diff --git a/include/info.h b/include/info.h
index d2964e6..306cbbb 100644
--- a/include/info.h
+++ b/include/info.h
@@ -1,4 +1,6 @@
 #pragma once
 
+void show_get_offset_usage(const char *prog_name, const char *cmd);
+void show_get_size_usage(const char *prog_name, const char *cmd);
 int do_get_offset(int argc, char *argv[]);
 int do_get_size(int argc, char *argv[]);
diff --git a/include/pack.h b/include/pack.h
index e8df87d..157002e 100644
--- a/include/pack.h
+++ b/include/pack.h
@@ -1,3 +1,4 @@
 #pragma once
 
+void show_pack_usage(const char *prog_name, const char *cmd);
 int do_pack(int argc, char *argv[]);
diff --git a/include/sign.h b/include/sign.h
index 25c2ccb..a0c08d8 100644
--- a/include/sign.h
+++ b/include/sign.h
@@ -1,3 +1,4 @@
 #pragma once
 
+void show_sign_usage(const char *prog_name, const char *cmd);
 int do_sign(int argc, char *argv[]);
diff --git a/include/unpack.h b/include/unpack.h
index b67750c..a7aac95 100644
--- a/include/unpack.h
+++ b/include/unpack.h
@@ -1,3 +1,4 @@
 #pragma once
 
+void show_unpack_usage(const char *prog_name, const char *cmd);
 int do_unpack(int argc, char *argv[]);
diff --git a/include/verify.h b/include/verify.h
index a59eb11..bc0730a 100644
--- a/include/verify.h
+++ b/include/verify.h
@@ -1,3 +1,4 @@
 #pragma once
 
+void show_verify_usage(const char *prog_name, const char *cmd);
 int do_verify(int argc, char *argv[]);
diff --git a/source/info.c b/source/info.c
index 3c73071..5da1793 100644
--- a/source/info.c
+++ b/source/info.c
@@ -8,6 +8,7 @@
 #include <image.h>
 #include <info.h>
 #include <macros.h>
+#include <option.h>
 #include <sign.h>
 
 enum {
@@ -19,6 +20,12 @@ struct args {
 	const char *image;
 };
 
+static struct opt_desc opt_descs[] = {
+	{ "-i", "--image=PATH", "the image in ROTS format" },
+	{ "-h", "--help", "display this help and exit" },
+	{ NULL, NULL, NULL },
+};
+
 static int parse_args(struct args *args, int argc, char *argv[])
 {
 	struct option options[] = {
@@ -40,6 +47,20 @@ static int parse_args(struct args *args, int argc, char *argv[])
 	return 0;
 }
 
+void show_get_offset_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"print the offset of the payload within an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
+void show_get_size_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"print the size of the payload within an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
 int do_get_offset(int argc, char *argv[])
 {
 	struct args args;
@@ -47,7 +68,7 @@ int do_get_offset(int argc, char *argv[])
 	FILE *fp;
 
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_get_offset_usage(argv[0], argv[1]);
 		return -1;
 	}
 
@@ -78,7 +99,7 @@ int do_get_size(int argc, char *argv[])
 	FILE *fp;
 
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_get_size_usage(argv[0], argv[1]);
 		return -1;
 	}
 
diff --git a/source/main.c b/source/main.c
index b44d6f7..cef6585 100644
--- a/source/main.c
+++ b/source/main.c
@@ -11,23 +11,24 @@
 struct entry {
 	const char *cmd;
 	int (* main)(int argc, char *argv[]);
+	void (* show_usage)(const char *prog_name, const char *cmd);
 	char *desc;
 };
 
 struct entry entries[] = {
-	{ "get-offset", do_get_offset,
+	{ "get-offset", do_get_offset, show_get_offset_usage,
 		"print the offset of the payload within an image" },
-	{ "get-size", do_get_size,
+	{ "get-size", do_get_size, show_get_size_usage,
 		"print the size of the payload within an image" },
-	{ "pack", do_pack,
+	{ "pack", do_pack, show_pack_usage,
 		"pack the payload within an image" },
-	{ "sign", do_sign,
+	{ "sign", do_sign, show_sign_usage,
 		"sign an image" },
-	{ "unpack", do_unpack,
+	{ "unpack", do_unpack, show_unpack_usage,
 		"unpack the payload of an image" },
-	{ "verify", do_verify,
+	{ "verify", do_verify, show_verify_usage,
 		"verify the signatures of an image" },
-	{ NULL, NULL, NULL },
+	{ NULL, NULL, NULL, NULL },
 };
 
 static void show_usage(const char *prog_name)
@@ -39,10 +40,28 @@ static void show_usage(const char *prog_name)
 		"Available commands:\n", prog_name);
 
 	for (entry = entries; entry->cmd; ++entry) {
-		fprintf(stderr, "\t%-16s %-80s\n", entry->cmd, entry->desc);
+		fprintf(stderr, "\t%-16s %s\n", entry->cmd, entry->desc);
 	}
 }
 
