patchpretty: Add %(trailer:X) to display single trailer

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 6109ef09aa..a46d0c0717 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -211,6 +211,10 @@ endif::git-rev-list[]
   If the `unfold` option is given, behave as if interpret-trailer's
   `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
   both.
+- %(trailer:<t>): display the specified trailer in parentheses (like
+  %d does for refnames). If there are multiple entries of that trailer
+  they are shown comma separated. If there are no matching trailers
+  nothing is displayed.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index 8ca29e9281..61ae34ced4 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1324,6 +1324,22 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 	}
 
+	if (skip_prefix(placeholder, "(trailer:", &arg)) {
+		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		opts.no_divider = 1;
+		opts.only_trailers = 1;
+		opts.unfold = 1;
+
+		const char *end = strchr(arg, ')');
+		if (!end)
+			return 0;
+
+		opts.filter_trailer = xstrndup(arg, end - arg);
+		format_trailers_from_commit(sb, msg + c->subject_off, &opts);
+		free(opts.filter_trailer);
+		return end - placeholder + 1;
+	}
+
 	return 0;	/* unknown placeholder */
 }
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66ff..e929f820e7 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -598,6 +598,46 @@ test_expect_success ':only and :unfold work together' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailer:foo) shows that trailer' '
+	git log --no-walk --pretty="%(trailer:Acked-By)" >actual &&
+	{
+		echo "(Acked-By: A U Thor <author@example.com>)"
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailer:nonexistant) becomes empty' '
+	git log --no-walk --pretty="x%(trailer:Nacked-By)x" >actual &&
+	{
+		echo "xx"
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '% (trailer:nonexistant) with space becomes empty' '
+	git log --no-walk --pretty="x% (trailer:Nacked-By)x" >actual &&
+	{
+		echo "xx"
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '% (trailer:foo) with space adds space before' '
+	git log --no-walk --pretty="x% (trailer:Acked-By)x" >actual &&
+	{
+		echo "x (Acked-By: A U Thor <author@example.com>)x"
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailer:foo) with multiple lines becomes comma separated and unwrapped' '
+	git log --no-walk --pretty="%(trailer:Signed-Off-By)" >actual &&
+	{
+		echo "(Signed-Off-By: A U Thor <author@example.com>, A U Thor <author@example.com>)"
+	} >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 0796f326b3..d337bca8dd 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1138,6 +1138,7 @@ static void format_trailer_info(struct strbuf *out,
 		return;
 	}
 
+	int printed_first = 0;
 	for (i = 0; i < info->trailer_nr; i++) {
 		char *trailer = info->trailers[i];
 		ssize_t separator_pos = find_separator(trailer, separators);
@@ -1150,7 +1151,19 @@ static void format_trailer_info(struct strbuf *out,
 			if (opts->unfold)
 				unfold_value(&val);
 
-			strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			if (opts->filter_trailer) {
+				if (!strcasecmp (tok.buf, opts->filter_trailer)) {
+					if (!printed_first) {
+						strbuf_addf(out, "(%s: ", opts->filter_trailer);
+						printed_first = 1;
+					} else {
+						strbuf_addstr(out, ", ");
+					}
+					strbuf_addstr(out, val.buf);
+				}
+			} else {
+				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
@@ -1158,7 +1171,8 @@ static void format_trailer_info(struct strbuf *out,
 			strbuf_addstr(out, trailer);
 		}
 	}
-
+	if (printed_first)
+		strbuf_addstr(out, ")");
 }
 
 void format_trailers_from_commit(struct strbuf *out, const char *msg,
diff --git a/trailer.h b/trailer.h
index b997739649..852c79d449 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	char *filter_trailer;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

diff --git a/pretty.c b/pretty.c
index b83a3ecd2..aa03d5b23 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1312,6 +1312,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		size_t ret = 0;
 
 		opts.no_divider = 1;
 
@@ -1328,8 +1329,9 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 		if (*arg == ')') {
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
-			return arg - placeholder + 1;
+			ret = arg - placeholder + 1;
 		}
+		return ret;
 	}
 
 	return 0;	/* unknown placeholder */

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 417b638cd..8326fc45e 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -209,11 +209,14 @@ endif::git-rev-list[]
   respectively, but padding both sides (i.e. the text is centered)
 - %(trailers[:options]): display the trailers of the body as interpreted
   by linkgit:git-interpret-trailers[1]. The `trailers` string may be
-  followed by a colon and zero or more comma-separated options. If the
-  `only` option is given, omit non-trailer lines from the trailer block.
-  If the `unfold` option is given, behave as if interpret-trailer's
-  `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
-  both.
+  followed by a colon and zero or more comma-separated options. The
+  allowed options are `only` which omits non-trailer lines from the
+  trailer block, `unfold` to make it behave as if interpret-trailer's
+  `--unfold` option was given, and `key=T` to only show trailers with
+  specified key (matching is done
+  case-insensitively). E.g. `%(trailers:only,unfold)` unfolds and
+  shows all trailer lines, `%(trailers:key=Reviewed-by,unfold)`
+  unfolds and shows trailer lines with key `Reviewed-by`.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index aa03d5b23..cdca9dce2 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1323,7 +1323,19 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.only_trailers = 1;
 				else if (match_placeholder_arg(arg, "unfold", &arg))
 					opts.unfold = 1;
-				else
+				else if (skip_prefix(arg, "key=", &arg)) {
+					const char *end = arg + strcspn(arg, ",)");
+
+					if (opts.filter_key)
+						free(opts.filter_key);
+
+					opts.filter_key = xstrndup(arg, end - arg);
+					arg = end;
+					if (*arg == ',')
+						arg++;
+
+					opts.only_trailers = 1;
+				} else
 					break;
 			}
 		}
@@ -1331,6 +1343,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 			ret = arg - placeholder + 1;
 		}
+		free(opts.filter_key);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66f..0f5207242 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -598,6 +598,51 @@ test_expect_success ':only and :unfold work together' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:key=foo) shows that trailer' '
+	git log --no-walk --pretty="%(trailers:key=Acked-by)" >actual &&
+	{
+		echo "Acked-by: A U Thor <author@example.com>" &&
+		echo
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) is case insensitive' '
+	git log --no-walk --pretty="%(trailers:key=AcKed-bY)" >actual &&
+	{
+		echo "Acked-by: A U Thor <author@example.com>" &&
+		echo
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=nonexistant) becomes empty' '
+	git log --no-walk --pretty="x%(trailers:key=Nacked-by)x" >actual &&
+	{
+		echo "xx"
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo) handles multiple lines even if folded' '
+	git log --no-walk --pretty="%(trailers:key=Signed-Off-by)" >actual &&
+	{
+		grep -v patch.description <trailers | grep -v Acked-by &&
+		echo
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
+	git log --no-walk --pretty="%(trailers:key=Signed-Off-by,unfold)" >actual &&
+	{
+		echo "Signed-off-by: A U Thor <author@example.com>" &&
+		echo "Signed-off-by: A U Thor <author@example.com>" &&
+		echo
+	} >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 0796f326b..cbbb553e4 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1147,10 +1147,12 @@ static void format_trailer_info(struct strbuf *out,
 			struct strbuf val = STRBUF_INIT;
 
 			parse_trailer(&tok, &val, NULL, trailer, separator_pos);
-			if (opts->unfold)
-				unfold_value(&val);
+			if (!opts->filter_key || !strcasecmp (tok.buf, opts->filter_key)) {
+				if (opts->unfold)
+					unfold_value(&val);
 
-			strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
diff --git a/trailer.h b/trailer.h
index b99773964..d052d02ae 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	char *filter_key;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 8326fc45e..e115e355d 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -212,11 +212,12 @@ endif::git-rev-list[]
   followed by a colon and zero or more comma-separated options. The
   allowed options are `only` which omits non-trailer lines from the
   trailer block, `unfold` to make it behave as if interpret-trailer's
-  `--unfold` option was given, and `key=T` to only show trailers with
-  specified key (matching is done
-  case-insensitively). E.g. `%(trailers:only,unfold)` unfolds and
-  shows all trailer lines, `%(trailers:key=Reviewed-by,unfold)`
-  unfolds and shows trailer lines with key `Reviewed-by`.
+  `--unfold` option was given, `key=T` to only show trailers with
+  specified key (matching is done case-insensitively), and `nokey`
+  which makes it skip over the key part of the trailer and only show
+  value. E.g. `%(trailers:only,unfold)` unfolds and shows all trailer
+  lines, `%(trailers:key=Reviewed-by,unfold)` unfolds and shows
+  trailer lines with key `Reviewed-by`.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index cdca9dce2..f87ba4f18 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1323,6 +1323,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.only_trailers = 1;
 				else if (match_placeholder_arg(arg, "unfold", &arg))
 					opts.unfold = 1;
+				else if (match_placeholder_arg(arg, "nokey", &arg))
+					opts.no_key = 1;
 				else if (skip_prefix(arg, "key=", &arg)) {
 					const char *end = arg + strcspn(arg, ",)");
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 0f5207242..e7de3b18a 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -643,6 +643,15 @@ test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:key=foo,nokey) shows only value' '
+	git log --no-walk --pretty="%(trailers:key=Acked-by,nokey)" >actual &&
+	{
+		echo "A U Thor <author@example.com>" &&
+		echo
+	} >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index cbbb553e4..4f19c34cb 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1150,8 +1150,10 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter_key || !strcasecmp (tok.buf, opts->filter_key)) {
 				if (opts->unfold)
 					unfold_value(&val);
-
-				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				if (opts->no_key)
+					strbuf_addf(out, "%s\n", val.buf);
+				else
+					strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
diff --git a/trailer.h b/trailer.h
index d052d02ae..83de87ee9 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int no_key;
 	char *filter_key;
 };
 

diff --git a/pretty.c b/pretty.c
index f87ba4f18..9fdddce9d 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1074,6 +1074,27 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+static size_t format_fundamental(struct strbuf *sb, /* in UTF-8 */
+				 const char *placeholder,
+				 void *context)
+{
+	int ch;
+
+	switch (placeholder[0]) {
+	case 'n':		/* newline */
+		strbuf_addch(sb, '\n');
+		return 1;
+	case 'x':
+		/* %x00 == NUL, %x0a == LF, etc. */
+		ch = hex2chr(placeholder + 1);
+		if (ch < 0)
+			return 0;
+		strbuf_addch(sb, ch);
+		return 3;
+	}
+	return 0;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1083,9 +1104,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	const char *msg = c->message;
 	struct commit_list *p;
 	const char *arg;
-	int ch;
+	size_t res;
 
 	/* these are independent of the commit */
+	res = format_fundamental(sb, placeholder, NULL);
+	if (res)
+		return res;
+
 	switch (placeholder[0]) {
 	case 'C':
 		if (starts_with(placeholder + 1, "(auto)")) {
@@ -1104,16 +1129,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			 */
 			return ret;
 		}
-	case 'n':		/* newline */
-		strbuf_addch(sb, '\n');
-		return 1;
-	case 'x':
-		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
-		if (ch < 0)
-			return 0;
-		strbuf_addch(sb, ch);
-		return 3;
 	case 'w':
 		if (placeholder[1] == '(') {
 			unsigned long width = 0, indent1 = 0, indent2 = 0;

