v3, 17 partstrailer: process trailers from file and arguments

Makefile, trailer.c

diff --git a/Makefile b/Makefile
index b4af1e2..ec90feb 100644
--- a/Makefile
+++ b/Makefile
@@ -871,6 +871,7 @@ LIB_OBJS += submodule.o
 LIB_OBJS += symlinks.o
 LIB_OBJS += tag.o
 LIB_OBJS += trace.o
+LIB_OBJS += trailer.o
 LIB_OBJS += transport.o
 LIB_OBJS += transport-helper.o
 LIB_OBJS += tree-diff.o
diff --git a/trailer.c b/trailer.c
new file mode 100644
index 0000000..d80d047
--- /dev/null
+++ b/trailer.c
@@ -0,0 +1,47 @@
+#include "cache.h"
+/*
+ * Copyright (c) 2013 Christian Couder <chriscool@tuxfamily.org>
+ */
+
+enum action_where { WHERE_AFTER, WHERE_BEFORE };
+enum action_if_exist { EXIST_ADD_IF_DIFFERENT, EXIST_ADD_IF_DIFFERENT_NEIGHBOR,
+		       EXIST_ADD, EXIST_OVERWRITE, EXIST_DO_NOTHING };
+enum action_if_missing { MISSING_ADD, MISSING_DO_NOTHING };
+
+struct conf_info {
+	char *name;
+	char *key;
+	char *command;
+	enum action_where where;
+	enum action_if_exist if_exist;
+	enum action_if_missing if_missing;
+};
+
+struct trailer_item {
+	struct trailer_item *previous;
+	struct trailer_item *next;
+	const char *token;
+	const char *value;
+	struct conf_info *conf;
+};
+
+static int same_token(struct trailer_item *a, struct trailer_item *b, int alnum_len)
+{
+	return !strncasecmp(a->token, b->token, alnum_len);
+}
+
+static int same_value(struct trailer_item *a, struct trailer_item *b)
+{
+	return !strcasecmp(a->value, b->value);
+}
+
+static int same_trailer(struct trailer_item *a, struct trailer_item *b, int alnum_len)
+{
+	return same_token(a, b, alnum_len) && same_value(a, b);
+}
+
+/* Get the length of buf from its beginning until its last alphanumeric character */
+static size_t alnum_len(const char *buf, size_t len) {
+	while (--len >= 0 && !isalnum(buf[len]));
+	return len + 1;
+}

diff --git a/trailer.c b/trailer.c
index d80d047..d88de3f 100644
--- a/trailer.c
+++ b/trailer.c
@@ -45,3 +45,192 @@ static size_t alnum_len(const char *buf, size_t len) {
 	while (--len >= 0 && !isalnum(buf[len]));
 	return len + 1;
 }
+
+static void add_arg_to_infile(struct trailer_item *infile_tok,
+			      struct trailer_item *arg_tok)
+{
+	if (arg_tok->conf->where == WHERE_AFTER) {
+		arg_tok->next = infile_tok->next;
+		infile_tok->next = arg_tok;
+		arg_tok->previous = infile_tok;
+		if (arg_tok->next)
+			arg_tok->next->previous = arg_tok;
+	} else {
+		arg_tok->previous = infile_tok->previous;
+		infile_tok->previous = arg_tok;
+		arg_tok->next = infile_tok;
+		if (arg_tok->previous)
+			arg_tok->previous->next = arg_tok;
+	}
+}
+
+static int check_if_different(struct trailer_item *infile_tok,
+			      struct trailer_item *arg_tok,
+			      int alnum_len, int check_all)
+{
+	enum action_where where = arg_tok->conf->where;
+	do {
+		if (!infile_tok)
+			return 1;
+		if (same_trailer(infile_tok, arg_tok, alnum_len))
+			return 0;
+		/*
+		 * if we want to add a trailer after another one,
+		 * we have to check those before this one
+		 */
+		infile_tok = (where == WHERE_AFTER) ? infile_tok->previous : infile_tok->next;
+	} while (check_all);
+	return 1;
+}
+
+static void apply_arg_if_exist(struct trailer_item *infile_tok,
+			       struct trailer_item *arg_tok,
+			       int alnum_len)
+{
+	switch(arg_tok->conf->if_exist) {
+	case EXIST_DO_NOTHING:
+		free(arg_tok);
+		break;
+	case EXIST_OVERWRITE:
+		free((char *)infile_tok->value);
+		infile_tok->value = xstrdup(arg_tok->value);
+		free(arg_tok);
+		break;
+	case EXIST_ADD:
+		add_arg_to_infile(infile_tok, arg_tok);
+		break;
+	case EXIST_ADD_IF_DIFFERENT:
+		if (check_if_different(infile_tok, arg_tok, alnum_len, 1))
+			add_arg_to_infile(infile_tok, arg_tok);
+		else
+			free(arg_tok);
+		break;
+	case EXIST_ADD_IF_DIFFERENT_NEIGHBOR:
+		if (check_if_different(infile_tok, arg_tok, alnum_len, 0))
+			add_arg_to_infile(infile_tok, arg_tok);
+		else
+			free(arg_tok);
+		break;
+	}
+}
+
+static void remove_from_list(struct trailer_item *item,
+			     struct trailer_item **first)
+{
+	if (item->next)
+		item->next->previous = item->previous;
+	if (item->previous)
+		item->previous->next = item->next;
+	else
+		*first = item->next;
+}
+
+static struct trailer_item *remove_first(struct trailer_item **first)
+{
+	struct trailer_item *item = *first;
+	*first = item->next;
+	if (item->next) {
+		item->next->previous = NULL;
+		item->next = NULL;
+	}
+	return item;
+}
+
+static void process_infile_tok(struct trailer_item *infile_tok,
+			       struct trailer_item **arg_tok_first,
+			       enum action_where where)
+{
+	struct trailer_item *arg_tok;
+	struct trailer_item *next_arg;
+
+	int tok_alnum_len = alnum_len(infile_tok->token, strlen(infile_tok->token));
+	for (arg_tok = *arg_tok_first; arg_tok; arg_tok = next_arg) {
+		next_arg = arg_tok->next;
+		if (same_token(infile_tok, arg_tok, tok_alnum_len) &&
+		    arg_tok->conf->where == where) {
+			/* Remove arg_tok from list */
+			remove_from_list(arg_tok, arg_tok_first);
+			/* Apply arg */
+			apply_arg_if_exist(infile_tok, arg_tok, tok_alnum_len);
+			/*
+			 * If arg has been added to infile,
+			 * then we need to process it too now.
+			 */
+			if ((where == WHERE_AFTER ? infile_tok->next : infile_tok->previous) == arg_tok)
+				infile_tok = arg_tok;
+		}
+	}
+}
+
+static void update_last(struct trailer_item **last)
+{
+	if (*last)
+		while((*last)->next != NULL)
+			*last = (*last)->next;
+}
+
+static void update_first(struct trailer_item **first)
+{
+	if (*first)
+		while((*first)->previous != NULL)
+			*first = (*first)->previous;
+}
+
+static void apply_arg_if_missing(struct trailer_item **infile_tok_first,
+				 struct trailer_item **infile_tok_last,
+				 struct trailer_item *arg_tok)
+{
+	struct trailer_item **infile_tok;
+	enum action_where where;
+
+	switch(arg_tok->conf->if_missing) {
+	case MISSING_DO_NOTHING:
+		free(arg_tok);
+		break;
+	case MISSING_ADD:
+		where = arg_tok->conf->where;
+		infile_tok = (where == WHERE_AFTER) ? infile_tok_last : infile_tok_first;
+		if (*infile_tok) {
+			add_arg_to_infile(*infile_tok, arg_tok);
+			*infile_tok = arg_tok;
+		} else {
+			*infile_tok_first = arg_tok;
+			*infile_tok_last = arg_tok;
+		}
+		break;
+	}
+}
+
+static void process_trailers_lists(struct trailer_item **infile_tok_first,
+				   struct trailer_item **infile_tok_last,
+				   struct trailer_item **arg_tok_first)
+{
+	struct trailer_item *infile_tok;
+	struct trailer_item *arg_tok;
+
+	if (!*arg_tok_first)
+		return;
+
+	/* Process infile from end to start */
+	for (infile_tok = *infile_tok_last; infile_tok; infile_tok = infile_tok->previous) {
+		process_infile_tok(infile_tok, arg_tok_first, WHERE_AFTER);
+	}
+
+	update_last(infile_tok_last);
+
+	if (!*arg_tok_first)
+		return;
+
+	/* Process infile from start to end */
+	for (infile_tok = *infile_tok_first; infile_tok; infile_tok = infile_tok->next) {
+		process_infile_tok(infile_tok, arg_tok_first, WHERE_BEFORE);
+	}
+
+	update_first(infile_tok_first);
+
+	/* Process args left */
+	while (*arg_tok_first) {
+		arg_tok = remove_first(arg_tok_first);
+		apply_arg_if_missing(infile_tok_first, infile_tok_last, arg_tok);
+	}
+}