+static void show_cmd_usage(char *prog_name, char *cmd)
+{
+	struct entry *entry;
+
+	for (entry = entries; entry->cmd; ++entry) {
+		if (strcmp(entry->cmd, cmd) != 0)
+			continue;
+
+		if (!entry->show_usage)
+			break;
+
+		entry->show_usage(prog_name, cmd);
+		return;
+	}
+
+	show_usage(prog_name);
+}
+
 int main(int argc, char *argv[])
 {
 	struct entry *entry;
@@ -52,6 +71,15 @@ int main(int argc, char *argv[])
 		return -1;
 	}
 
+	if (strcmp(argv[1], "-h") == 0 || strcmp(argv[1], "--help") == 0) {
+		if (argc < 3)
+			show_usage(argv[0]);
+		else
+			show_cmd_usage(argv[0], argv[2]);
+
+		return -1;
+	}
+
 	for (entry = entries; entry->cmd; ++entry) {
 		if (strcmp(entry->cmd, argv[1]) == 0)
 			return entry->main(argc, argv);
diff --git a/source/pack.c b/source/pack.c
index c14466d..8071908 100644
--- a/source/pack.c
+++ b/source/pack.c
@@ -6,6 +6,7 @@
 #include <file.h>
 #include <image.h>
 #include <macros.h>
+#include <option.h>
 #include <pack.h>
 
 enum {
@@ -17,6 +18,12 @@ struct args {
 	const char *input, *output;
 };
 
+static struct opt_desc opt_descs[] = {
+	{ "-o", "--output", "the file to contain the image in ROTS format" },
+	{ "-h", "--help", "display this help and exit" },
+	{ NULL, NULL, NULL },
+};
+
 static int parse_args(struct args *args, int argc, char *argv[])
 {
 	struct option options[] = {
@@ -31,10 +38,13 @@ static int parse_args(struct args *args, int argc, char *argv[])
 		switch (ret) {
 		case OPTION_HELP: return -1;
 		case OPTION_OUTPUT: args->output = optarg; break;
-		default: break;
+		default: return -1;
 		}
 	}
 
+	if (!args->output)
+		return -1;
+
 	if (optind >= argc)
 		return -1;
 
@@ -48,6 +58,13 @@ static int parse_args(struct args *args, int argc, char *argv[])
 	return 0;
 }
 
+void show_pack_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"pack the payload within an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
 int do_pack(int argc, char *argv[])
 {
 	char data[4096];
@@ -57,7 +74,7 @@ int do_pack(int argc, char *argv[])
 	size_t nbytes, size;
 
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_pack_usage(argv[0], argv[1]);
 		return -1;
 	}
 
diff --git a/source/sign.c b/source/sign.c
index 55098e5..63e58b5 100644
--- a/source/sign.c
+++ b/source/sign.c
@@ -14,6 +14,7 @@
 
 #include <image.h>
 #include <macros.h>
+#include <option.h>
 #include <sign.h>
 #include <x509.h>
 
@@ -29,6 +30,16 @@ struct args {
 	const char *image, *digest, *key, *cert;
 };
 
+static struct opt_desc opt_descs[] = {
+	{ "-i", "--image=PATH","the image in ROTS format to sign" },
+	{ "-d", "--digest={ripemd160,sha224,sha256,sha384,sha512,whirlpool}\n",
+		"\t\tthe digest algorithm to use for signing" },
+	{ "-k", "--key=PATH", "the private key" },
+	{ "-c", "--cert=PATH", "the certificate containing the public key" },
+	{ "-h", "--help", "display this help and exit" },
+	{ NULL, NULL, NULL },
+};
+
 static int parse_args(struct args *args, int argc, char *argv[])
 {
 	struct option options[] = {
@@ -53,6 +64,9 @@ static int parse_args(struct args *args, int argc, char *argv[])
 		}
 	}
 