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index e115e355d..3312850e6 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -213,11 +213,14 @@ endif::git-rev-list[]
   allowed options are `only` which omits non-trailer lines from the
   trailer block, `unfold` to make it behave as if interpret-trailer's
   `--unfold` option was given, `key=T` to only show trailers with
-  specified key (matching is done case-insensitively), and `nokey`
-  which makes it skip over the key part of the trailer and only show
-  value. E.g. `%(trailers:only,unfold)` unfolds and shows all trailer
-  lines, `%(trailers:key=Reviewed-by,unfold)` unfolds and shows
-  trailer lines with key `Reviewed-by`.
+  specified key (matching is done case-insensitively), `nokey` which
+  makes it skip over the key part of the trailer and only show value
+  and `separator` which allows specifying an alternative separator
+  than the default line
+  break. E.g. `%(trailers:only,unfold,separator=%x00)` unfolds and
+  shows all trailer lines separated by NUL character,
+  `%(trailers:key=Reviewed-by,unfold)` unfolds and shows trailer lines
+  with key `Reviewed-by`.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index 9fdddce9d..f73a2b0dc 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1327,6 +1327,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		struct strbuf sepbuf = STRBUF_INIT;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1352,6 +1353,17 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 						arg++;
 
 					opts.only_trailers = 1;
+				} else if (skip_prefix(arg, "separator=", &arg)) {
+					size_t seplen = strcspn(arg, ",)");
+					strbuf_reset(&sepbuf);
+					char *fmt = xstrndup(arg, seplen);
+					strbuf_expand(&sepbuf, fmt, format_fundamental, NULL);
+					free(fmt);
+					opts.separator = &sepbuf;
+
+					arg += seplen;
+					if (*arg == ',')
+						arg++;
 				} else
 					break;
 			}
@@ -1360,6 +1372,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 			ret = arg - placeholder + 1;
 		}
+		strbuf_release (&sepbuf);
 		free(opts.filter_key);
 		return ret;
 	}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index e7de3b18a..71218d22e 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -652,6 +652,12 @@ test_expect_success '%(trailers:key=foo,nokey) shows only value' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:separator) changes separator' '
+	git log --no-walk --pretty=format:"X%(trailers:separator=%x00,unfold)X" >actual &&
+	printf "XSigned-off-by: A U Thor <author@example.com>\0Acked-by: A U Thor <author@example.com>\0[ v2 updated patch description ]\0Signed-off-by: A U Thor <author@example.com>X" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 4f19c34cb..a79e4e36a 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1129,10 +1129,11 @@ static void format_trailer_info(struct strbuf *out,
 				const struct trailer_info *info,
 				const struct process_trailer_options *opts)
 {
+	int first_printed = 0;
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold) {
+	if (!opts->only_trailers && !opts->unfold && !opts->separator) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1150,16 +1151,29 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter_key || !strcasecmp (tok.buf, opts->filter_key)) {
 				if (opts->unfold)
 					unfold_value(&val);
+				if (opts->separator && first_printed)
+					strbuf_addbuf(out, opts->separator);
 				if (opts->no_key)
-					strbuf_addf(out, "%s\n", val.buf);
+					strbuf_addf(out, "%s", val.buf);
 				else
-					strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+					strbuf_addf(out, "%s: %s", tok.buf, val.buf);
+				if (!opts->separator)
+					strbuf_addch(out, '\n');
+
+				first_printed = 1;
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
 		} else if (!opts->only_trailers) {
+			if (opts->separator && first_printed) {
+				strbuf_addbuf(out, opts->separator);
+			}
 			strbuf_addstr(out, trailer);
+			if (opts->separator) {
+				strbuf_rtrim(out);
+			}
+			first_printed = 1;
 		}
 	}
 
diff --git a/trailer.h b/trailer.h
index 83de87ee9..0e9d89660 100644
--- a/trailer.h
+++ b/trailer.h
@@ -74,6 +74,7 @@ struct process_trailer_options {
 	int no_divider;
 	int no_key;
 	char *filter_key;
+	const struct strbuf *separator;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 8cc8c3f9f..30e238338 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -218,9 +218,16 @@ endif::git-rev-list[]
      is given multiple times only last one is used.
   ** 'valueonly': skip over the key part of the trailer and only show
      the its value part.
-  ** Examples: `%(trailers:only,unfold)` unfolds and shows all trailer
-     lines, `%(trailers:key=Reviewed-by,unfold)` unfolds and shows
-     trailer lines with key `Reviewed-by`.
+  ** 'separator=<SEP>': specifying an alternative separator than the
+     default line feed character. SEP may can contain the literal
+     formatting codes %n and %xNN allowing it to contain characters
+     that are hard to type such as %x00, or comma and end-parenthesis
+     which would break parsing. If option is given multiple times only
+     the last one is used.
+  ** Examples: `%(trailers:only,unfold,separator=%x00)` unfolds and
+     shows all trailer lines separated by NUL character,
+     `%(trailers:key=Reviewed-by,unfold)` unfolds and shows trailer
+     lines with key `Reviewed-by`.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index 819c5c50a..5b22a7237 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1318,6 +1318,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
 		struct format_trailer_match_data filter_data;
+		struct strbuf sepbuf = STRBUF_INIT;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1348,6 +1349,19 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					arg = end;
 					if (*arg == ',')
 						arg++;
+				} else if (skip_prefix(arg, "separator=", &arg)) {
+					size_t seplen = strcspn(arg, ",)");
+					char *fmt;
+
+					strbuf_reset(&sepbuf);
+					fmt = xstrndup(arg, seplen);
+					strbuf_expand(&sepbuf, fmt, strbuf_expand_literal_cb, NULL);
+					free(fmt);
+					opts.separator = &sepbuf;
+
+					arg += seplen;
+					if (*arg == ',')
+						arg++;
 				} else
 					break;
 			}
@@ -1356,6 +1370,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 			ret = arg - placeholder + 1;
 		}
+		strbuf_release(&sepbuf);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 095208d6b..562b56dda 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -640,6 +640,42 @@ test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:separator) changes separator' '
+	git log --no-walk --pretty=format:"X%(trailers:separator=%x00,unfold)X" >actual &&
+	printf "XSigned-off-by: A U Thor <author@example.com>\0Acked-by: A U Thor <author@example.com>\0[ v2 updated patch description ]\0Signed-off-by: A U Thor <author@example.com>X" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers) combining separator/key/valueonly' '
+	git commit --allow-empty -F - <<-\EOF &&
+	Important fix
+
+	The fix is explained here
+
+	Closes: #1234
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Another fix
+
+	The fix is explained here
+
+	Closes: #567
+	Closes: #890
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Does not close any tickets
+	EOF
+
+	git log --pretty="%s% (trailers:separator=%x2c%x20,key=Closes,valueonly)" HEAD~3.. >actual &&
+	test_write_lines \
+		"Does not close any tickets" \
+		"Another fix #567, #890" \
+		"Important fix #1234" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 662c7ff03..85cd2e52e 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1129,10 +1129,11 @@ static void format_trailer_info(struct strbuf *out,
 				const struct trailer_info *info,
 				const struct process_trailer_options *opts)
 {
+	size_t origlen = out->len;
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold) {
+	if (!opts->only_trailers && !opts->unfold && !opts->separator) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1150,16 +1151,26 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
+
+				if (opts->separator && out->len != origlen)
+					strbuf_addbuf(out, opts->separator);
 				if (!opts->value_only)
 					strbuf_addf(out, "%s: ", tok.buf);
 				strbuf_addbuf(out, &val);
-				strbuf_addch(out, '\n');
+				if (!opts->separator)
+					strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
 		} else if (!opts->only_trailers) {
+			if (opts->separator && out->len != origlen) {
+				strbuf_addbuf(out, opts->separator);
+			}
 			strbuf_addstr(out, trailer);
+			if (opts->separator) {
+				strbuf_rtrim(out);
+			}
 		}
 	}
 
diff --git a/trailer.h b/trailer.h
index 06d417fe9..203acf4ee 100644
--- a/trailer.h
+++ b/trailer.h
@@ -73,6 +73,7 @@ struct process_trailer_options {
 	int unfold;
 	int no_divider;
 	int value_only;
+	const struct strbuf *separator;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 417b638cd..75c2e838d 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -207,13 +207,18 @@ endif::git-rev-list[]
   than given and there are spaces on its left, use those spaces
 - '%><(<N>)', '%><|(<N>)': similar to '%<(<N>)', '%<|(<N>)'
   respectively, but padding both sides (i.e. the text is centered)
-- %(trailers[:options]): display the trailers of the body as interpreted
+- '%(trailers[:options])': display the trailers of the body as interpreted
   by linkgit:git-interpret-trailers[1]. The `trailers` string may be
-  followed by a colon and zero or more comma-separated options. If the
-  `only` option is given, omit non-trailer lines from the trailer block.
-  If the `unfold` option is given, behave as if interpret-trailer's
-  `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
-  both.
+  followed by a colon and zero or more comma-separated options:
+  ** 'only': omit non-trailer lines from the trailer block.
+  ** 'unfold': make it behave as if interpret-trailer's `--unfold`
+     option was given.
+  ** 'key=<K>': only show trailers with specified key. Matching is
+     done case-insensitively and trailing colon is optional. If option
+     is given multiple times only last one is used.
+  ** Examples: `%(trailers:only,unfold)` unfolds and shows all trailer
+     lines, `%(trailers:key=Reviewed-by,unfold)` unfolds and shows
+     trailer lines with key `Reviewed-by`.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index aa03d5b23..aaedc8447 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1074,6 +1074,17 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+struct format_trailer_match_data {
+	const char *trailer;
+	size_t trailer_len;
+};
+
+static int format_trailer_match_cb(const struct strbuf *sb, void *ud)
+{
+	struct format_trailer_match_data *data = ud;
+	return data->trailer_len == sb->len && !strncasecmp (data->trailer, sb->buf, sb->len);
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1312,6 +1323,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		struct format_trailer_match_data filter_data;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1323,7 +1335,24 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.only_trailers = 1;
 				else if (match_placeholder_arg(arg, "unfold", &arg))
 					opts.unfold = 1;
-				else
+				else if (skip_prefix(arg, "key=", &arg)) {
+					const char *end = arg + strcspn(arg, ",)");
+
+					filter_data.trailer = arg;
+					filter_data.trailer_len = end - arg;
+
+					if (filter_data.trailer_len &&
+					    filter_data.trailer[filter_data.trailer_len - 1] == ':')
+						filter_data.trailer_len--;
+
+					opts.filter = format_trailer_match_cb;
+					opts.filter_data = &filter_data;
+					opts.only_trailers = 1;
+
+					arg = end;
+					if (*arg == ',')
+						arg++;
+				} else
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66f..aba7ba206 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -598,6 +598,42 @@ test_expect_success ':only and :unfold work together' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:key=foo) shows that trailer' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) is case insensitive' '
+	git log --no-walk --pretty="format:%(trailers:key=AcKed-bY)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo:) trailing colon also works' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by:)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=nonexistant) becomes empty' '
+	git log --no-walk --pretty="x%(trailers:key=Nacked-by)x" >actual &&
+	echo "xx" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo) handles multiple lines even if folded' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by)" >actual &&
+	grep -v patch.description <trailers | grep -v Acked-by >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by,unfold)" >actual &&
+	unfold <trailers | grep Signed-off-by >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 0796f326b..97c8f2762 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1147,10 +1147,12 @@ static void format_trailer_info(struct strbuf *out,
 			struct strbuf val = STRBUF_INIT;
 
 			parse_trailer(&tok, &val, NULL, trailer, separator_pos);
-			if (opts->unfold)
-				unfold_value(&val);
+			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
+				if (opts->unfold)
+					unfold_value(&val);
 