diff --git a/trailer.c b/trailer.c
index d88de3f..e7d8244 100644
--- a/trailer.c
+++ b/trailer.c
@@ -25,6 +25,8 @@ struct trailer_item {
 	struct conf_info *conf;
 };
 
+static struct trailer_item *first_conf_item;
+
 static int same_token(struct trailer_item *a, struct trailer_item *b, int alnum_len)
 {
 	return !strncasecmp(a->token, b->token, alnum_len);
@@ -234,3 +236,130 @@ static void process_trailers_lists(struct trailer_item **infile_tok_first,
 		apply_arg_if_missing(infile_tok_first, infile_tok_last, arg_tok);
 	}
 }
+
+static int set_where(struct conf_info *item, const char *value)
+{
+	if (!strcasecmp("after", value)) {
+		item->where = WHERE_AFTER;
+	} else if (!strcasecmp("before", value)) {
+		item->where = WHERE_BEFORE;
+	} else
+		return 1;
+	return 0;
+}
+
+static int set_if_exist(struct conf_info *item, const char *value)
+{
+	if (!strcasecmp("addIfDifferent", value)) {
+		item->if_exist = EXIST_ADD_IF_DIFFERENT;
+	} else if (!strcasecmp("addIfDifferentNeighbor", value)) {
+		item->if_exist = EXIST_ADD_IF_DIFFERENT_NEIGHBOR;
+	} else if (!strcasecmp("add", value)) {
+		item->if_exist = EXIST_ADD;
+	} else if (!strcasecmp("overwrite", value)) {
+		item->if_exist = EXIST_OVERWRITE;
+	} else if (!strcasecmp("doNothing", value)) {
+		item->if_exist = EXIST_DO_NOTHING;
+	} else
+		return 1;
+	return 0;
+}
+
+static int set_if_missing(struct conf_info *item, const char *value)
+{
+	if (!strcasecmp("doNothing", value)) {
+		item->if_missing = MISSING_DO_NOTHING;
+	} else if (!strcasecmp("add", value)) {
+		item->if_missing = MISSING_ADD;
+	} else
+		return 1;
+	return 0;
+}
+
+enum trailer_info_type { TRAILER_VALUE, TRAILER_COMMAND, TRAILER_WHERE,
+			 TRAILER_IF_EXIST, TRAILER_IF_MISSING };
+
+static int set_name_and_type(const char *conf_key, const char *suffix,
+			     enum trailer_info_type type,
+			     char **pname, enum trailer_info_type *ptype)
+{
+	int ret = ends_with(conf_key, suffix);
+	if (ret) {
+		*pname = xstrndup(conf_key, strlen(conf_key) - strlen(suffix));
+		*ptype = type;
+	}
+	return ret;
+}
+
+static struct trailer_item *get_conf_item(char *name)
+{
+	struct trailer_item *item;
+	struct trailer_item *previous;
+
+	/* Look up item with same name */
+	for (previous = NULL, item = first_conf_item;
+	     item;
+	     previous = item, item = item->next)
+	{
+		if (!strcasecmp(item->conf->name, name))
+			return item;
+	}
+
+	/* Item does not already exists, create it */
+	item = xcalloc(sizeof(struct trailer_item), 1);
+	item->conf = xcalloc(sizeof(struct conf_info), 1);
+	item->conf->name = xstrdup(name);
+
+	if (!previous) {
+		first_conf_item = item;
+	} else {
+		previous->next = item;
+		item->previous = previous;
+	}
+
+	return item;
+}
+
+static int git_trailer_config(const char *conf_key, const char *value, void *cb)
+{
+	if (starts_with(conf_key, "trailer.")) {
+		const char *orig_conf_key = conf_key;
+		struct trailer_item *item;
+		struct conf_info *conf;
+		char *name;
+		enum trailer_info_type type;
+
+		conf_key += 8;
+		if (!set_name_and_type(conf_key, ".key", TRAILER_VALUE, &name, &type) &&
+		    !set_name_and_type(conf_key, ".command", TRAILER_COMMAND, &name, &type) &&
+		    !set_name_and_type(conf_key, ".where", TRAILER_WHERE, &name, &type) &&
+		    !set_name_and_type(conf_key, ".ifexist", TRAILER_IF_EXIST, &name, &type) &&
+		    !set_name_and_type(conf_key, ".ifmissing", TRAILER_IF_MISSING, &name, &type))
+			return 0;
+
+		item = get_conf_item(name);
+		conf = item->conf;
+
+		if (type == TRAILER_VALUE) {
+			if (conf->key)
+				warning(_("more than one %s"), orig_conf_key);
+			conf->key = xstrdup(value);
+		} else if (type == TRAILER_COMMAND) {
+			if (conf->command)
+				warning(_("more than one %s"), orig_conf_key);
+			conf->command = xstrdup(value);
+		} else if (type == TRAILER_WHERE) {
+			if (set_where(conf, value))
+				warning(_("unknown value '%s' for key '%s'"), value, orig_conf_key);
+		} else if (type == TRAILER_IF_EXIST) {
+			if (set_if_exist(conf, value))
+				warning(_("unknown value '%s' for key '%s'"), value, orig_conf_key);
+		} else if (type == TRAILER_IF_MISSING) {
+			if (set_if_missing(conf, value))
+				warning(_("unknown value '%s' for key '%s'"), value, orig_conf_key);
+		} else {
+			die("internal bug in trailer.c");
+		}
+	}
+	return 0;
+}

