..bc711dc00a 100644
--- a/Documentation/git-ls-tree.txt
+++ b/Documentation/git-ls-tree.txt

@@ -10,7 +10,8 @@ SYNOPSIS
 --------
 [verse]
 'git ls-tree' [-d] [-r] [-t] [-l] [-z]
-	    [--name-only] [--name-status] [--full-name] [--full-tree] [--abbrev[=<n>]]
+	    [--name-only] [--name-status] [--oid-only]

+	    [--full-name] [--full-tree] [--abbrev[=<n>]]
 	    <tree-ish> [<path>...]
 
 DESCRIPTION

@@ -59,7 +60,10 @@ OPTIONS
 --name-only::
 --name-status::
 	List only filenames (instead of the "long" output), one per line.
-
+	Cannot be used with `--oid-only` together.
+--oid-only::
+	List only OIDs of the objects, one per line. Cannot be used with
+	`--name-only` or `--name-status` together.
 --abbrev[=<n>]::
 	Instead of showing the full 40-byte hexadecimal object
 	lines, show the shortest prefix that is at least '<n>'

diff --git a/builtin/ls-tree.c b/builtin/ls-tree.c
index 3a442631c7..1e4a82e669 100644
--- a/builtin/ls-tree.c
+++ b/builtin/ls-tree.c

@@ -18,19 +18,26 @@ static int line_termination = '\n';
 #define LS_RECURSIVE 1
 #define LS_TREE_ONLY 2
 #define LS_SHOW_TREES 4
-#define LS_NAME_ONLY 8
-#define LS_SHOW_SIZE 16
+#define LS_SHOW_SIZE 8
 static int abbrev;
 static int ls_options;
 static struct pathspec pathspec;
 static int chomp_prefix;
 static const char *ls_tree_prefix;
 
-static const  char * const ls_tree_usage[] = {
+static const char * const ls_tree_usage[] = {
 	N_("git ls-tree [<options>] <tree-ish> [<path>...]"),
 	NULL
 };
 
+enum {
+	MODE_UNSPECIFIED = 0,
+	MODE_NAME_ONLY,
+	MODE_OID_ONLY
+};
+
+static int cmdmode = MODE_UNSPECIFIED;
+
 static int show_recursive(const char *base, int baselen, const char *pathname)
 {
 	int i;

@@ -90,7 +97,12 @@ static int show_tree(const struct object_id *oid, struct strbuf *base,
 	else if (ls_options & LS_TREE_ONLY)
 		return 0;
 
-	if (!(ls_options & LS_NAME_ONLY)) {
+	if (cmdmode == 2) {

 		if (ls_options & LS_SHOW_SIZE) {
 			char size_text[24];
 			if (!strcmp(type, blob_type)) {

@@ -135,10 +147,9 @@ int cmd_ls_tree(int argc, const char **argv, const char *prefix)
 			    N_("terminate entries with NUL byte"), 0),
 		OPT_BIT('l', "long", &ls_options, N_("include object size"),
 			LS_SHOW_SIZE),
-		OPT_BIT(0, "name-only", &ls_options, N_("list only filenames"),
-			LS_NAME_ONLY),
-		OPT_BIT(0, "name-status", &ls_options, N_("list only filenames"),
-			LS_NAME_ONLY),
+		OPT_CMDMODE('n', "name-only", &cmdmode, N_("list only filenames"), MODE_NAME_ONLY),
+		OPT_CMDMODE('s', "name-status", &cmdmode, N_("list only filenames"), MODE_NAME_ONLY),
+		OPT_CMDMODE('o', "oid-only", &cmdmode, N_("list only oids"), MODE_OID_ONLY),
 		OPT_SET_INT(0, "full-name", &chomp_prefix,
 			    N_("use full path names"), 0),
 		OPT_BOOL(0, "full-tree", &full_tree,

diff --git a/t/t3104-ls-tree-oid.sh b/t/t3104-ls-tree-oid.sh
new file mode 100755
index 0000000000..4c02cdd3c3
--- /dev/null
+++ b/t/t3104-ls-tree-oid.sh

@@ -0,0 +1,40 @@
+#!/bin/sh
+
+test_description='git ls-tree oids handling.'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo 111 >1.txt &&
+	echo 222 >2.txt &&
+	mkdir -p path0/a/b/c &&
+	echo 333 >path0/a/b/c/3.txt &&
+	find *.txt path* \( -type f -o -type l \) -print |
+	xargs git update-index --add &&
+	tree=$(git write-tree) &&
+	echo $tree
+'
+
+test_expect_success 'usage: --oid-only' '
+	git ls-tree --oid-only $tree >current &&
+	git ls-tree $tree | awk "{print \$3}" >expected &&
+	test_cmp current expected
+'
+
+test_expect_success 'usage: --oid-only with -r' '
+	git ls-tree --oid-only -r $tree >current &&
+	git ls-tree -r $tree | awk "{print \$3}" >expected &&
+	test_cmp current expected
+'
+
+test_expect_success 'usage: --oid-only with --abbrev' '
+	git ls-tree --oid-only --abbrev=6 $tree >current &&
+	git ls-tree --abbrev=6 $tree | awk "{print \$3}" > expected &&
+	test_cmp current expected
+'
+
+test_expect_failure 'usage: incompatible options: --name-only with --oid-only' '
+	test_incompatible_usage git ls-tree --oid-only --name-only
+'
+
+test_done