-			strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
diff --git a/trailer.h b/trailer.h
index b99773964..5255b676d 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,8 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int (*filter)(const struct strbuf *, void *);
+	void *filter_data;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

pretty.c, trailer.c, Documentation/pretty-formats.txt

diff --git a/pretty.c b/pretty.c
index 302e67fa8..f45ccaf51 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1360,7 +1360,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */

                                        opts.filter = format_trailer_match_cb;
                                        opts.filter_data = &filter_list;
-                                       opts.only_trailers = 1;
                                } else
                                        break;
                        }
diff --git a/trailer.c b/trailer.c
index 97c8f2762..07ca2b2c6 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1132,7 +1132,7 @@ static void format_trailer_info(struct strbuf *out,
        size_t i;

        /* If we want the whole block untouched, we can take the fast path. */
-       if (!opts->only_trailers && !opts->unfold) {
+       if (!opts->only_trailers && !opts->unfold && !opts->filter) {
                strbuf_add(out, info->trailer_start,
                           info->trailer_end - info->trailer_start);
                return;
@@ -1156,7 +1156,7 @@ static void format_trailer_info(struct strbuf *out,
                        strbuf_release(&tok);
                        strbuf_release(&val);

-               } else if (!opts->only_trailers) {
+               } else if (!opts->only_trailers && !opts->filter) {
                        strbuf_addstr(out, trailer);
                }
        }
diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 7548e1d38..ea3cd5b28 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -228,9 +228,9 @@ endif::git-rev-list[]
 ** 'key=<K>': only show trailers with specified key. Matching is done
    case-insensitively and trailing colon is optional. If option is
    given multiple times trailer lines matching any of the keys are
-   shown. Non-trailer lines in the trailer block are also hidden
-   (i.e. 'key' implies 'only'). E.g., `%(trailers:key=Reviewed-by)`
-   shows trailer lines with key `Reviewed-by`.
+   shown. Non-trailer lines in the trailer block are also hidden.
+   E.g., `%(trailers:key=Reviewed-by)` shows trailer lines with key
+   `Reviewed-by`.
 ** 'only': omit non-trailer lines from the trailer block.
 ** 'unfold': make it behave as if interpret-trailer's `--unfold`
    option was given. E.g., `%(trailers:only,unfold)` unfolds and

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 75c2e838d..8cc8c3f9f 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -216,6 +216,8 @@ endif::git-rev-list[]
   ** 'key=<K>': only show trailers with specified key. Matching is
      done case-insensitively and trailing colon is optional. If option
      is given multiple times only last one is used.
+  ** 'valueonly': skip over the key part of the trailer and only show
+     the its value part.
   ** Examples: `%(trailers:only,unfold)` unfolds and shows all trailer
      lines, `%(trailers:key=Reviewed-by,unfold)` unfolds and shows
      trailer lines with key `Reviewed-by`.
diff --git a/pretty.c b/pretty.c
index aaedc8447..2e99f2418 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1335,6 +1335,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.only_trailers = 1;
 				else if (match_placeholder_arg(arg, "unfold", &arg))
 					opts.unfold = 1;
+				else if (match_placeholder_arg(arg, "valueonly", &arg))
+					opts.value_only = 1;
 				else if (skip_prefix(arg, "key=", &arg)) {
 					const char *end = arg + strcspn(arg, ",)");
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index aba7ba206..095208d6b 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -634,6 +634,12 @@ test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by,valueonly)" >actual &&
+	echo "A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 97c8f2762..662c7ff03 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1150,8 +1150,10 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
-
-				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				if (!opts->value_only)
+					strbuf_addf(out, "%s: ", tok.buf);
+				strbuf_addbuf(out, &val);
+				strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
diff --git a/trailer.h b/trailer.h
index 5255b676d..06d417fe9 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int value_only;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

diff --git a/pretty.c b/pretty.c
index b83a3ecd2..aa03d5b23 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1312,6 +1312,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		size_t ret = 0;
 
 		opts.no_divider = 1;
 
@@ -1328,8 +1329,9 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 		if (*arg == ')') {
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
-			return arg - placeholder + 1;
+			ret = arg - placeholder + 1;
 		}
+		return ret;
 	}
 
 	return 0;	/* unknown placeholder */

pretty.c, strbuf.c, strbuf.h

diff --git a/pretty.c b/pretty.c
index 2e99f2418..819c5c50a 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1094,9 +1094,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	const char *msg = c->message;
 	struct commit_list *p;
 	const char *arg;
-	int ch;
+	size_t res;
 
 	/* these are independent of the commit */
+	res = strbuf_expand_literal_cb(sb, placeholder, NULL);
+	if (res)
+		return res;
+
 	switch (placeholder[0]) {
 	case 'C':
 		if (starts_with(placeholder + 1, "(auto)")) {
@@ -1115,16 +1119,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			 */
 			return ret;
 		}
-	case 'n':		/* newline */
-		strbuf_addch(sb, '\n');
-		return 1;
-	case 'x':
-		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
-		if (ch < 0)
-			return 0;
-		strbuf_addch(sb, ch);
-		return 3;
 	case 'w':
 		if (placeholder[1] == '(') {
 			unsigned long width = 0, indent1 = 0, indent2 = 0;
diff --git a/strbuf.c b/strbuf.c
index f6a6cf78b..78eecd29f 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -380,6 +380,27 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 	}
 }
 
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context)
+{
+	int ch;
+
+	switch (placeholder[0]) {
+	case 'n':		/* newline */
+		strbuf_addch(sb, '\n');
+		return 1;
+	case 'x':
+		/* %x00 == NUL, %x0a == LF, etc. */
+		ch = hex2chr(placeholder + 1);
+		if (ch < 0)
+			return 0;
+		strbuf_addch(sb, ch);
+		return 3;
+	}
+	return 0;
+}
+
 size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
 		void *context)
 {
diff --git a/strbuf.h b/strbuf.h
index fc40873b6..52e44c9ab 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -320,6 +320,14 @@ void strbuf_expand(struct strbuf *sb,
 		   expand_fn_t fn,
 		   void *context);
 
+/**
+ * Used as callback for `strbuf_expand` to only expand literals
+ * (i.e. %n and %xNN). The context argument is ignored.
+ */
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context);
+
 /**
  * Used as callback for `strbuf_expand()`, expects an array of
  * struct strbuf_expand_dict_entry as context, i.e. pairs of

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 86d804fe97..d33b072eb2 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -225,10 +225,16 @@ endif::git-rev-list[]
                           linkgit:git-interpret-trailers[1]. The
                           `trailers` string may be followed by a colon
                           and zero or more comma-separated options:
-** 'only': omit non-trailer lines from the trailer block.
-** 'unfold': make it behave as if interpret-trailer's `--unfold`
-   option was given. E.g., `%(trailers:only,unfold)` unfolds and
-   shows all trailer lines.
+** 'only[=val]': select whether non-trailer lines from the trailer
+   block should be included. The `only` keyword may optionally be
+   followed by an equal sign and one of `true`, `on`, `yes` to omit or
+   `false`, `off`, `no` to show the non-trailer lines. If option is
+   given without value it is enabled. If given multiple times the last
+   value is used.
+** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
+   option was given. In same way as to for `only` it can be followed
+   by an equal sign and explicit value. E.g.,
+   `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index b83a3ecd23..26efdba73a 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1074,6 +1074,31 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+static int match_placeholder_bool_arg(const char *to_parse, const char *candidate,
+				      const char **end, int *val)
+{
+	const char *p;
+	if (!skip_prefix(to_parse, candidate, &p))
+		return 0;
+
+	if (match_placeholder_arg(p, "=no", end) ||
+	    match_placeholder_arg(p, "=off", end) ||
+	    match_placeholder_arg(p, "=false", end)) {
+		*val = 0;
+		return 1;
+	}
+
+	if (match_placeholder_arg(p, "", end) ||
+	    match_placeholder_arg(p, "=yes", end) ||
+	    match_placeholder_arg(p, "=on", end) ||
+	    match_placeholder_arg(p, "=true", end)) {
+		*val = 1;
+		return 1;
+	}
+
+	return 0;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1318,11 +1343,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		if (*arg == ':') {
 			arg++;
 			for (;;) {
-				if (match_placeholder_arg(arg, "only", &arg))
-					opts.only_trailers = 1;
-				else if (match_placeholder_arg(arg, "unfold", &arg))
-					opts.unfold = 1;
-				else
+				if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
+				    !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66ff..63730a4ec0 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -578,6 +578,24 @@ test_expect_success '%(trailers:only) shows only "key: value" trailers' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:only=yes) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no) shows all trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=no)" >actual &&
+	cat trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no,only=true) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success '%(trailers:unfold) unfolds trailers' '
 	git log --no-walk --pretty="%(trailers:unfold)" >actual &&
 	{

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 417b638cd8..86d804fe97 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -102,118 +102,133 @@ The title was >>t4119: test autocomputing -p<n> for traditional diff input.<<
 +
 The placeholders are:
 