diff --git a/trailer.c b/trailer.c
index e7d8244..89377f2 100644
--- a/trailer.c
+++ b/trailer.c
@@ -363,3 +363,80 @@ static int git_trailer_config(const char *conf_key, const char *value, void *cb)
 	}
 	return 0;
 }
+
+static void parse_trailer(struct strbuf *tok, struct strbuf *val, const char *trailer)
+{
+	char *end = strchr(trailer, '=');
+	if (!end)
+		end = strchr(trailer, ':');
+	if (end) {
+		strbuf_add(tok, trailer, end - trailer);
+		strbuf_trim(tok);
+		strbuf_addstr(val, end + 1);
+		strbuf_trim(val);
+	} else {
+		strbuf_addstr(tok, trailer);
+		strbuf_trim(tok);
+	}
+}
+
+static struct trailer_item *create_trailer_item(const char *string)
+{
+	struct strbuf tok = STRBUF_INIT;
+	struct strbuf val = STRBUF_INIT;
+	struct trailer_item *new;
+	struct trailer_item *item;
+	int tok_alnum_len;
+
+	parse_trailer(&tok, &val, string);
+
+	tok_alnum_len = alnum_len(tok.buf, tok.len);
+
+	/* Lookup if the token matches something in the config */
+	for (item = first_conf_item; item; item = item->next) {
+		if (!strncasecmp(tok.buf, item->conf->key, tok_alnum_len) ||
+		    !strncasecmp(tok.buf, item->conf->name, tok_alnum_len)) {
+			new = xcalloc(sizeof(struct trailer_item), 1);
+			new->conf = item->conf;
+			new->token = xstrdup(item->conf->key);
+			new->value = strbuf_detach(&val, NULL);
+			strbuf_release(&tok);
+			return new;
+		}
+	}
+
+	new = xcalloc(sizeof(struct trailer_item), 1);
+	new->conf = xcalloc(sizeof(struct conf_info), 1);
+	new->token = strbuf_detach(&tok, NULL);
+	new->value = strbuf_detach(&val, NULL);
+
+	return new;
+}
+
+static void add_trailer_item(struct trailer_item **first,
+			     struct trailer_item **last,
+			     struct trailer_item *new)
+{
+	if (!*last) {
+		*first = new;
+		*last = new;
+	} else {
+		(*last)->next = new;
+		new->previous = *last;
+		*last = new;
+	}
+}
+
+static struct trailer_item *process_command_line_args(int argc, const char **argv)
+{
+	int i;
+	struct trailer_item *arg_tok_first = NULL;
+	struct trailer_item *arg_tok_last = NULL;
+
+	for (i = 0; i < argc; i++) {
+		struct trailer_item *new = create_trailer_item(argv[i]);
+		add_trailer_item(&arg_tok_first, &arg_tok_last, new);
+	}
+
+	return arg_tok_first;
+}

strbuf.c, strbuf.h

diff --git a/strbuf.c b/strbuf.c
index 83caf4a..2124bb8 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -124,6 +124,13 @@ void strbuf_ltrim(struct strbuf *sb)
 	sb->buf[sb->len] = '\0';
 }
 
+int strbuf_isspace(struct strbuf *sb)
+{
+	char *b;
+	for (b = sb->buf; *b && isspace(*b); b++);
+	return !*b;
+}
+
 struct strbuf **strbuf_split_buf(const char *str, size_t slen,
 				 int terminator, int max)
 {
diff --git a/strbuf.h b/strbuf.h
index 73e80ce..02bff3a 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -42,6 +42,7 @@ static inline void strbuf_setlen(struct strbuf *sb, size_t len) {
 extern void strbuf_trim(struct strbuf *);
 extern void strbuf_rtrim(struct strbuf *);
 extern void strbuf_ltrim(struct strbuf *);
+extern int strbuf_isspace(struct strbuf *);
 extern int strbuf_cmp(const struct strbuf *, const struct strbuf *);
 
 /*

diff --git a/trailer.c b/trailer.c
index 89377f2..9ea08a7 100644
--- a/trailer.c
+++ b/trailer.c
@@ -440,3 +440,65 @@ static struct trailer_item *process_command_line_args(int argc, const char **arg
 
 	return arg_tok_first;
 }
+
+static struct strbuf **read_input_file(const char *infile)
+{
+	struct strbuf sb = STRBUF_INIT;
+
+	if (strbuf_read_file(&sb, infile, 0) < 0)
+		die_errno(_("could not read input file '%s'"), infile);
+
+	return strbuf_split(&sb, '\n');
+}
+
+/*
+ * Return the the (0 based) index of the first trailer line
+ * or the line count if there are no trailers.
+ */
+static int find_trailer_start(struct strbuf **lines)
+{
+	int count, start, empty = 1;
+
+	/* Get the line count */
+	for (count = 0; lines[count]; count++);
+
+	/*
+	 * Get the start of the trailers by looking starting from the end
+	 * for a line with only spaces before lines with one ':'.
+	 */
+	for (start = count - 1; start >= 0; start--) {
+		if (strbuf_isspace(lines[start])) {
+			if (empty)
+				continue;
+			return start + 1;
+		}
+		if (strchr(lines[start]->buf, ':')) {
+			if (empty)
+				empty = 0;
+			continue;
+		}
+		return count;
+	}
+
+	return empty ? count : start + 1;
+}
+
+static void process_input_file(const char *infile,
+			       struct trailer_item **infile_tok_first,
+			       struct trailer_item **infile_tok_last)
+{
+	struct strbuf **lines = read_input_file(infile);
+	int start = find_trailer_start(lines);
+	int i;
+
+	/* Print non trailer lines as is */
+	for (i = 0; lines[i] && i < start; i++) {
+		printf("%s", lines[i]->buf);
+	}
+
+	/* Parse trailer lines */
+	for (i = start; lines[i]; i++) {
+		struct trailer_item *new = create_trailer_item(lines[i]->buf);
+		add_trailer_item(infile_tok_first, infile_tok_last, new);
+	}
+}

diff --git a/trailer.c b/trailer.c
index 9ea08a7..2678b3e 100644
--- a/trailer.c
+++ b/trailer.c
@@ -48,6 +48,26 @@ static size_t alnum_len(const char *buf, size_t len) {
 	return len + 1;
 }
 