+	if (!args->image || !args->digest || !args->key || !args->cert)
+		return -1;
+
 	return 0;
 }
 
@@ -158,6 +172,13 @@ err_close_image:
 	return -1;
 }
 
+void show_sign_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"sign an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
 int do_sign(int argc, char *argv[])
 {
 	struct args args;
@@ -166,7 +187,7 @@ int do_sign(int argc, char *argv[])
 	char *cn;
 
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_sign_usage(argv[0], argv[1]);
 		return -1;
 	}
 
diff --git a/source/unpack.c b/source/unpack.c
index 9331161..bf7fbdc 100644
--- a/source/unpack.c
+++ b/source/unpack.c
@@ -6,6 +6,7 @@
 
 #include <image.h>
 #include <macros.h>
+#include <option.h>
 #include <unpack.h>
 
 enum {
@@ -18,6 +19,13 @@ struct args {
 	const char *image, *output;
 };
 
+static struct opt_desc opt_descs[] = {
+	{ "-i", "--image=PATH", "the image in ROTS format to unpack" },
+	{ "-o", "--output=PATH", "the file to contain the payload" },
+	{ "-h", "--help", "display this help and exit" },
+	{ NULL, NULL, NULL },
+};
+
 static int parse_args(struct args *args, int argc, char *argv[])
 {
 	struct option options[] = {
@@ -34,13 +42,23 @@ static int parse_args(struct args *args, int argc, char *argv[])
 		case OPTION_HELP: return -1;
 		case OPTION_IMAGE: args->image = optarg; break;
 		case OPTION_OUTPUT: args->output = optarg; break;
-		default: break;
+		default: return -1;
 		}
 	}
 
+	if (!args->image || !args->output)
+		return -1;
+
 	return 0;
 }
 
+void show_unpack_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"unpack the payload of an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
 int do_unpack(int argc, char *argv[])
 {
 	char data[512];
@@ -49,8 +67,10 @@ int do_unpack(int argc, char *argv[])
 	struct args args;
 	size_t nbytes, size;
 
+	memset(&args, 0, sizeof args);
+
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_unpack_usage(argv[0], argv[1]);
 		return -1;
 	}
 
diff --git a/source/verify.c b/source/verify.c
index 4d750df..939d0bd 100644
--- a/source/verify.c
+++ b/source/verify.c
@@ -15,6 +15,7 @@
 
 #include <image.h>
 #include <macros.h>
+#include <option.h>
 #include <verify.h>
 #include <x509.h>
 
@@ -28,6 +29,13 @@ struct args {
 	const char *image, *digest, *certs;
 };
 
+static struct opt_desc opt_descs[] = {
+	{ "-i", "--image=PATH", "the image in ROTS format to verify" },
+	{ "-c", "--certs=PATH", "the directory containing the certificates" },
+	{ "-h", "--help", "display this help and exit" },
+	{ NULL, NULL, NULL },
+};
+
 static int parse_args(struct args *args, int argc, char *argv[])
 {
 	struct option options[] = {
@@ -38,7 +46,7 @@ static int parse_args(struct args *args, int argc, char *argv[])
 	};
 	int ret;
 
-	while ((ret = getopt_long(argc, (char * const *)argv, "hi:d:c:", options,
+	while ((ret = getopt_long(argc, (char * const *)argv, "hi:c:", options,
 		NULL)) >= 0) {
 		switch (ret) {
 		case OPTION_HELP: return -1;
@@ -48,6 +56,9 @@ static int parse_args(struct args *args, int argc, char *argv[])
 		}
 	}
 
+	if (!args->image || !args->certs)
+		return -1;
+
 	return 0;
 }
 
@@ -220,13 +231,20 @@ err_close_image:
 	return -1;
 }
 
+void show_verify_usage(const char *prog_name, const char *cmd)
+{
+	fprintf(stderr, "usage: %s %s [option]...\n\n"
+		"verify the signature of an image\n\n", prog_name, cmd);
+	format_options(opt_descs);
+}
+
 int do_verify(int argc, char *argv[])
 {
 	struct args args;
 	size_t count, total;
 
 	if (parse_args(&args, argc, argv) < 0) {
-		fprintf(stderr, "invalid\n");
+		show_verify_usage(argv[0], argv[1]);
 		return -1;
 	}