-- '%H': commit hash
-- '%h': abbreviated commit hash
-- '%T': tree hash
-- '%t': abbreviated tree hash
-- '%P': parent hashes
-- '%p': abbreviated parent hashes
-- '%an': author name
-- '%aN': author name (respecting .mailmap, see linkgit:git-shortlog[1]
-  or linkgit:git-blame[1])
-- '%ae': author email
-- '%aE': author email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ad': author date (format respects --date= option)
-- '%aD': author date, RFC2822 style
-- '%ar': author date, relative
-- '%at': author date, UNIX timestamp
-- '%ai': author date, ISO 8601-like format
-- '%aI': author date, strict ISO 8601 format
-- '%cn': committer name
-- '%cN': committer name (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ce': committer email
-- '%cE': committer email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%cd': committer date (format respects --date= option)
-- '%cD': committer date, RFC2822 style
-- '%cr': committer date, relative
-- '%ct': committer date, UNIX timestamp
-- '%ci': committer date, ISO 8601-like format
-- '%cI': committer date, strict ISO 8601 format
-- '%d': ref names, like the --decorate option of linkgit:git-log[1]
-- '%D': ref names without the " (", ")" wrapping.
-- '%e': encoding
-- '%s': subject
-- '%f': sanitized subject line, suitable for a filename
-- '%b': body
-- '%B': raw body (unwrapped subject and body)
+- Placeholders that expand to a single literal character:
+'%n':: newline
+'%%':: a raw '%'
+'%x00':: print a byte from a hex code
+
+- Placeholders that affect formatting of later placeholders:
+'%Cred':: switch color to red
+'%Cgreen':: switch color to green
+'%Cblue':: switch color to blue
+'%Creset':: reset color
+'%C(...)':: color specification, as described under Values in the
+            "CONFIGURATION FILE" section of linkgit:git-config[1].  By
+            default, colors are shown only when enabled for log output
+            (by `color.diff`, `color.ui`, or `--color`, and respecting
+            the `auto` settings of the former if we are going to a
+            terminal). `%C(auto,...)` is accepted as a historical
+            synonym for the default (e.g., `%C(auto,red)`). Specifying
+            `%C(always,...) will show the colors even when color is
+            not otherwise enabled (though consider just using
+            `--color=always` to enable color for the whole output,
+            including this format and anything else git might color).
+            `auto` alone (i.e. `%C(auto)`) will turn on auto coloring
+            on the next placeholders until the color is switched
+            again.
+'%m':: left (`<`), right (`>`) or boundary (`-`) mark
+'%w([<w>[,<i1>[,<i2>]]])':: switch line wrapping, like the -w option of
+                            linkgit:git-shortlog[1].
+'%<(<N>[,trunc|ltrunc|mtrunc])':: make the next placeholder take at
+                                  least N columns, padding spaces on
+                                  the right if necessary.  Optionally
+                                  truncate at the beginning (ltrunc),
+                                  the middle (mtrunc) or the end
+                                  (trunc) if the output is longer than
+                                  N columns.  Note that truncating
+                                  only works correctly with N >= 2.
+'%<|(<N>)':: make the next placeholder take at least until Nth
+             columns, padding spaces on the right if necessary
+'%>(<N>)', '%>|(<N>)':: similar to '%<(<N>)', '%<|(<N>)' respectively,
+                        but padding spaces on the left
+'%>>(<N>)', '%>>|(<N>)':: similar to '%>(<N>)', '%>|(<N>)'
+                          respectively, except that if the next
+                          placeholder takes more spaces than given and
+                          there are spaces on its left, use those
+                          spaces
+'%><(<N>)', '%><|(<N>)':: similar to '%<(<N>)', '%<|(<N>)'
+                          respectively, but padding both sides
+                          (i.e. the text is centered)
+
+- Placeholders that expand to information extracted from the commit:
+'%H':: commit hash
+'%h':: abbreviated commit hash
+'%T':: tree hash
+'%t':: abbreviated tree hash
+'%P':: parent hashes
+'%p':: abbreviated parent hashes
+'%an':: author name
+'%aN':: author name (respecting .mailmap, see linkgit:git-shortlog[1]
+        or linkgit:git-blame[1])
+'%ae':: author email
+'%aE':: author email (respecting .mailmap, see linkgit:git-shortlog[1]
+        or linkgit:git-blame[1])
+'%ad':: author date (format respects --date= option)
+'%aD':: author date, RFC2822 style
+'%ar':: author date, relative
+'%at':: author date, UNIX timestamp
+'%ai':: author date, ISO 8601-like format
+'%aI':: author date, strict ISO 8601 format
+'%cn':: committer name
+'%cN':: committer name (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%ce':: committer email
+'%cE':: committer email (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%cd':: committer date (format respects --date= option)
+'%cD':: committer date, RFC2822 style
+'%cr':: committer date, relative
+'%ct':: committer date, UNIX timestamp
+'%ci':: committer date, ISO 8601-like format
+'%cI':: committer date, strict ISO 8601 format
+'%d':: ref names, like the --decorate option of linkgit:git-log[1]
+'%D':: ref names without the " (", ")" wrapping.
+'%e':: encoding
+'%s':: subject
+'%f':: sanitized subject line, suitable for a filename
+'%b':: body
+'%B':: raw body (unwrapped subject and body)
 ifndef::git-rev-list[]
-- '%N': commit notes
+'%N':: commit notes
 endif::git-rev-list[]
-- '%GG': raw verification message from GPG for a signed commit
-- '%G?': show "G" for a good (valid) signature,
-  "B" for a bad signature,
-  "U" for a good signature with unknown validity,
-  "X" for a good signature that has expired,
-  "Y" for a good signature made by an expired key,
-  "R" for a good signature made by a revoked key,
-  "E" if the signature cannot be checked (e.g. missing key)
-  and "N" for no signature
-- '%GS': show the name of the signer for a signed commit
-- '%GK': show the key used to sign a signed commit
-- '%GF': show the fingerprint of the key used to sign a signed commit
-- '%GP': show the fingerprint of the primary key whose subkey was used
-  to sign a signed commit
-- '%gD': reflog selector, e.g., `refs/stash@{1}` or
-  `refs/stash@{2 minutes ago`}; the format follows the rules described
-  for the `-g` option. The portion before the `@` is the refname as
-  given on the command line (so `git log -g refs/heads/master` would
-  yield `refs/heads/master@{0}`).
-- '%gd': shortened reflog selector; same as `%gD`, but the refname
-  portion is shortened for human readability (so `refs/heads/master`
-  becomes just `master`).
-- '%gn': reflog identity name
-- '%gN': reflog identity name (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ge': reflog identity email
-- '%gE': reflog identity email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%gs': reflog subject
-- '%Cred': switch color to red
-- '%Cgreen': switch color to green
-- '%Cblue': switch color to blue
-- '%Creset': reset color
-- '%C(...)': color specification, as described under Values in the
-  "CONFIGURATION FILE" section of linkgit:git-config[1].
-  By default, colors are shown only when enabled for log output (by
-  `color.diff`, `color.ui`, or `--color`, and respecting the `auto`
-  settings of the former if we are going to a terminal). `%C(auto,...)`
-  is accepted as a historical synonym for the default (e.g.,
-  `%C(auto,red)`). Specifying `%C(always,...) will show the colors
-  even when color is not otherwise enabled (though consider
-  just using `--color=always` to enable color for the whole output,
-  including this format and anything else git might color).  `auto`
-  alone (i.e. `%C(auto)`) will turn on auto coloring on the next
-  placeholders until the color is switched again.
-- '%m': left (`<`), right (`>`) or boundary (`-`) mark
-- '%n': newline
-- '%%': a raw '%'
-- '%x00': print a byte from a hex code
-- '%w([<w>[,<i1>[,<i2>]]])': switch line wrapping, like the -w option of
-  linkgit:git-shortlog[1].
-- '%<(<N>[,trunc|ltrunc|mtrunc])': make the next placeholder take at
-  least N columns, padding spaces on the right if necessary.
-  Optionally truncate at the beginning (ltrunc), the middle (mtrunc)
-  or the end (trunc) if the output is longer than N columns.
-  Note that truncating only works correctly with N >= 2.
-- '%<|(<N>)': make the next placeholder take at least until Nth
-  columns, padding spaces on the right if necessary
-- '%>(<N>)', '%>|(<N>)': similar to '%<(<N>)', '%<|(<N>)'
-  respectively, but padding spaces on the left
-- '%>>(<N>)', '%>>|(<N>)': similar to '%>(<N>)', '%>|(<N>)'
-  respectively, except that if the next placeholder takes more spaces
-  than given and there are spaces on its left, use those spaces
-- '%><(<N>)', '%><|(<N>)': similar to '%<(<N>)', '%<|(<N>)'
-  respectively, but padding both sides (i.e. the text is centered)
-- %(trailers[:options]): display the trailers of the body as interpreted
-  by linkgit:git-interpret-trailers[1]. The `trailers` string may be
-  followed by a colon and zero or more comma-separated options. If the
-  `only` option is given, omit non-trailer lines from the trailer block.
-  If the `unfold` option is given, behave as if interpret-trailer's
-  `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
-  both.
+'%GG':: raw verification message from GPG for a signed commit
+'%G?':: show "G" for a good (valid) signature,
+        "B" for a bad signature,
+        "U" for a good signature with unknown validity,
+        "X" for a good signature that has expired,
+        "Y" for a good signature made by an expired key,
+        "R" for a good signature made by a revoked key,
+        "E" if the signature cannot be checked (e.g. missing key)
+        and "N" for no signature
+'%GS':: show the name of the signer for a signed commit
+'%GK':: show the key used to sign a signed commit
+'%GF':: show the fingerprint of the key used to sign a signed commit
+'%GP':: show the fingerprint of the primary key whose subkey was used
+        to sign a signed commit
+'%gD':: reflog selector, e.g., `refs/stash@{1}` or `refs/stash@{2
+        minutes ago`}; the format follows the rules described for the
+        `-g` option. The portion before the `@` is the refname as
+        given on the command line (so `git log -g refs/heads/master`
+        would yield `refs/heads/master@{0}`).
+'%gd':: shortened reflog selector; same as `%gD`, but the refname
+        portion is shortened for human readability (so
+        `refs/heads/master` becomes just `master`).
+'%gn':: reflog identity name
+'%gN':: reflog identity name (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%ge':: reflog identity email
+'%gE':: reflog identity email (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%gs':: reflog subject
+'%(trailers[:options])':: display the trailers of the body as
+                          interpreted by
+                          linkgit:git-interpret-trailers[1]. The
+                          `trailers` string may be followed by a colon
+                          and zero or more comma-separated options:
+** 'only': omit non-trailer lines from the trailer block.
+** 'unfold': make it behave as if interpret-trailer's `--unfold`
+   option was given. E.g., `%(trailers:only,unfold)` unfolds and
+   shows all trailer lines.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will

diff --git a/pretty.c b/pretty.c
index 26efdba73a..044447e6c0 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1337,6 +1337,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		size_t ret = 0;
 
 		opts.no_divider = 1;
 
@@ -1350,8 +1351,9 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 		if (*arg == ')') {
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
-			return arg - placeholder + 1;
+			ret = arg - placeholder + 1;
 		}
+		return ret;
 	}
 
 	return 0;	/* unknown placeholder */

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d6add831c0..a920dd15b1 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -243,6 +243,8 @@ endif::git-rev-list[]
    option was given. In same way as to for `only` it can be followed
    by an equal sign and explicit value. E.g.,
    `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
+** 'valueonly[=val]': skip over the key part of the trailer line and only
+   show the value part. Also this optionally allows explicit value.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index 541a553ccc..c508357606 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1383,7 +1383,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.filter_data = &filter_list;
 					opts.only_trailers = 1;
 				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
-					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
+					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold) &&
+					   !match_placeholder_bool_arg(arg, "valueonly", &arg, &opts.value_only))
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 54239290cf..22336c5485 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -667,6 +667,12 @@ test_expect_success 'pretty format %(trailers:key=foo,only=no) also includes non
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by,valueonly)" >actual &&
+	echo "A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index d6da555cd7..d0d9e91631 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1150,8 +1150,10 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
-
-				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				if (!opts->value_only)
+					strbuf_addf(out, "%s: ", tok.buf);
+				strbuf_addbuf(out, &val);
+				strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
diff --git a/trailer.h b/trailer.h
index 5255b676de..06d417fe93 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int value_only;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

pretty.c, strbuf.c, strbuf.h

diff --git a/pretty.c b/pretty.c
index c508357606..50d0b5830d 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1133,9 +1133,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	const char *msg = c->message;
 	struct commit_list *p;
 	const char *arg;
-	int ch;
+	size_t res;
 