+static void print_tok_val(const char *tok, const char *val)
+{
+	char c = tok[strlen(tok) - 1];
+	if (isalnum(c))
+		printf("%s: %s\n", tok, val);
+	else if (isspace(c) || c == '#')
+		printf("%s%s\n", tok, val);
+	else
+		printf("%s %s\n", tok, val);
+}
+
+static void print_all(struct trailer_item *first, int trim_empty)
+{
+	struct trailer_item *item;
+	for (item = first; item; item = item->next) {
+		if (!trim_empty || strlen(item->value) > 0)
+			print_tok_val(item->token, item->value);
+	}
+}
+
 static void add_arg_to_infile(struct trailer_item *infile_tok,
 			      struct trailer_item *arg_tok)
 {
@@ -502,3 +522,23 @@ static void process_input_file(const char *infile,
 		add_trailer_item(infile_tok_first, infile_tok_last, new);
 	}
 }
+
+void process_trailers(const char *infile, int trim_empty, int argc, const char **argv)
+{
+	struct trailer_item *infile_tok_first = NULL;
+	struct trailer_item *infile_tok_last = NULL;
+	struct trailer_item *arg_tok_first;
+
+	git_config(git_trailer_config, NULL);
+
+	/* Print the non trailer part of infile */
+	if (infile) {
+		process_input_file(infile, &infile_tok_first, &infile_tok_last);
+	}
+
+	arg_tok_first = process_command_line_args(argc, argv);
+
+	process_trailers_lists(&infile_tok_first, &infile_tok_last, &arg_tok_first);
+
+	print_all(infile_tok_first, trim_empty);
+}

.gitignore, Makefile, builtin.h, builtin/interpret-trailers.c, git.c, trailer.h

diff --git a/.gitignore b/.gitignore
index b5f9def..c870ada 100644
--- a/.gitignore
+++ b/.gitignore
@@ -74,6 +74,7 @@
 /git-index-pack
 /git-init
 /git-init-db
+/git-interpret-trailers
 /git-instaweb
 /git-log
 /git-ls-files
diff --git a/Makefile b/Makefile
index ec90feb..a91465e 100644
--- a/Makefile
+++ b/Makefile
@@ -935,6 +935,7 @@ BUILTIN_OBJS += builtin/hash-object.o
 BUILTIN_OBJS += builtin/help.o
 BUILTIN_OBJS += builtin/index-pack.o
 BUILTIN_OBJS += builtin/init-db.o
+BUILTIN_OBJS += builtin/interpret-trailers.o
 BUILTIN_OBJS += builtin/log.o
 BUILTIN_OBJS += builtin/ls-files.o
 BUILTIN_OBJS += builtin/ls-remote.o
diff --git a/builtin.h b/builtin.h
index d4afbfe..30f4c30 100644
--- a/builtin.h
+++ b/builtin.h
@@ -71,6 +71,7 @@ extern int cmd_hash_object(int argc, const char **argv, const char *prefix);
 extern int cmd_help(int argc, const char **argv, const char *prefix);
 extern int cmd_index_pack(int argc, const char **argv, const char *prefix);
 extern int cmd_init_db(int argc, const char **argv, const char *prefix);
+extern int cmd_interpret_trailers(int argc, const char **argv, const char *prefix);
 extern int cmd_log(int argc, const char **argv, const char *prefix);
 extern int cmd_log_reflog(int argc, const char **argv, const char *prefix);
 extern int cmd_ls_files(int argc, const char **argv, const char *prefix);
diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
new file mode 100644
index 0000000..f79bffa
--- /dev/null
+++ b/builtin/interpret-trailers.c
@@ -0,0 +1,36 @@
+/*
+ * Builtin "git interpret-trailers"
+ *
+ * Copyright (c) 2013 Christian Couder <chriscool@tuxfamily.org>
+ *
+ */
+
+#include "cache.h"
+#include "builtin.h"
+#include "parse-options.h"
+#include "strbuf.h"
+#include "trailer.h"
+
+static const char * const git_interpret_trailers_usage[] = {
+	N_("git interpret-trailers [--trim-empty] [--infile=file] [<token[=value]>...]"),
+	NULL
+};
+
+int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
+{
+	const char *infile = NULL;
+	int trim_empty = 0;
+
+	struct option options[] = {
+		OPT_BOOL(0, "trim-empty", &trim_empty, N_("trim empty trailers")),
+		OPT_FILENAME(0, "infile", &infile, N_("use message from file")),
+		OPT_END()
+	};
+
+	argc = parse_options(argc, argv, prefix, options,
+			     git_interpret_trailers_usage, 0);
+
+	process_trailers(infile, trim_empty, argc, argv);
+
+	return 0;
+}
diff --git a/git.c b/git.c
index 3799514..1420b58 100644
--- a/git.c
+++ b/git.c
@@ -383,6 +383,7 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "index-pack", cmd_index_pack, RUN_SETUP_GENTLY },
 		{ "init", cmd_init_db },
 		{ "init-db", cmd_init_db },
+		{ "interpret-trailers", cmd_interpret_trailers, RUN_SETUP },
 		{ "log", cmd_log, RUN_SETUP },
 		{ "ls-files", cmd_ls_files, RUN_SETUP },
 		{ "ls-remote", cmd_ls_remote, RUN_SETUP_GENTLY },
diff --git a/trailer.h b/trailer.h
new file mode 100644
index 0000000..9db4459
--- /dev/null
+++ b/trailer.h
@@ -0,0 +1,6 @@
+#ifndef TRAILER_H
+#define TRAILER_H
+
+void process_trailers(const char *infile, int trim_empty, int argc, const char **argv);
+
+#endif /* TRAILER_H */

