mosquitto_sub now only needs `-t` or `-U` to run

This means that `-t` is not required in all situations.

ChangeLog.txt

@@ -60,6 +60,8 @@ Clients:
 - Add `-w`/`--watch` to mosquitto_sub which means messages will be printed on a
   fixed line number based on the topic and order in which messages were
   received. Requires ANSI escape code support in the terminal.
+- mosquitto_sub now only needs `-t` or `-U` to run - this means that `-t` is
+  not required in all situations.
 
 
 2.0.10 - 2021-04-03

client/client_shared.c

@@ -472,8 +472,8 @@ int client_config_load(struct mosq_config *cfg, int pub_or_sub, int argc, char *
 	}
 
 	if(pub_or_sub == CLIENT_SUB){
-		if(cfg->topic_count == 0){
-			fprintf(stderr, "Error: You must specify a topic to subscribe to.\n");
+		if(cfg->topic_count == 0 && cfg->unsub_topic_count == 0){
+			fprintf(stderr, "Error: You must specify a topic to subscribe to (-t) or unsubscribe from (-U).\n");
 			return 1;
 		}
 	}

client/sub_client.c

@@ -192,9 +192,9 @@ static void print_usage(void)
 	mosquitto_lib_version(&major, &minor, &revision);
 	printf("mosquitto_sub is a simple mqtt client that will subscribe to a set of topics and print all messages it receives.\n");
 	printf("mosquitto_sub version %s running on libmosquitto %d.%d.%d.\n\n", VERSION, major, minor, revision);
-	printf("Usage: mosquitto_sub {[-h host] [--unix path] [-p port] [-u username] [-P password] -t topic | -L URL [-t topic]}\n");
+	printf("Usage: mosquitto_sub {[-h host] [--unix path] [-p port] [-u username] [-P password] {-t topic | -U topic} | -L URL [-t topic]}\n");
 	printf("                     [-c] [-k keepalive] [-q qos] [-x session-expiry-interval]\n");
-	printf("                     [-C msg_count] [-E] [-R] [--retained-only] [--remove-retained] [-T filter_out] [-U topic ...]\n");
+	printf("                     [-C msg_count] [-E] [-R] [--retained-only] [--remove-retained] [-T filter_out]\n");
 	printf("                     [-F format]\n");
 #ifndef WIN32
 	printf("                     [-W timeout_secs]\n");

man/mosquitto_sub.1.xml

@@ -24,7 +24,10 @@
 					<arg><option>-p</option> <replaceable>port-number</replaceable></arg>
 					<arg><option>-u</option> <replaceable>username</replaceable></arg>
 					<arg><option>-P</option> <replaceable>password</replaceable></arg>
-					<arg choice='plain' rep='repeat'><option>-t</option> <replaceable>message-topic</replaceable></arg>
+					<group choice='req'>
+						<arg choice='plain' rep='repeat'><option>-t</option> <replaceable>message-topic</replaceable></arg>
+						<arg choice='plain' rep='repeat'><option>-U</option> <replaceable>unsubscribe-topic</replaceable></arg>
+					</group>
 				</arg>
 				<arg choice='plain'>
 					<arg choice='plain'><option>-L</option> <replaceable>URL</replaceable></arg>
@@ -53,7 +56,6 @@
 			<arg><option>--retain-as-published</option></arg>
 			<arg><option>-S</option></arg>
 			<arg choice='opt' rep='repeat'><option>-T</option> <replaceable>filter-out</replaceable></arg>
-			<arg choice='opt' rep='repeat'><option>-U</option> <replaceable>unsub-topic</replaceable></arg>
 			<arg><option>-v</option></arg>
 			<arg><option>-V</option> <replaceable>protocol-version</replaceable></arg>
 			<arg><option>-W</option> <replaceable>message-processing-timeout</replaceable></arg>