 	/* these are independent of the commit */
+	res = strbuf_expand_literal_cb(sb, placeholder, NULL);
+	if (res)
+		return res;
+
 	switch (placeholder[0]) {
 	case 'C':
 		if (starts_with(placeholder + 1, "(auto)")) {
@@ -1154,16 +1158,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			 */
 			return ret;
 		}
-	case 'n':		/* newline */
-		strbuf_addch(sb, '\n');
-		return 1;
-	case 'x':
-		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
-		if (ch < 0)
-			return 0;
-		strbuf_addch(sb, ch);
-		return 3;
 	case 'w':
 		if (placeholder[1] == '(') {
 			unsigned long width = 0, indent1 = 0, indent2 = 0;
diff --git a/strbuf.c b/strbuf.c
index f6a6cf78b9..78eecd29f7 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -380,6 +380,27 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 	}
 }
 
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context)
+{
+	int ch;
+
+	switch (placeholder[0]) {
+	case 'n':		/* newline */
+		strbuf_addch(sb, '\n');
+		return 1;
+	case 'x':
+		/* %x00 == NUL, %x0a == LF, etc. */
+		ch = hex2chr(placeholder + 1);
+		if (ch < 0)
+			return 0;
+		strbuf_addch(sb, ch);
+		return 3;
+	}
+	return 0;
+}
+
 size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
 		void *context)
 {
diff --git a/strbuf.h b/strbuf.h
index fc40873b65..52e44c9ab8 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -320,6 +320,14 @@ void strbuf_expand(struct strbuf *sb,
 		   expand_fn_t fn,
 		   void *context);
 
+/**
+ * Used as callback for `strbuf_expand` to only expand literals
+ * (i.e. %n and %xNN). The context argument is ignored.
+ */
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context);
+
 /**
  * Used as callback for `strbuf_expand()`, expects an array of
  * struct strbuf_expand_dict_entry as context, i.e. pairs of

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index a920dd15b1..ce087dee80 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -239,6 +239,15 @@ endif::git-rev-list[]
    `false`, `off`, `no` to show the non-trailer lines. If option is
    given without value it is enabled. If given multiple times the last
    value is used.
+** 'separator=<SEP>': specify a separator inserted between trailer
+   lines. When this option is not given each trailer line is
+   terminated with a line feed character. The string SEP may contain
+   the literal formatting codes described above. To use comma as
+   separator one must use `%x2C` as it would otherwise be parsed as
+   next option. If separator option is given multiple times only the
+   last one is used. E.g., `%(trailers:key=Ticket,separator=%x2C )`
+   shows all trailer lines whose key is "Ticket" separated by a comma
+   and a space.
 ** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
    option was given. In same way as to for `only` it can be followed
    by an equal sign and explicit value. E.g.,
diff --git a/pretty.c b/pretty.c
index 50d0b5830d..c7609493ee 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1357,6 +1357,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
 		struct string_list filter_list = STRING_LIST_INIT_NODUP;
+		struct strbuf sepbuf = STRBUF_INIT;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1376,6 +1377,14 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.filter = format_trailer_match_cb;
 					opts.filter_data = &filter_list;
 					opts.only_trailers = 1;
+				} else if (match_placeholder_arg_value(arg, "separator", &arg, &argval, &arglen)) {
+					char *fmt;
+
+					strbuf_reset(&sepbuf);
+					fmt = xstrndup(argval, arglen);
+					strbuf_expand(&sepbuf, fmt, strbuf_expand_literal_cb, NULL);
+					free(fmt);
+					opts.separator = &sepbuf;
 				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
 					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold) &&
 					   !match_placeholder_bool_arg(arg, "valueonly", &arg, &opts.value_only))
@@ -1387,6 +1396,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			ret = arg - placeholder + 1;
 		}
 		string_list_clear (&filter_list, 0);
+		strbuf_release(&sepbuf);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 22336c5485..282369dac0 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -673,6 +673,42 @@ test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:separator) changes separator' '
+	git log --no-walk --pretty=format:"X%(trailers:separator=%x00,unfold)X" >actual &&
+	printf "XSigned-off-by: A U Thor <author@example.com>\0Acked-by: A U Thor <author@example.com>\0[ v2 updated patch description ]\0Signed-off-by: A U Thor <author@example.com>X" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers) combining separator/key/valueonly' '
+	git commit --allow-empty -F - <<-\EOF &&
+	Important fix
+
+	The fix is explained here
+
+	Closes: #1234
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Another fix
+
+	The fix is explained here
+
+	Closes: #567
+	Closes: #890
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Does not close any tickets
+	EOF
+
+	git log --pretty="%s% (trailers:separator=%x2c%x20,key=Closes,valueonly)" HEAD~3.. >actual &&
+	test_write_lines \
+		"Does not close any tickets" \
+		"Another fix #567, #890" \
+		"Important fix #1234" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index d0d9e91631..0c414f2fed 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1129,10 +1129,11 @@ static void format_trailer_info(struct strbuf *out,
 				const struct trailer_info *info,
 				const struct process_trailer_options *opts)
 {
+	size_t origlen = out->len;
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold && !opts->filter) {
+	if (!opts->only_trailers && !opts->unfold && !opts->filter && !opts->separator) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1150,16 +1151,26 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
+
+				if (opts->separator && out->len != origlen)
+					strbuf_addbuf(out, opts->separator);
 				if (!opts->value_only)
 					strbuf_addf(out, "%s: ", tok.buf);
 				strbuf_addbuf(out, &val);
-				strbuf_addch(out, '\n');
+				if (!opts->separator)
+					strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
 		} else if (!opts->only_trailers) {
+			if (opts->separator && out->len != origlen) {
+				strbuf_addbuf(out, opts->separator);
+			}
 			strbuf_addstr(out, trailer);
+			if (opts->separator) {
+				strbuf_rtrim(out);
+			}
 		}
 	}
 
diff --git a/trailer.h b/trailer.h
index 06d417fe93..203acf4ee1 100644
--- a/trailer.h
+++ b/trailer.h
@@ -73,6 +73,7 @@ struct process_trailer_options {
 	int unfold;
 	int no_divider;
 	int value_only;
+	const struct strbuf *separator;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d33b072eb2..d6add831c0 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -225,6 +225,14 @@ endif::git-rev-list[]
                           linkgit:git-interpret-trailers[1]. The
                           `trailers` string may be followed by a colon
                           and zero or more comma-separated options:
+** 'key=<K>': only show trailers with specified key. Matching is done
+   case-insensitively and trailing colon is optional. If option is
+   given multiple times trailer lines matching any of the keys are
+   shown. This option automatically enables the `only` option so that
+   non-trailer lines in the trailer block are hidden. If that is not
+   desired it can be disabled with `only=false`.  E.g.,
+   `%(trailers:key=Reviewed-by)` shows trailer lines with key
+   `Reviewed-by`.
 ** 'only[=val]': select whether non-trailer lines from the trailer
    block should be included. The `only` keyword may optionally be
    followed by an equal sign and one of `true`, `on`, `yes` to omit or
diff --git a/pretty.c b/pretty.c
index 044447e6c0..541a553ccc 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1056,13 +1056,20 @@ static size_t parse_padding_placeholder(struct strbuf *sb,
 	return 0;
 }
 
-static int match_placeholder_arg(const char *to_parse, const char *candidate,
-				 const char **end)
+static int match_placeholder_arg_value(const char *to_parse, const char *candidate,
+				       const char **end, const char **valuestart, size_t *valuelen)
 {
 	const char *p;
 
 	if (!(skip_prefix(to_parse, candidate, &p)))
 		return 0;
+	if (valuestart) {
+		if (*p != '=')
+			return 0;
+		*valuestart = p + 1;
+		*valuelen = strcspn(*valuestart, ",)");
+		p = *valuestart + *valuelen;
+	}
 	if (*p == ',') {
 		*end = p + 1;
 		return 1;
@@ -1074,6 +1081,12 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+static int match_placeholder_arg(const char *to_parse, const char *candidate,
+				 const char **end)
+{
+	return match_placeholder_arg_value(to_parse, candidate, end, NULL, NULL);
+}
+
 static int match_placeholder_bool_arg(const char *to_parse, const char *candidate,
 				      const char **end, int *val)
 {
@@ -1095,7 +1108,19 @@ static int match_placeholder_bool_arg(const char *to_parse, const char *candidat
 		*val = 1;
 		return 1;
 	}
+	return 0;
+}
 
+static int format_trailer_match_cb(const struct strbuf *key, void *ud)
+{
+	const struct string_list *list = ud;
+	const struct string_list_item *item;
+
+	for_each_string_list_item (item, list) {
+		if (key->len == (uintptr_t)item->util &&
+		    !strncasecmp (item->string, key->buf, key->len))
+			return 1;
+	}
 	return 0;
 }
 
@@ -1337,6 +1362,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		struct string_list filter_list = STRING_LIST_INIT_NODUP;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1344,8 +1370,20 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		if (*arg == ':') {
 			arg++;
 			for (;;) {
-				if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
-				    !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
+				const char *argval;
+				size_t arglen;
+
+				if (match_placeholder_arg_value(arg, "key", &arg, &argval, &arglen)) {
+					uintptr_t len = arglen;
+					if (len && argval[len - 1] == ':')
+						len--;
+					string_list_append(&filter_list, argval)->util = (char *)len;
+
+					opts.filter = format_trailer_match_cb;
+					opts.filter_data = &filter_list;
+					opts.only_trailers = 1;
+				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
+					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
 					break;
 			}
 		}
@@ -1353,6 +1391,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 			ret = arg - placeholder + 1;
 		}
+		string_list_clear (&filter_list, 0);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 63730a4ec0..54239290cf 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -616,6 +616,57 @@ test_expect_success ':only and :unfold work together' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:key=foo) shows that trailer' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) is case insensitive' '
+	git log --no-walk --pretty="format:%(trailers:key=AcKed-bY)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo:) trailing colon also works' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by:)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) multiple keys' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by:,key=Signed-off-By)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=nonexistant) becomes empty' '
+	git log --no-walk --pretty="x%(trailers:key=Nacked-by)x" >actual &&
+	echo "xx" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo) handles multiple lines even if folded' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by)" >actual &&
+	grep -v patch.description <trailers | grep -v Acked-by >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by,unfold)" >actual &&
+	unfold <trailers | grep Signed-off-by >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo,only=no) also includes nontrailer lines' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by,only=no)" >actual &&
+	{
+		echo "Acked-by: A U Thor <author@example.com>" &&
+		grep patch.description <trailers
+	} >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 0796f326b3..d6da555cd7 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1132,7 +1132,7 @@ static void format_trailer_info(struct strbuf *out,
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold) {
+	if (!opts->only_trailers && !opts->unfold && !opts->filter) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1147,10 +1147,12 @@ static void format_trailer_info(struct strbuf *out,
 			struct strbuf val = STRBUF_INIT;
 
 			parse_trailer(&tok, &val, NULL, trailer, separator_pos);
-			if (opts->unfold)
-				unfold_value(&val);
+			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
+				if (opts->unfold)
+					unfold_value(&val);
 