diff --git a/t/t7513-interpret-trailers.sh b/t/t7513-interpret-trailers.sh
new file mode 100755
index 0000000..8be333c
--- /dev/null
+++ b/t/t7513-interpret-trailers.sh
@@ -0,0 +1,208 @@
+#!/bin/sh
+#
+# Copyright (c) 2013 Christian Couder
+#
+
+test_description='git interpret-trailers'
+
+. ./test-lib.sh
+
+cat >basic_message <<'EOF'
+subject
+
+body
+EOF
+
+cat >complex_message_body <<'EOF'
+my subject
+
+my body which is long
+and contains some special
+chars like : = ? !
+
+EOF
+
+# We want one trailing space at the end of each line.
+# Let's use sed to make sure that these spaces are not removed
+# by any automatic tool.
+sed -e 's/ Z$/ /' >complex_message_trailers <<-\EOF
+Fixes: Z
+Acked-by: Z
+Reviewed-by: Z
+Signed-off-by: Z
+EOF
+
+test_expect_success 'without config' '
+	printf "ack: Peff\nReviewed-by: \nAcked-by: Johan\n" >expected &&
+	git interpret-trailers "ack = Peff" "Reviewed-by" "Acked-by: Johan" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success '--trim-empty without config' '
+	printf "ack: Peff\nAcked-by: Johan\n" >expected &&
+	git interpret-trailers --trim-empty "ack = Peff" "Reviewed-by" "Acked-by: Johan" "sob:" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with config setup' '
+	git config trailer.ack.key "Acked-by: " &&
+	printf "Acked-by: Peff\n" >expected &&
+	git interpret-trailers --trim-empty "ack = Peff" >actual &&
+	test_cmp expected actual &&
+	git interpret-trailers --trim-empty "Acked-by = Peff" >actual &&
+	test_cmp expected actual &&
+	git interpret-trailers --trim-empty "Acked-by :Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with config setup and = sign' '
+	git config trailer.ack.key "Acked-by= " &&
+	printf "Acked-by= Peff\n" >expected &&
+	git interpret-trailers --trim-empty "ack = Peff" >actual &&
+	test_cmp expected actual &&
+	git interpret-trailers --trim-empty "Acked-by= Peff" >actual &&
+	test_cmp expected actual &&
+	git interpret-trailers --trim-empty "Acked-by : Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with config setup and # sign' '
+	git config trailer.bug.key "Bug #" &&
+	printf "Bug #42\n" >expected &&
+	git interpret-trailers --trim-empty "bug = 42" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with commit basic message' '
+	git interpret-trailers --infile basic_message >actual &&
+	test_cmp basic_message actual
+'
+
+test_expect_success 'with commit complex message' '
+	cat complex_message_body complex_message_trailers >complex_message &&
+	cat complex_message_body >expected &&
+	printf "Fixes: \nAcked-by= \nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with commit complex message and args' '
+	cat complex_message_body >expected &&
+	printf "Fixes: \nAcked-by= \nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \nBug #42\n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "bug: 42" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with commit complex message, args and --trim-empty' '
+	cat complex_message_body >expected &&
+	printf "Acked-by= Peff\nBug #42\n" >>expected &&
+	git interpret-trailers --trim-empty --infile complex_message "ack: Peff" "bug: 42" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "where = before"' '
+	git config trailer.bug.where "before" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "bug: 42" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "where = before" for a token in the middle of infile' '
+	git config trailer.review.key "Reviewed-by:" &&
+	git config trailer.review.where "before" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nReviewed-by: Johan\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "bug: 42" "review: Johan" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "where = before" and --trim-empty' '
+	cat complex_message_body >expected &&
+	printf "Bug #46\nBug #42\nAcked-by= Peff\nReviewed-by: Johan\n" >>expected &&
+	git interpret-trailers --infile complex_message --trim-empty "ack: Peff" "bug: 42" "review: Johan" "Bug: 46"  >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'the default is "ifExist = addIfDifferent"' '
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "review:" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = addIfDifferent"' '
+	git config trailer.review.ifExist "addIfDifferent" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "review:" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = addIfDifferentNeighbor"' '
+	git config trailer.ack.ifExist "addIfDifferentNeighbor" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "review:" "ack: Junio" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = addIfDifferentNeighbor" and --trim-empty' '
+	git config trailer.ack.ifExist "addIfDifferentNeighbor" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nAcked-by= Peff\nAcked-by= Junio\nAcked-by= Peff\n" >>expected &&
+	git interpret-trailers --infile complex_message --trim-empty "ack: Peff" "Acked-by= Peff" "review:" "ack: Junio" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = add"' '
+	git config trailer.ack.ifExist "add" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Peff\nAcked-by= Peff\nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "ack: Peff" "Acked-by= Peff" "review:" "ack: Junio" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = overwrite"' '
+	git config trailer.fix.key "Fixes:" &&
+	git config trailer.fix.ifExist "overwrite" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: 22\nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "review:" "fix=53" "ack: Junio" "fix=22" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifExist = doNothing"' '
+	git config trailer.fix.ifExist "doNothing" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "review:" "fix=53" "ack: Junio" "fix=22" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'the default is "ifMissing = add"' '
+	git config trailer.cc.key "Cc: " &&
+	git config trailer.Cc.where "before" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nCc: Linus\nFixes: \nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "review:" "fix=53" "cc=Linus" "ack: Junio" "fix=22" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifMissing = add"' '
+	git config trailer.Cc.ifMissing "add" &&
+	cat complex_message_body >expected &&
+	printf "Cc: Linus\nBug #42\nFixes: \nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "review:" "fix=53" "ack: Junio" "fix=22" "bug: 42" "cc=Linus" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'using "ifMissing = doNothing"' '
+	git config trailer.Cc.ifMissing "doNothing" &&
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers --infile complex_message "review:" "fix=53" "cc=Linus" "ack: Junio" "fix=22" "bug: 42" "ack: Peff" >actual &&
+	test_cmp expected actual
+'
+
+test_done

builtin/interpret-trailers.c, t/t7513-interpret-trailers.sh, trailer.c

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index f79bffa..37237f7 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -23,7 +23,7 @@ int cmd_interpret_trailers(int argc, const char **argv, const char *prefix)
 
 	struct option options[] = {
 		OPT_BOOL(0, "trim-empty", &trim_empty, N_("trim empty trailers")),
-		OPT_FILENAME(0, "infile", &infile, N_("use message from file")),
+		OPT_FILENAME(0, "infile", &infile, N_("use message from file, instead of stdin")),
 		OPT_END()
 	};
 
diff --git a/t/t7513-interpret-trailers.sh b/t/t7513-interpret-trailers.sh
index 8be333c..f5ef81f 100755
--- a/t/t7513-interpret-trailers.sh
+++ b/t/t7513-interpret-trailers.sh
@@ -205,4 +205,11 @@ test_expect_success 'using "ifMissing = doNothing"' '
 	test_cmp expected actual
 '
 