-			strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
diff --git a/trailer.h b/trailer.h
index b997739649..5255b676de 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,8 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int (*filter)(const struct strbuf *, void *);
+	void *filter_data;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 86d804fe97..d33b072eb2 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -225,10 +225,16 @@ endif::git-rev-list[]
                           linkgit:git-interpret-trailers[1]. The
                           `trailers` string may be followed by a colon
                           and zero or more comma-separated options:
-** 'only': omit non-trailer lines from the trailer block.
-** 'unfold': make it behave as if interpret-trailer's `--unfold`
-   option was given. E.g., `%(trailers:only,unfold)` unfolds and
-   shows all trailer lines.
+** 'only[=val]': select whether non-trailer lines from the trailer
+   block should be included. The `only` keyword may optionally be
+   followed by an equal sign and one of `true`, `on`, `yes` to omit or
+   `false`, `off`, `no` to show the non-trailer lines. If option is
+   given without value it is enabled. If given multiple times the last
+   value is used.
+** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
+   option was given. In same way as to for `only` it can be followed
+   by an equal sign and explicit value. E.g.,
+   `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index b83a3ecd23..b8d71a57c9 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1056,13 +1056,25 @@ static size_t parse_padding_placeholder(struct strbuf *sb,
 	return 0;
 }
 
-static int match_placeholder_arg(const char *to_parse, const char *candidate,
-				 const char **end)
+static int match_placeholder_arg_value(const char *to_parse, const char *candidate,
+				       const char **end, const char **valuestart, size_t *valuelen)
 {
 	const char *p;
 
 	if (!(skip_prefix(to_parse, candidate, &p)))
 		return 0;
+	if (valuestart) {
+		if (*p == '=') {
+			*valuestart = p + 1;
+			*valuelen = strcspn(*valuestart, ",)");
+			p = *valuestart + *valuelen;
+		} else {
+			if (*p != ',' && *p != ')')
+				return 0;
+			*valuestart = NULL;
+			*valuelen = 0;
+		}
+	}
 	if (*p == ',') {
 		*end = p + 1;
 		return 1;
@@ -1074,6 +1086,35 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+static int match_placeholder_bool_arg(const char *to_parse, const char *candidate,
+				      const char **end, int *val)
+{
+	char buf[8];
+	const char *strval;
+	size_t len;
+	int v;
+
+	if (!match_placeholder_arg_value(to_parse, candidate, end, &strval, &len))
+		return 0;
+
+	if (!strval) {
+		*val = 1;
+		return 1;
+	}
+
+	strlcpy(buf, strval, sizeof(buf));
+	if (len < sizeof(buf))
+		buf[len] = 0;
+
+	v = git_parse_maybe_bool(buf);
+	if (v == -1)
+		return 0;
+
+	*val = v;
+
+	return 1;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1318,11 +1359,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		if (*arg == ':') {
 			arg++;
 			for (;;) {
-				if (match_placeholder_arg(arg, "only", &arg))
-					opts.only_trailers = 1;
-				else if (match_placeholder_arg(arg, "unfold", &arg))
-					opts.unfold = 1;
-				else
+				if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
+				    !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66ff..63730a4ec0 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -578,6 +578,24 @@ test_expect_success '%(trailers:only) shows only "key: value" trailers' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:only=yes) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no) shows all trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=no)" >actual &&
+	cat trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no,only=true) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success '%(trailers:unfold) unfolds trailers' '
 	git log --no-walk --pretty="%(trailers:unfold)" >actual &&
 	{

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 86d804fe97..d33b072eb2 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -225,10 +225,16 @@ endif::git-rev-list[]
                           linkgit:git-interpret-trailers[1]. The
                           `trailers` string may be followed by a colon
                           and zero or more comma-separated options:
-** 'only': omit non-trailer lines from the trailer block.
-** 'unfold': make it behave as if interpret-trailer's `--unfold`
-   option was given. E.g., `%(trailers:only,unfold)` unfolds and
-   shows all trailer lines.
+** 'only[=val]': select whether non-trailer lines from the trailer
+   block should be included. The `only` keyword may optionally be
+   followed by an equal sign and one of `true`, `on`, `yes` to omit or
+   `false`, `off`, `no` to show the non-trailer lines. If option is
+   given without value it is enabled. If given multiple times the last
+   value is used.
+** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
+   option was given. In same way as to for `only` it can be followed
+   by an equal sign and explicit value. E.g.,
+   `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index b83a3ecd23..4dfbd38cf6 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1056,13 +1056,26 @@ static size_t parse_padding_placeholder(struct strbuf *sb,
 	return 0;
 }
 
-static int match_placeholder_arg(const char *to_parse, const char *candidate,
-				 const char **end)
+static int match_placeholder_arg_value(const char *to_parse, const char *candidate,
+				       const char **end, const char **valuestart,
+				       size_t *valuelen)
 {
 	const char *p;
 
 	if (!(skip_prefix(to_parse, candidate, &p)))
 		return 0;
+	if (valuestart) {
+		if (*p == '=') {
+			*valuestart = p + 1;
+			*valuelen = strcspn(*valuestart, ",)");
+			p = *valuestart + *valuelen;
+		} else {
+			if (*p != ',' && *p != ')')
+				return 0;
+			*valuestart = NULL;
+			*valuelen = 0;
+		}
+	}
 	if (*p == ',') {
 		*end = p + 1;
 		return 1;
@@ -1074,6 +1087,34 @@ static int match_placeholder_arg(const char *to_parse, const char *candidate,
 	return 0;
 }
 
+static int match_placeholder_bool_arg(const char *to_parse, const char *candidate,
+				      const char **end, int *val)
+{
+	const char *argval;
+	char *strval;
+	size_t arglen;
+	int v;
+
+	if (!match_placeholder_arg_value(to_parse, candidate, end, &argval, &arglen))
+		return 0;
+
+	if (!argval) {
+		*val = 1;
+		return 1;
+	}
+
+	strval = xstrndup(argval, arglen);
+	v = git_parse_maybe_bool(strval);
+	free(strval);
+
+	if (v == -1)
+		return 0;
+
+	*val = v;
+
+	return 1;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1318,11 +1359,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		if (*arg == ':') {
 			arg++;
 			for (;;) {
-				if (match_placeholder_arg(arg, "only", &arg))
-					opts.only_trailers = 1;
-				else if (match_placeholder_arg(arg, "unfold", &arg))
-					opts.unfold = 1;
-				else
+				if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
+				    !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 978a8a66ff..63730a4ec0 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -578,6 +578,24 @@ test_expect_success '%(trailers:only) shows only "key: value" trailers' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:only=yes) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no) shows all trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=no)" >actual &&
+	cat trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:only=no,only=true) shows only "key: value" trailers' '
+	git log --no-walk --pretty=format:"%(trailers:only=yes)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success '%(trailers:unfold) unfolds trailers' '
 	git log --no-walk --pretty="%(trailers:unfold)" >actual &&
 	{

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d6add831c0..a920dd15b1 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -243,6 +243,8 @@ endif::git-rev-list[]
    option was given. In same way as to for `only` it can be followed
    by an equal sign and explicit value. E.g.,
    `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
+** 'valueonly[=val]': skip over the key part of the trailer line and only
+   show the value part. Also this optionally allows explicit value.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will
diff --git a/pretty.c b/pretty.c
index d3dd2d6254..ed25845c98 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1391,7 +1391,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.filter_data = &filter_list;
 					opts.only_trailers = 1;
 				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
-					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
+					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold) &&
+					   !match_placeholder_bool_arg(arg, "valueonly", &arg, &opts.value_only))
 					break;
 			}
 		}
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index d87201afbe..1ad6834781 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -673,6 +673,12 @@ test_expect_success '%(trailers:key) without value is error' '
 	test_cmp expect actual
 '
 
+test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by,valueonly)" >actual &&
+	echo "A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index d6da555cd7..d0d9e91631 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1150,8 +1150,10 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
-
-				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				if (!opts->value_only)
+					strbuf_addf(out, "%s: ", tok.buf);
+				strbuf_addbuf(out, &val);
+				strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
diff --git a/trailer.h b/trailer.h
index 5255b676de..06d417fe93 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,7 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int value_only;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index a920dd15b1..ce087dee80 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -239,6 +239,15 @@ endif::git-rev-list[]
    `false`, `off`, `no` to show the non-trailer lines. If option is
    given without value it is enabled. If given multiple times the last
    value is used.
+** 'separator=<SEP>': specify a separator inserted between trailer
+   lines. When this option is not given each trailer line is
+   terminated with a line feed character. The string SEP may contain
+   the literal formatting codes described above. To use comma as
+   separator one must use `%x2C` as it would otherwise be parsed as
+   next option. If separator option is given multiple times only the
+   last one is used. E.g., `%(trailers:key=Ticket,separator=%x2C )`
+   shows all trailer lines whose key is "Ticket" separated by a comma
+   and a space.
 ** 'unfold[=val]': make it behave as if interpret-trailer's `--unfold`
    option was given. In same way as to for `only` it can be followed
    by an equal sign and explicit value. E.g.,
diff --git a/pretty.c b/pretty.c
index 7baa4c1c26..99b66ccf5a 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1361,6 +1361,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
 		struct string_list filter_list = STRING_LIST_INIT_NODUP;
+		struct strbuf sepbuf = STRBUF_INIT;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1384,6 +1385,14 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 					opts.filter = format_trailer_match_cb;
 					opts.filter_data = &filter_list;
 					opts.only_trailers = 1;
+				} else if (match_placeholder_arg_value(arg, "separator", &arg, &argval, &arglen)) {
+					char *fmt;
+
+					strbuf_reset(&sepbuf);
+					fmt = xstrndup(argval, arglen);
+					strbuf_expand(&sepbuf, fmt, strbuf_expand_literal_cb, NULL);
+					free(fmt);
+					opts.separator = &sepbuf;
 				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
 					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold) &&
 					   !match_placeholder_bool_arg(arg, "valueonly", &arg, &opts.value_only))
@@ -1396,6 +1405,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 	trailer_out:
 		string_list_clear(&filter_list, 0);
+		strbuf_release(&sepbuf);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 1ad6834781..99f50fa401 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -679,6 +679,42 @@ test_expect_success '%(trailers:key=foo,valueonly) shows only value' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:separator) changes separator' '
+	git log --no-walk --pretty=format:"X%(trailers:separator=%x00,unfold)X" >actual &&
+	printf "XSigned-off-by: A U Thor <author@example.com>\0Acked-by: A U Thor <author@example.com>\0[ v2 updated patch description ]\0Signed-off-by: A U Thor <author@example.com>X" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers) combining separator/key/valueonly' '
+	git commit --allow-empty -F - <<-\EOF &&
+	Important fix
+
+	The fix is explained here
+
+	Closes: #1234
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Another fix
+
+	The fix is explained here
+
+	Closes: #567
+	Closes: #890
+	EOF
+
+	git commit --allow-empty -F - <<-\EOF &&
+	Does not close any tickets
+	EOF
+
+	git log --pretty="%s% (trailers:separator=%x2c%x20,key=Closes,valueonly)" HEAD~3.. >actual &&
+	test_write_lines \
+		"Does not close any tickets" \
+		"Another fix #567, #890" \
+		"Important fix #1234" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index d0d9e91631..0c414f2fed 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1129,10 +1129,11 @@ static void format_trailer_info(struct strbuf *out,
 				const struct trailer_info *info,
 				const struct process_trailer_options *opts)
 {
+	size_t origlen = out->len;
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold && !opts->filter) {
+	if (!opts->only_trailers && !opts->unfold && !opts->filter && !opts->separator) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1150,16 +1151,26 @@ static void format_trailer_info(struct strbuf *out,
 			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
 				if (opts->unfold)
 					unfold_value(&val);
+
+				if (opts->separator && out->len != origlen)
+					strbuf_addbuf(out, opts->separator);
 				if (!opts->value_only)
 					strbuf_addf(out, "%s: ", tok.buf);
 				strbuf_addbuf(out, &val);
-				strbuf_addch(out, '\n');
+				if (!opts->separator)
+					strbuf_addch(out, '\n');
 			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
 		} else if (!opts->only_trailers) {
+			if (opts->separator && out->len != origlen) {
+				strbuf_addbuf(out, opts->separator);
+			}
 			strbuf_addstr(out, trailer);
+			if (opts->separator) {
+				strbuf_rtrim(out);
+			}
 		}
 	}
 
diff --git a/trailer.h b/trailer.h
index 06d417fe93..203acf4ee1 100644
--- a/trailer.h
+++ b/trailer.h
@@ -73,6 +73,7 @@ struct process_trailer_options {
 	int unfold;
 	int no_divider;
 	int value_only;
+	const struct strbuf *separator;
 	int (*filter)(const struct strbuf *, void *);
 	void *filter_data;
 };

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index 417b638cd8..86d804fe97 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -102,118 +102,133 @@ The title was >>t4119: test autocomputing -p<n> for traditional diff input.<<
 +
 The placeholders are:
 
-- '%H': commit hash
-- '%h': abbreviated commit hash
-- '%T': tree hash
-- '%t': abbreviated tree hash
-- '%P': parent hashes
-- '%p': abbreviated parent hashes
-- '%an': author name
-- '%aN': author name (respecting .mailmap, see linkgit:git-shortlog[1]
-  or linkgit:git-blame[1])
-- '%ae': author email
-- '%aE': author email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ad': author date (format respects --date= option)
-- '%aD': author date, RFC2822 style
-- '%ar': author date, relative
-- '%at': author date, UNIX timestamp
-- '%ai': author date, ISO 8601-like format
-- '%aI': author date, strict ISO 8601 format
-- '%cn': committer name
-- '%cN': committer name (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ce': committer email
-- '%cE': committer email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%cd': committer date (format respects --date= option)
-- '%cD': committer date, RFC2822 style
-- '%cr': committer date, relative
-- '%ct': committer date, UNIX timestamp
-- '%ci': committer date, ISO 8601-like format
-- '%cI': committer date, strict ISO 8601 format
-- '%d': ref names, like the --decorate option of linkgit:git-log[1]
-- '%D': ref names without the " (", ")" wrapping.
-- '%e': encoding
-- '%s': subject
-- '%f': sanitized subject line, suitable for a filename
-- '%b': body
-- '%B': raw body (unwrapped subject and body)
+- Placeholders that expand to a single literal character:
+'%n':: newline
+'%%':: a raw '%'
+'%x00':: print a byte from a hex code
+
+- Placeholders that affect formatting of later placeholders:
+'%Cred':: switch color to red
+'%Cgreen':: switch color to green
+'%Cblue':: switch color to blue
+'%Creset':: reset color
+'%C(...)':: color specification, as described under Values in the
+            "CONFIGURATION FILE" section of linkgit:git-config[1].  By
+            default, colors are shown only when enabled for log output
+            (by `color.diff`, `color.ui`, or `--color`, and respecting
+            the `auto` settings of the former if we are going to a
+            terminal). `%C(auto,...)` is accepted as a historical
+            synonym for the default (e.g., `%C(auto,red)`). Specifying
+            `%C(always,...) will show the colors even when color is
+            not otherwise enabled (though consider just using
+            `--color=always` to enable color for the whole output,
+            including this format and anything else git might color).
+            `auto` alone (i.e. `%C(auto)`) will turn on auto coloring
+            on the next placeholders until the color is switched
+            again.
+'%m':: left (`<`), right (`>`) or boundary (`-`) mark
+'%w([<w>[,<i1>[,<i2>]]])':: switch line wrapping, like the -w option of
+                            linkgit:git-shortlog[1].
+'%<(<N>[,trunc|ltrunc|mtrunc])':: make the next placeholder take at
+                                  least N columns, padding spaces on
+                                  the right if necessary.  Optionally
+                                  truncate at the beginning (ltrunc),
+                                  the middle (mtrunc) or the end
+                                  (trunc) if the output is longer than
+                                  N columns.  Note that truncating
+                                  only works correctly with N >= 2.
+'%<|(<N>)':: make the next placeholder take at least until Nth
+             columns, padding spaces on the right if necessary
+'%>(<N>)', '%>|(<N>)':: similar to '%<(<N>)', '%<|(<N>)' respectively,
+                        but padding spaces on the left
+'%>>(<N>)', '%>>|(<N>)':: similar to '%>(<N>)', '%>|(<N>)'
+                          respectively, except that if the next
+                          placeholder takes more spaces than given and
+                          there are spaces on its left, use those
+                          spaces
+'%><(<N>)', '%><|(<N>)':: similar to '%<(<N>)', '%<|(<N>)'
+                          respectively, but padding both sides
+                          (i.e. the text is centered)
+
+- Placeholders that expand to information extracted from the commit:
+'%H':: commit hash
+'%h':: abbreviated commit hash
+'%T':: tree hash
+'%t':: abbreviated tree hash
+'%P':: parent hashes
+'%p':: abbreviated parent hashes
+'%an':: author name
+'%aN':: author name (respecting .mailmap, see linkgit:git-shortlog[1]
+        or linkgit:git-blame[1])
+'%ae':: author email
+'%aE':: author email (respecting .mailmap, see linkgit:git-shortlog[1]
+        or linkgit:git-blame[1])
+'%ad':: author date (format respects --date= option)
+'%aD':: author date, RFC2822 style
+'%ar':: author date, relative
+'%at':: author date, UNIX timestamp
+'%ai':: author date, ISO 8601-like format
+'%aI':: author date, strict ISO 8601 format
+'%cn':: committer name
+'%cN':: committer name (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%ce':: committer email
+'%cE':: committer email (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%cd':: committer date (format respects --date= option)
+'%cD':: committer date, RFC2822 style
+'%cr':: committer date, relative
+'%ct':: committer date, UNIX timestamp
+'%ci':: committer date, ISO 8601-like format
+'%cI':: committer date, strict ISO 8601 format
+'%d':: ref names, like the --decorate option of linkgit:git-log[1]
+'%D':: ref names without the " (", ")" wrapping.
+'%e':: encoding
+'%s':: subject
+'%f':: sanitized subject line, suitable for a filename
+'%b':: body
+'%B':: raw body (unwrapped subject and body)
 ifndef::git-rev-list[]
-- '%N': commit notes
+'%N':: commit notes
 endif::git-rev-list[]
-- '%GG': raw verification message from GPG for a signed commit
-- '%G?': show "G" for a good (valid) signature,
-  "B" for a bad signature,
-  "U" for a good signature with unknown validity,
-  "X" for a good signature that has expired,
-  "Y" for a good signature made by an expired key,
-  "R" for a good signature made by a revoked key,
-  "E" if the signature cannot be checked (e.g. missing key)
-  and "N" for no signature
-- '%GS': show the name of the signer for a signed commit
-- '%GK': show the key used to sign a signed commit
-- '%GF': show the fingerprint of the key used to sign a signed commit
-- '%GP': show the fingerprint of the primary key whose subkey was used
-  to sign a signed commit
-- '%gD': reflog selector, e.g., `refs/stash@{1}` or
-  `refs/stash@{2 minutes ago`}; the format follows the rules described
-  for the `-g` option. The portion before the `@` is the refname as
-  given on the command line (so `git log -g refs/heads/master` would
-  yield `refs/heads/master@{0}`).
-- '%gd': shortened reflog selector; same as `%gD`, but the refname
-  portion is shortened for human readability (so `refs/heads/master`
-  becomes just `master`).
-- '%gn': reflog identity name
-- '%gN': reflog identity name (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%ge': reflog identity email
-- '%gE': reflog identity email (respecting .mailmap, see
-  linkgit:git-shortlog[1] or linkgit:git-blame[1])
-- '%gs': reflog subject
-- '%Cred': switch color to red
-- '%Cgreen': switch color to green
-- '%Cblue': switch color to blue
-- '%Creset': reset color
-- '%C(...)': color specification, as described under Values in the
-  "CONFIGURATION FILE" section of linkgit:git-config[1].
-  By default, colors are shown only when enabled for log output (by
-  `color.diff`, `color.ui`, or `--color`, and respecting the `auto`
-  settings of the former if we are going to a terminal). `%C(auto,...)`
-  is accepted as a historical synonym for the default (e.g.,
-  `%C(auto,red)`). Specifying `%C(always,...) will show the colors
-  even when color is not otherwise enabled (though consider
-  just using `--color=always` to enable color for the whole output,
-  including this format and anything else git might color).  `auto`
-  alone (i.e. `%C(auto)`) will turn on auto coloring on the next
-  placeholders until the color is switched again.
-- '%m': left (`<`), right (`>`) or boundary (`-`) mark
-- '%n': newline
-- '%%': a raw '%'
-- '%x00': print a byte from a hex code
-- '%w([<w>[,<i1>[,<i2>]]])': switch line wrapping, like the -w option of
-  linkgit:git-shortlog[1].
-- '%<(<N>[,trunc|ltrunc|mtrunc])': make the next placeholder take at
-  least N columns, padding spaces on the right if necessary.
-  Optionally truncate at the beginning (ltrunc), the middle (mtrunc)
-  or the end (trunc) if the output is longer than N columns.
-  Note that truncating only works correctly with N >= 2.
-- '%<|(<N>)': make the next placeholder take at least until Nth
-  columns, padding spaces on the right if necessary
-- '%>(<N>)', '%>|(<N>)': similar to '%<(<N>)', '%<|(<N>)'
-  respectively, but padding spaces on the left
-- '%>>(<N>)', '%>>|(<N>)': similar to '%>(<N>)', '%>|(<N>)'
-  respectively, except that if the next placeholder takes more spaces
-  than given and there are spaces on its left, use those spaces
-- '%><(<N>)', '%><|(<N>)': similar to '%<(<N>)', '%<|(<N>)'
-  respectively, but padding both sides (i.e. the text is centered)
-- %(trailers[:options]): display the trailers of the body as interpreted
-  by linkgit:git-interpret-trailers[1]. The `trailers` string may be
-  followed by a colon and zero or more comma-separated options. If the
-  `only` option is given, omit non-trailer lines from the trailer block.
-  If the `unfold` option is given, behave as if interpret-trailer's
-  `--unfold` option was given.  E.g., `%(trailers:only,unfold)` to do
-  both.
+'%GG':: raw verification message from GPG for a signed commit
+'%G?':: show "G" for a good (valid) signature,
+        "B" for a bad signature,
+        "U" for a good signature with unknown validity,
+        "X" for a good signature that has expired,
+        "Y" for a good signature made by an expired key,
+        "R" for a good signature made by a revoked key,
+        "E" if the signature cannot be checked (e.g. missing key)
+        and "N" for no signature
+'%GS':: show the name of the signer for a signed commit
+'%GK':: show the key used to sign a signed commit
+'%GF':: show the fingerprint of the key used to sign a signed commit
+'%GP':: show the fingerprint of the primary key whose subkey was used
+        to sign a signed commit
+'%gD':: reflog selector, e.g., `refs/stash@{1}` or `refs/stash@{2
+        minutes ago`}; the format follows the rules described for the
+        `-g` option. The portion before the `@` is the refname as
+        given on the command line (so `git log -g refs/heads/master`
+        would yield `refs/heads/master@{0}`).
+'%gd':: shortened reflog selector; same as `%gD`, but the refname
+        portion is shortened for human readability (so
+        `refs/heads/master` becomes just `master`).
+'%gn':: reflog identity name
+'%gN':: reflog identity name (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%ge':: reflog identity email
+'%gE':: reflog identity email (respecting .mailmap, see
+        linkgit:git-shortlog[1] or linkgit:git-blame[1])
+'%gs':: reflog subject
+'%(trailers[:options])':: display the trailers of the body as
+                          interpreted by
+                          linkgit:git-interpret-trailers[1]. The
+                          `trailers` string may be followed by a colon
+                          and zero or more comma-separated options:
+** 'only': omit non-trailer lines from the trailer block.
+** 'unfold': make it behave as if interpret-trailer's `--unfold`
+   option was given. E.g., `%(trailers:only,unfold)` unfolds and
+   shows all trailer lines.
 