+test_expect_success 'with input from stdin' '
+	cat complex_message_body >expected &&
+	printf "Bug #42\nFixes: \nAcked-by= \nAcked-by= Junio\nAcked-by= Peff\nReviewed-by: \nSigned-off-by: \n" >>expected &&
+	git interpret-trailers "review:" "fix=53" "cc=Linus" "ack: Junio" "fix=22" "bug: 42" "ack: Peff" < complex_message >actual &&
+	test_cmp expected actual
+'
+
 test_done
diff --git a/trailer.c b/trailer.c
index 2678b3e..d581371 100644
--- a/trailer.c
+++ b/trailer.c
@@ -465,8 +465,13 @@ static struct strbuf **read_input_file(const char *infile)
 {
 	struct strbuf sb = STRBUF_INIT;
 
-	if (strbuf_read_file(&sb, infile, 0) < 0)
-		die_errno(_("could not read input file '%s'"), infile);
+	if (infile) {
+		if (strbuf_read_file(&sb, infile, 0) < 0)
+			die_errno(_("could not read input file '%s'"), infile);
+	} else {
+		if (strbuf_read(&sb, fileno(stdin), 0) < 0)
+			die_errno(_("could not read from stdin"));
+	}
 
 	return strbuf_split(&sb, '\n');
 }
@@ -531,10 +536,8 @@ void process_trailers(const char *infile, int trim_empty, int argc, const char *
 
 	git_config(git_trailer_config, NULL);
 
-	/* Print the non trailer part of infile */
-	if (infile) {
-		process_input_file(infile, &infile_tok_first, &infile_tok_last);
-	}
+	/* Print the non trailer part of infile (or stdin if infile is NULL) */
+	process_input_file(infile, &infile_tok_first, &infile_tok_last);
 
 	arg_tok_first = process_command_line_args(argc, argv);
 

diff --git a/trailer.c b/trailer.c
index d581371..6eec3ce 100644
--- a/trailer.c
+++ b/trailer.c
@@ -400,11 +400,27 @@ static void parse_trailer(struct strbuf *tok, struct strbuf *val, const char *tr
 	}
 }
 
+static struct trailer_item *new_trailer_item(struct trailer_item *conf_item,
+					     const char* tok, const char* val)
+{
+	struct trailer_item *new = xcalloc(sizeof(struct trailer_item), 1);
+	new->value = val;
+
+	if (conf_item) {
+		new->conf = conf_item->conf;
+		new->token = xstrdup(conf_item->conf->key);
+	} else {
+		new->conf = xcalloc(sizeof(struct conf_info), 1);
+		new->token = tok;
+	}
+
+	return new;
+}
+
 static struct trailer_item *create_trailer_item(const char *string)
 {
 	struct strbuf tok = STRBUF_INIT;
 	struct strbuf val = STRBUF_INIT;
-	struct trailer_item *new;
 	struct trailer_item *item;
 	int tok_alnum_len;
 
@@ -416,21 +432,12 @@ static struct trailer_item *create_trailer_item(const char *string)
 	for (item = first_conf_item; item; item = item->next) {
 		if (!strncasecmp(tok.buf, item->conf->key, tok_alnum_len) ||
 		    !strncasecmp(tok.buf, item->conf->name, tok_alnum_len)) {
-			new = xcalloc(sizeof(struct trailer_item), 1);
-			new->conf = item->conf;
-			new->token = xstrdup(item->conf->key);
-			new->value = strbuf_detach(&val, NULL);
 			strbuf_release(&tok);
-			return new;
+			return new_trailer_item(item, NULL, strbuf_detach(&val, NULL));
 		}
 	}
 
-	new = xcalloc(sizeof(struct trailer_item), 1);
-	new->conf = xcalloc(sizeof(struct conf_info), 1);
-	new->token = strbuf_detach(&tok, NULL);
-	new->value = strbuf_detach(&val, NULL);
-
-	return new;
+	return new_trailer_item(NULL, strbuf_detach(&tok, NULL), strbuf_detach(&val, NULL));;
 }
 
 static void add_trailer_item(struct trailer_item **first,

strbuf.c, strbuf.h

diff --git a/strbuf.c b/strbuf.c
index 2124bb8..e45e513 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -197,6 +197,13 @@ void strbuf_splice(struct strbuf *sb, size_t pos, size_t len,
 	strbuf_setlen(sb, sb->len + dlen - len);
 }
 
+void strbuf_replace(struct strbuf *sb, const char *a, const char *b)
+{
+	char *ptr = strstr(sb->buf, a);
+	if (ptr)
+		strbuf_splice(sb, ptr - sb->buf, strlen(a), b, strlen(b));
+}
+
 void strbuf_insert(struct strbuf *sb, size_t pos, const void *data, size_t len)
 {
 	strbuf_splice(sb, pos, 0, data, len);
diff --git a/strbuf.h b/strbuf.h
index 02bff3a..38faf70 100644
--- a/strbuf.h
+++ b/strbuf.h
@@ -111,6 +111,9 @@ extern void strbuf_remove(struct strbuf *, size_t pos, size_t len);
 extern void strbuf_splice(struct strbuf *, size_t pos, size_t len,
                           const void *, size_t);
 
+/* first occurence of a replaced with b */
+extern void strbuf_replace(struct strbuf *, const char *a, const char *b);
+
 extern void strbuf_add_commented_lines(struct strbuf *out, const char *buf, size_t size);
 
 extern void strbuf_add(struct strbuf *, const void *, size_t);

diff --git a/trailer.c b/trailer.c
index 6eec3ce..dc81a01 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "run-command.h"
 /*
  * Copyright (c) 2013 Christian Couder <chriscool@tuxfamily.org>
  */
@@ -12,11 +13,14 @@ struct conf_info {
 	char *name;
 	char *key;
 	char *command;
+	unsigned command_uses_arg : 1;
 	enum action_where where;
 	enum action_if_exist if_exist;
 	enum action_if_missing if_missing;
 };
 
+#define TRAILER_ARG_STRING "$ARG"
+
 struct trailer_item {
 	struct trailer_item *previous;
 	struct trailer_item *next;
@@ -368,6 +372,7 @@ static int git_trailer_config(const char *conf_key, const char *value, void *cb)
 			if (conf->command)
 				warning(_("more than one %s"), orig_conf_key);
 			conf->command = xstrdup(value);
+			conf->command_uses_arg = !!strstr(conf->command, TRAILER_ARG_STRING);
 		} else if (type == TRAILER_WHERE) {
 			if (set_where(conf, value))
 				warning(_("unknown value '%s' for key '%s'"), value, orig_conf_key);
@@ -400,6 +405,45 @@ static void parse_trailer(struct strbuf *tok, struct strbuf *val, const char *tr
 	}
 }
 
+static int read_from_command(struct child_process *cp, struct strbuf *buf)
+{
+	if (run_command(cp))
+		return error("running trailer command '%s' failed", cp->argv[0]);
+	if (strbuf_read(buf, cp->out, 1024) < 1)
+		return error("reading from trailer command '%s' failed", cp->argv[0]);
+	strbuf_trim(buf);
+	return 0;
+}
+
+static const char *apply_command(const char *command, const char *arg)
+{
+	struct strbuf cmd = STRBUF_INIT;
+	struct strbuf buf = STRBUF_INIT;
+	struct child_process cp;
+	const char *argv[] = {NULL, NULL};
+	const char *result = "";
+
+	strbuf_addstr(&cmd, command);
+	if (arg)
+		strbuf_replace(&cmd, TRAILER_ARG_STRING, arg);
+
+	argv[0] = cmd.buf;
+	memset(&cp, 0, sizeof(cp));
+	cp.argv = argv;
+	cp.env = local_repo_env;
+	cp.no_stdin = 1;
+	cp.out = -1;
+	cp.use_shell = 1;
+
+	if (read_from_command(&cp, &buf))
+		strbuf_release(&buf);
+	else
+		result = strbuf_detach(&buf, NULL);
+
+	strbuf_release(&cmd);
+	return result;
+}
+
 static struct trailer_item *new_trailer_item(struct trailer_item *conf_item,
 					     const char* tok, const char* val)
 {
@@ -409,6 +453,8 @@ static struct trailer_item *new_trailer_item(struct trailer_item *conf_item,
 	if (conf_item) {
 		new->conf = conf_item->conf;
 		new->token = xstrdup(conf_item->conf->key);
+		if (conf_item->conf->command_uses_arg || !val)
+			new->value = apply_command(conf_item->conf->command, val);
 	} else {
 		new->conf = xcalloc(sizeof(struct conf_info), 1);
 		new->token = tok;
@@ -459,12 +505,22 @@ static struct trailer_item *process_command_line_args(int argc, const char **arg
 	int i;
 	struct trailer_item *arg_tok_first = NULL;
 	struct trailer_item *arg_tok_last = NULL;
+	struct trailer_item *item;
 
 	for (i = 0; i < argc; i++) {
 		struct trailer_item *new = create_trailer_item(argv[i]);
 		add_trailer_item(&arg_tok_first, &arg_tok_last, new);
 	}
 
+	/* Add conf commands that don't use $ARG */
+	for (item = first_conf_item; item; item = item->next) {
+		if (item->conf->command && !item->conf->command_uses_arg)
+		{
+			struct trailer_item *new = new_trailer_item(item, NULL, NULL);
+			add_trailer_item(&arg_tok_first, &arg_tok_last, new);
+		}
+	}
+
 	return arg_tok_first;
 }
 

diff --git a/t/t7513-interpret-trailers.sh b/t/t7513-interpret-trailers.sh
index f5ef81f..2d50b7a 100755
--- a/t/t7513-interpret-trailers.sh
+++ b/t/t7513-interpret-trailers.sh
@@ -212,4 +212,31 @@ test_expect_success 'with input from stdin' '
 	test_cmp expected actual
 '
 
+test_expect_success 'with simple command' '
+	git config trailer.sign.key "Signed-off-by: " &&
+	git config trailer.sign.where "after" &&
+	git config trailer.sign.ifExist "addIfDifferentNeighbor" &&
+	git config trailer.sign.command "echo \"A U Thor <author@example.com>\"" &&
+	cat complex_message_body >expected &&
+	printf "Fixes: \nAcked-by= \nReviewed-by: \nSigned-off-by: \nSigned-off-by: A U Thor <author@example.com>\n" >>expected &&
+	git interpret-trailers "review:" "fix=22" < complex_message >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'setup a commit' '
+	echo "Content of the first commit." > a.txt &&
+	git add a.txt &&
+	git commit -m "Add file a.txt"
+'
+
+test_expect_success 'with command using $ARG' '
+	git config trailer.fix.ifExist "overwrite" &&
+	git config trailer.fix.command "git log -1 --oneline --format=\"%h (%s)\" --abbrev-commit --abbrev=14 \$ARG" &&
+	FIXED=$(git log -1 --oneline --format="%h (%s)" --abbrev-commit --abbrev=14 HEAD) &&
+	cat complex_message_body >expected &&
+	printf "Fixes: $FIXED\nAcked-by= \nReviewed-by: \nSigned-off-by: \nSigned-off-by: A U Thor <author@example.com>\n" >>expected &&
+	git interpret-trailers "review:" "fix=HEAD" < complex_message >actual &&
+	test_cmp expected actual
+'
+
 test_done

diff --git a/trailer.c b/trailer.c
index dc81a01..6c2a2b9 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "run-command.h"
+#include "argv-array.h"
 /*
  * Copyright (c) 2013 Christian Couder <chriscool@tuxfamily.org>
  */
@@ -415,14 +416,40 @@ static int read_from_command(struct child_process *cp, struct strbuf *buf)
 	return 0;
 }
 
+static void setup_ac_env(struct argv_array *env, const char *ac_name, const char *ac_mail, const char *(*read)(int))
+{
+	if (!getenv(ac_name) || !getenv(ac_mail)) {
+		struct ident_split ident;
+		const char *namebuf, *mailbuf;
+		int namelen, maillen;
+		const char *ac_info = read(IDENT_NO_DATE);
+
+		if (split_ident_line(&ident, ac_info, strlen(ac_info)))
+			return;
+
+		namelen = ident.name_end - ident.name_begin;
+		namebuf = ident.name_begin;
+
+		maillen = ident.mail_end - ident.mail_begin;
+		mailbuf = ident.mail_begin;
+
+		argv_array_pushf(env, "%s=%.*s", ac_name, namelen, namebuf);
+		argv_array_pushf(env, "%s=%.*s", ac_mail, maillen, mailbuf);
+	}
+}
+
 static const char *apply_command(const char *command, const char *arg)
 {
+	struct argv_array env = ARGV_ARRAY_INIT;
 	struct strbuf cmd = STRBUF_INIT;
 	struct strbuf buf = STRBUF_INIT;
 	struct child_process cp;
 	const char *argv[] = {NULL, NULL};
 	const char *result = "";
 