 NOTE: Some placeholders may depend on other options given to the
 revision traversal engine. For example, the `%g*` reflog options will

pretty.c, strbuf.c, strbuf.h

diff --git a/pretty.c b/pretty.c
index ed25845c98..7baa4c1c26 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1137,9 +1137,13 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 	const char *msg = c->message;
 	struct commit_list *p;
 	const char *arg;
-	int ch;
+	size_t res;
 
 	/* these are independent of the commit */
+	res = strbuf_expand_literal_cb(sb, placeholder, NULL);
+	if (res)
+		return res;
+
 	switch (placeholder[0]) {
 	case 'C':
 		if (starts_with(placeholder + 1, "(auto)")) {
@@ -1158,16 +1162,6 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			 */
 			return ret;
 		}
-	case 'n':		/* newline */
-		strbuf_addch(sb, '\n');
-		return 1;
-	case 'x':
-		/* %x00 == NUL, %x0a == LF, etc. */
-		ch = hex2chr(placeholder + 1);
-		if (ch < 0)
-			return 0;
-		strbuf_addch(sb, ch);
-		return 3;
 	case 'w':
 		if (placeholder[1] == '(') {
 			unsigned long width = 0, indent1 = 0, indent2 = 0;
diff --git a/strbuf.c b/strbuf.c
index f6a6cf78b9..78eecd29f7 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -380,6 +380,27 @@ void strbuf_expand(struct strbuf *sb, const char *format, expand_fn_t fn,
 	}
 }
 
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context)
+{
+	int ch;
+
+	switch (placeholder[0]) {
+	case 'n':		/* newline */
+		strbuf_addch(sb, '\n');
+		return 1;
+	case 'x':
+		/* %x00 == NUL, %x0a == LF, etc. */
+		ch = hex2chr(placeholder + 1);
+		if (ch < 0)
+			return 0;
+		strbuf_addch(sb, ch);
+		return 3;
+	}
+	return 0;
+}
+
 size_t strbuf_expand_dict_cb(struct strbuf *sb, const char *placeholder,
 		void *context)
 {
diff --git a/strbuf.h b/strbuf.h
index fc40873b65..52e44c9ab8 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -320,6 +320,14 @@ void strbuf_expand(struct strbuf *sb,
 		   expand_fn_t fn,
 		   void *context);
 
+/**
+ * Used as callback for `strbuf_expand` to only expand literals
+ * (i.e. %n and %xNN). The context argument is ignored.
+ */
+size_t strbuf_expand_literal_cb(struct strbuf *sb,
+				const char *placeholder,
+				void *context);
+
 /**
  * Used as callback for `strbuf_expand()`, expects an array of
  * struct strbuf_expand_dict_entry as context, i.e. pairs of

Documentation/pretty-formats.txt, pretty.c, t/t4205-log-pretty-formats.sh, trailer.c, trailer.h

diff --git a/Documentation/pretty-formats.txt b/Documentation/pretty-formats.txt
index d33b072eb2..d6add831c0 100644
--- a/Documentation/pretty-formats.txt
+++ b/Documentation/pretty-formats.txt
@@ -225,6 +225,14 @@ endif::git-rev-list[]
                           linkgit:git-interpret-trailers[1]. The
                           `trailers` string may be followed by a colon
                           and zero or more comma-separated options:
+** 'key=<K>': only show trailers with specified key. Matching is done
+   case-insensitively and trailing colon is optional. If option is
+   given multiple times trailer lines matching any of the keys are
+   shown. This option automatically enables the `only` option so that
+   non-trailer lines in the trailer block are hidden. If that is not
+   desired it can be disabled with `only=false`.  E.g.,
+   `%(trailers:key=Reviewed-by)` shows trailer lines with key
+   `Reviewed-by`.
 ** 'only[=val]': select whether non-trailer lines from the trailer
    block should be included. The `only` keyword may optionally be
    followed by an equal sign and one of `true`, `on`, `yes` to omit or
diff --git a/pretty.c b/pretty.c
index 65a1b9bd82..d3dd2d6254 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1115,6 +1115,19 @@ static int match_placeholder_bool_arg(const char *to_parse, const char *candidat
 	return 1;
 }
 
+static int format_trailer_match_cb(const struct strbuf *key, void *ud)
+{
+	const struct string_list *list = ud;
+	const struct string_list_item *item;
+
+	for_each_string_list_item (item, list) {
+		if (key->len == (uintptr_t)item->util &&
+		    !strncasecmp(item->string, key->buf, key->len))
+			return 1;
+	}
+	return 0;
+}
+
 static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 				const char *placeholder,
 				void *context)
@@ -1353,6 +1366,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		struct string_list filter_list = STRING_LIST_INIT_NODUP;
 		size_t ret = 0;
 
 		opts.no_divider = 1;
@@ -1360,8 +1374,24 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		if (*arg == ':') {
 			arg++;
 			for (;;) {
-				if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
-				    !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
+				const char *argval;
+				size_t arglen;
+
+				if (match_placeholder_arg_value(arg, "key", &arg, &argval, &arglen)) {
+					uintptr_t len = arglen;
+
+					if (!argval)
+						goto trailer_out;
+
+					if (len && argval[len - 1] == ':')
+						len--;
+					string_list_append(&filter_list, argval)->util = (char *)len;
+
+					opts.filter = format_trailer_match_cb;
+					opts.filter_data = &filter_list;
+					opts.only_trailers = 1;
+				} else if (!match_placeholder_bool_arg(arg, "only", &arg, &opts.only_trailers) &&
+					   !match_placeholder_bool_arg(arg, "unfold", &arg, &opts.unfold))
 					break;
 			}
 		}
@@ -1369,6 +1399,8 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
 			ret = arg - placeholder + 1;
 		}
+	trailer_out:
+		string_list_clear(&filter_list, 0);
 		return ret;
 	}
 
diff --git a/t/t4205-log-pretty-formats.sh b/t/t4205-log-pretty-formats.sh
index 63730a4ec0..d87201afbe 100755
--- a/t/t4205-log-pretty-formats.sh
+++ b/t/t4205-log-pretty-formats.sh
@@ -616,6 +616,63 @@ test_expect_success ':only and :unfold work together' '
 	test_cmp expect actual
 '
 
+test_expect_success 'pretty format %(trailers:key=foo) shows that trailer' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) is case insensitive' '
+	git log --no-walk --pretty="format:%(trailers:key=AcKed-bY)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo:) trailing colon also works' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by:)" >actual &&
+	echo "Acked-by: A U Thor <author@example.com>" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo) multiple keys' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by:,key=Signed-off-By)" >actual &&
+	grep -v patch.description <trailers >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=nonexistant) becomes empty' '
+	git log --no-walk --pretty="x%(trailers:key=Nacked-by)x" >actual &&
+	echo "xx" >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo) handles multiple lines even if folded' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by)" >actual &&
+	grep -v patch.description <trailers | grep -v Acked-by >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key=foo,unfold) properly unfolds' '
+	git log --no-walk --pretty="format:%(trailers:key=Signed-Off-by,unfold)" >actual &&
+	unfold <trailers | grep Signed-off-by >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'pretty format %(trailers:key=foo,only=no) also includes nontrailer lines' '
+	git log --no-walk --pretty="format:%(trailers:key=Acked-by,only=no)" >actual &&
+	{
+		echo "Acked-by: A U Thor <author@example.com>" &&
+		grep patch.description <trailers
+	} >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success '%(trailers:key) without value is error' '
+	git log --no-walk --pretty="tformat:%(trailers:key)" >actual &&
+	echo "%(trailers:key)" >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'trailer parsing not fooled by --- line' '
 	git commit --allow-empty -F - <<-\EOF &&
 	this is the subject
diff --git a/trailer.c b/trailer.c
index 0796f326b3..d6da555cd7 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1132,7 +1132,7 @@ static void format_trailer_info(struct strbuf *out,
 	size_t i;
 
 	/* If we want the whole block untouched, we can take the fast path. */
-	if (!opts->only_trailers && !opts->unfold) {
+	if (!opts->only_trailers && !opts->unfold && !opts->filter) {
 		strbuf_add(out, info->trailer_start,
 			   info->trailer_end - info->trailer_start);
 		return;
@@ -1147,10 +1147,12 @@ static void format_trailer_info(struct strbuf *out,
 			struct strbuf val = STRBUF_INIT;
 
 			parse_trailer(&tok, &val, NULL, trailer, separator_pos);
-			if (opts->unfold)
-				unfold_value(&val);
+			if (!opts->filter || opts->filter(&tok, opts->filter_data)) {
+				if (opts->unfold)
+					unfold_value(&val);
 
-			strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+				strbuf_addf(out, "%s: %s\n", tok.buf, val.buf);
+			}
 			strbuf_release(&tok);
 			strbuf_release(&val);
 
diff --git a/trailer.h b/trailer.h
index b997739649..5255b676de 100644
--- a/trailer.h
+++ b/trailer.h
@@ -72,6 +72,8 @@ struct process_trailer_options {
 	int only_input;
 	int unfold;
 	int no_divider;
+	int (*filter)(const struct strbuf *, void *);
+	void *filter_data;
 };
 
 #define PROCESS_TRAILER_OPTIONS_INIT {0}

diff --git a/pretty.c b/pretty.c
index b8d71a57c9..65a1b9bd82 100644
--- a/pretty.c
+++ b/pretty.c
@@ -1353,6 +1353,7 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 
 	if (skip_prefix(placeholder, "(trailers", &arg)) {
 		struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+		size_t ret = 0;
 
 		opts.no_divider = 1;
 
@@ -1366,8 +1367,9 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
 		}
 		if (*arg == ')') {
 			format_trailers_from_commit(sb, msg + c->subject_off, &opts);
-			return arg - placeholder + 1;
+			ret = arg - placeholder + 1;
 		}
+		return ret;
 	}
 
 	return 0;	/* unknown placeholder */