+	setup_ac_env(&env, "GIT_AUTHOR_NAME", "GIT_AUTHOR_EMAIL", git_author_info);
+	setup_ac_env(&env, "GIT_COMMITTER_NAME", "GIT_COMMITTER_EMAIL", git_committer_info);
+
 	strbuf_addstr(&cmd, command);
 	if (arg)
 		strbuf_replace(&cmd, TRAILER_ARG_STRING, arg);
@@ -430,7 +457,7 @@ static const char *apply_command(const char *command, const char *arg)
 	argv[0] = cmd.buf;
 	memset(&cp, 0, sizeof(cp));
 	cp.argv = argv;
-	cp.env = local_repo_env;
+	cp.env = env.argv;
 	cp.no_stdin = 1;
 	cp.out = -1;
 	cp.use_shell = 1;
@@ -441,6 +468,7 @@ static const char *apply_command(const char *command, const char *arg)
 		result = strbuf_detach(&buf, NULL);
 
 	strbuf_release(&cmd);
+	argv_array_clear(&env);
 	return result;
 }
 

diff --git a/t/t7513-interpret-trailers.sh b/t/t7513-interpret-trailers.sh
index 2d50b7a..00894a8 100755
--- a/t/t7513-interpret-trailers.sh
+++ b/t/t7513-interpret-trailers.sh
@@ -223,6 +223,26 @@ test_expect_success 'with simple command' '
 	test_cmp expected actual
 '
 
+test_expect_success 'with command using commiter information' '
+	git config trailer.sign.ifExist "addIfDifferent" &&
+	git config trailer.sign.command "echo \"\$GIT_COMMITTER_NAME <\$GIT_COMMITTER_EMAIL>\"" &&
+	cat complex_message_body >expected &&
+	printf "Fixes: \nAcked-by= \nReviewed-by: \nSigned-off-by: \nSigned-off-by: C O Mitter <committer@example.com>\n" >>expected &&
+	git interpret-trailers "review:" "fix=22" < complex_message >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'with command using author information' '
+	git config trailer.sign.key "Signed-off-by: " &&
+	git config trailer.sign.where "after" &&
+	git config trailer.sign.ifExist "addIfDifferentNeighbor" &&
+	git config trailer.sign.command "echo \"\$GIT_AUTHOR_NAME <\$GIT_AUTHOR_EMAIL>\"" &&
+	cat complex_message_body >expected &&
+	printf "Fixes: \nAcked-by= \nReviewed-by: \nSigned-off-by: \nSigned-off-by: A U Thor <author@example.com>\n" >>expected &&
+	git interpret-trailers "review:" "fix=22" < complex_message >actual &&
+	test_cmp expected actual
+'
+
 test_expect_success 'setup a commit' '
 	echo "Content of the first commit." > a.txt &&
 	git add a.txt &&

diff --git a/Documentation/git-interpret-trailers.txt b/Documentation/git-interpret-trailers.txt
new file mode 100644
index 0000000..f74843e
--- /dev/null
+++ b/Documentation/git-interpret-trailers.txt
@@ -0,0 +1,137 @@
+git-interpret-trailers(1)
+=========================
+
+NAME
+----
+git-interpret-trailers - help add stuctured information into commit messages
+
+SYNOPSIS
+--------
+[verse]
+'git interpret-trailers' [--trim-empty] [--infile=file] [<token[=value]>...]
+
+DESCRIPTION
+-----------
+Help add RFC 822 like headers, called 'trailers', at the end of the
+otherwise free-form part of a commit message.
+
+Unless `--infile=file` is used, this command is a filter. It reads the
+standard input for a commit message and apply the `token` arguments,
+if any, to this message. The resulting message is emited on the
+standard output.
+
+Some configuration variables control the way the `token` arguments are
+applied to the message and the way any existing trailer in the message
+is changed. They also make it possible to automatically add some
+trailers.
+
+By default, a 'token=value' or 'token:value' argument will be added
+only if no trailer with the same (token, value) pair is already in the
+message. The 'token' and 'value' parts will be trimmed to remove
+starting and trailing white spaces, and the resulting trimmed 'token'
+and 'value' will appear in the message like this:
+
+------------------------------------------------
+token: value
+------------------------------------------------
+
+By default, if there are already trailers with the same 'token' the
+trailer will appear just after the last trailer with the same
+'token'. Otherwise it will appear at the end of the message.
+
+Note that 'trailers' do not follow and are not intended to follow many
+rules that are in RFC 822. For example they do not follow the line
+breaking rules, the encoding rules and probably many other rules.
+
+Trailers have become a de facto standard way to add helpful structured
+information into commit messages. For example the well known
+"Signed-off-by: " trailer is used by many projects like the Linux
+kernel and Git.
+
+OPTIONS
+-------
+--trim-empty::
+	If the 'value' part of any trailer contains onlywhite spaces,
+	the whole trailer will be removed from the resulting message.
+
+----infile=file::
+	Read the commit message from `file` instead of the standard
+	input.
+
+CONFIGURATION VARIABLES
+-----------------------
+
+trailer.<token>.key::
+	This 'key' will be used instead of 'token' in the
+	trailer. After some alphanumeric characters, it can contain
+	some non alphanumeric characters like ':', '=' or '#' that will
+	be used instead of ':' to separate the token from the value in
+	the trailer, though the default ':' is more standard.
+
+trailer.<token>.where::
+	This can be either `after`, which is the default, or
+	`before`. If it is `before`, then a trailer with the specified
+	token, will appear before, instead of after, other trailers
+	with the same token, or otherwise at the beginning, instead of
+	at the end, of all the trailers.
+
+trailer.<token>.ifexist::
+	This option makes it possible to chose what action will be
+	performed when there is already at least one trailer with the
+	same token in the message.
++
+The valid values for this option are: `addIfDifferent` (this is the
+default), `addIfDifferentNeighbor`, `add`, `overwrite` or `doNothing`.
++
+With `addIfDifferent`, a new trailer will be added only if no trailer
+with the same (token, value) pair is already in the message.
++
+With `addIfDifferentNeighbor`, a new trailer will be added only if no
+trailer with the same (token, value) pair is above or below the line
+where the new trailer will be added.
++
+With `add`, a new trailer will be added, even if some trailers with
+the same (token, value) pair are already in the message.
++
+With `overwrite`, the new trailer will overwrite an existing trailer
+with the same token.
++
+With `doNothing`, nothing will be done, that is no new trailer will be
+added if there is already one with the same token in the message.
+
+trailer.<token>.ifmissing::
+	This option makes it possible to chose what action will be
+	performed when there is not yet any trailer with the same
+	token in the message.
++
+The valid values for this option are: `add` (this is the default) and
+`doNothing`.
++
+With `add`, a new trailer will be added.
++
+With `doNothing`, nothing will be done.
+
+trailer.<token>.command::
+	This option can be used to specify a shell command that will
+	be used to automatically add or modify a trailer with the
+	specified 'token'.
++
+When this option is specified, it is like if a special 'token=value'
+argument is added at the end of the command line, where 'value' will
+be given by the standard output of the specified command.
++
+If the command contains the `$ARG` string, this string will be
+replaced with the 'value' part of an existing trailer with the same
+token, if any, before the command is launched.
++
+The following environment variables are set when the command is run:
+GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_COMMITTER_NAME,
+GIT_COMMITTER_EMAIL.
+
+SEE ALSO
+--------
+linkgit:git-commit[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite