diff --git a/src/emc/motion/command.c b/src/emc/motion/command.c
index b8094e03e9b..0ba29b8c4fd 100644
--- a/src/emc/motion/command.c
+++ b/src/emc/motion/command.c
@@ -664,8 +664,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	       Called  by task using [KINS]JOINTS= which is typically
 	       the same value as the motmod num_joints= parameter
 	    */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_NUM_JOINTS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->joint);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_NUM_JOINTS %d", emcmotCommand->joint);
 	    if (( emcmotCommand->joint <= 0 ) ||
 		( emcmotCommand->joint > EMCMOT_MAX_JOINTS )) {
 		break;
@@ -679,9 +678,8 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	       the number of spindles configured for the motion module
 	       (motion_num_spindles)
 	    */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_NUM_SPINDLES");
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_NUM_SPINDLES %d", emcmotCommand->spindle);
 	    emcmot_config_change();
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->spindle);
 	    if (   emcmotCommand->spindle > motion_num_spindles
 	        || emcmotCommand->spindle <= 0
 	        || emcmotCommand->spindle > EMCMOT_MAX_SPINDLES
@@ -706,8 +704,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    break;
 
 	case EMCMOT_SET_JOINT_HOMING_PARAMS:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_HOMING_PARAMS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_HOMING_PARAMS %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0) {
 		break;
@@ -727,8 +724,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    break;
 
 	case EMCMOT_UPDATE_JOINT_HOMING_PARAMS:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "UPDATE_JOINT_HOMING_PARAMS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "UPDATE_JOINT_HOMING_PARAMS %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0) {
 		break;
@@ -744,8 +740,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    /* this command can be issued with joint < 0 to re-enable
 	       limits, but they are automatically re-enabled at the
 	       end of the next jog */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "OVERRIDE_LIMITS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "OVERRIDE_LIMITS %d", joint_num);
 	    if (joint_num < 0) {
 		/* don't override limits */
 		rtapi_print_msg(RTAPI_MSG_DBG, "override off");
@@ -775,8 +770,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    break;
 
 	case EMCMOT_SET_JOINT_MOTOR_OFFSET:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MOTOR_OFFSET");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MOTOR_OFFSET %d", joint_num);
 	    if(joint == 0) {
 		break;
 	    }
@@ -786,8 +780,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_JOINT_POSITION_LIMITS:
 	    /* set the position limits for the joint */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_POSITION_LIMITS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_POSITION_LIMITS %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0) {
 		break;
@@ -799,8 +792,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_JOINT_BACKLASH:
 	    /* set the backlash for the joint */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_BACKLASH");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_BACKLASH %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0) {
 		break;
@@ -815,8 +807,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	       minFerror then OK else if ferror < limiting ferror then OK
 	       else ERROR */
 	case EMCMOT_SET_JOINT_MAX_FERROR:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MAX_FERROR");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MAX_FERROR %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0 || emcmotCommand->maxFerror < 0.0) {
 		break;
@@ -825,8 +816,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    break;
 
 	case EMCMOT_SET_JOINT_MIN_FERROR:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MIN_FERROR");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_MIN_FERROR %d", joint_num);
 	    emcmot_config_change();
 	    if (joint == 0 || emcmotCommand->minFerror < 0.0) {
 		break;
@@ -838,8 +828,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    /* do a continuous jog, implemented as an incremental jog to the
 	       limit.  When the user lets go of the button an abort will
 	       stop the jog. */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_CONT");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_CONT %d", joint_num);
 	    if (!GET_MOTION_ENABLE_FLAG()) {
 		reportError(_("Can't jog joint when not enabled."));
 		SET_JOINT_ERROR_FLAG(joint, 1);
@@ -907,8 +896,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 
 	case EMCMOT_JOG_INCR:
 	    /* do an incremental jog */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_INCR");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_INCR %d", joint_num);
 	    if (!GET_MOTION_ENABLE_FLAG()) {
 		reportError(_("Can't jog joint when not enabled."));
 		SET_JOINT_ERROR_FLAG(joint, 1);
@@ -984,8 +972,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 
 	case EMCMOT_JOG_ABS:
 	    /* do an absolute jog */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_ABS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOG_ABS %d", joint_num);
 	    if (joint == 0) {
 		break;
 	    }
@@ -1208,8 +1195,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_JOINT_VEL_LIMIT:
 	    /* set joint max velocity */
 	    /* can do it at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_VEL_LIMIT");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_VEL_LIMIT %d", joint_num);
 	    emcmot_config_change();
 	    /* check joint range */
 	    if (joint == 0) {
@@ -1221,8 +1207,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_JOINT_ACC_LIMIT:
 	    /* set joint max acceleration */
 	    /* can do it at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_ACC_LIMIT");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_ACC_LIMIT %d", joint_num);
 	    emcmot_config_change();
 	    /* check joint range */
 	    if (joint == 0) {
@@ -1234,8 +1219,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_JOINT_JERK_LIMIT:
 		/* set joint max jerk */
 		/* can do it at any time */
-		rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_JERK_LIMIT");
-		rtapi_print_msg(RTAPI_MSG_DBG, " j(%d) jerk(%f)", joint_num, emcmotCommand->jerk);
+		rtapi_print_msg(RTAPI_MSG_DBG, "SET_JOINT_JERK_LIMIT j(%d) jerk(%f)", joint_num, emcmotCommand->jerk);
 		emcmot_config_change();
 		if (joint == 0) {
 		break;
@@ -1483,8 +1467,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    /* make joint active, so that amps will be enabled when system is
 	       enabled or disabled */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_ACTIVATE");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_ACTIVATE %d", joint_num);
 	    if (joint == 0) {
 		break;
 	    }
@@ -1495,8 +1478,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    /* make joint inactive, so that amps won't be affected when system
 	       is enabled or disabled */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_DEACTIVATE");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_DEACTIVATE %d", joint_num);
 	    if (joint == 0) {
 		break;
 	    }
@@ -1508,8 +1490,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    /* need to be in free mode, enable on */
 	    /* this just sets the initial state, then the state machine in
 	       homing.c does the rest */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_HOME");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_HOME %d", joint_num);
 
 	    /* Normally homing requires free (joint) mode. The queued G28.2 path
 	     * (emctaskmain EMC_TASK_EXEC::WAITING_FOR_HOMING) dips motion into
@@ -1550,8 +1531,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 
 	case EMCMOT_JOINT_UNHOME:
             /* unhome the specified joint, or all joints if -1, or volatile joints if -2 */
-            rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_UNHOME");
-            rtapi_print_msg(RTAPI_MSG_DBG, " %d", joint_num);
+            rtapi_print_msg(RTAPI_MSG_DBG, "JOINT_UNHOME %d", joint_num);
 
             if (   (emcmotStatus->motion_state != EMCMOT_MOTION_FREE)
                 && (emcmotStatus->motion_state != EMCMOT_MOTION_DISABLED)) {
@@ -2043,8 +2023,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	case EMCMOT_SET_AXIS_POSITION_LIMITS:
 	    /* set the position limits for axis */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_POSITION_LIMITS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->axis);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_POSITION_LIMITS %d", emcmotCommand->axis);
 	    emcmot_config_change();
             if ((emcmotCommand->axis < 0) || (emcmotCommand->axis >= EMCMOT_MAX_AXIS)) {
                 break;
@@ -2056,8 +2035,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
         case EMCMOT_SET_AXIS_VEL_LIMIT:
 	    /* set the max axis vel */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_VEL_LIMITS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->axis);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_VEL_LIMITS %d", emcmotCommand->axis);
 	    emcmot_config_change();
             if ((emcmotCommand->axis < 0) || (emcmotCommand->axis >= EMCMOT_MAX_AXIS)) {
                 break;
@@ -2069,8 +2047,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
         case EMCMOT_SET_AXIS_ACC_LIMIT:
  	    /* set the max axis acc */
 	    /* can be done at any time */
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_ACC_LIMITS");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->axis);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_ACC_LIMITS %d", emcmotCommand->axis);
 	    emcmot_config_change();
             if ((emcmotCommand->axis < 0) || (emcmotCommand->axis >= EMCMOT_MAX_AXIS)) {
                 break;
@@ -2082,8 +2059,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 		case EMCMOT_SET_AXIS_JERK_LIMIT:
 			/* set the max axis jerk */
 			/* can be done at any time */
-			rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_JERK_LIMITS");
-			rtapi_print_msg(RTAPI_MSG_DBG, " %d jerk(%f)", emcmotCommand->axis, emcmotCommand->jerk);
+			rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_JERK_LIMITS %d jerk(%f)", emcmotCommand->axis, emcmotCommand->jerk);
 			emcmot_config_change();
 			if ((emcmotCommand->axis < 0) || (emcmotCommand->axis >= EMCMOT_MAX_AXIS)) {
 			break;
@@ -2092,8 +2068,7 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 			break;
 
         case EMCMOT_SET_AXIS_LOCKING_JOINT:
-	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_ACC_LOCKING_JOINT");
-	    rtapi_print_msg(RTAPI_MSG_DBG, " %d", emcmotCommand->axis);
+	    rtapi_print_msg(RTAPI_MSG_DBG, "SET_AXIS_ACC_LOCKING_JOINT %d", emcmotCommand->axis);
 	    emcmot_config_change();
             if ((emcmotCommand->axis < 0) || (emcmotCommand->axis >= EMCMOT_MAX_AXIS)) {
                 break;
@@ -2139,7 +2114,6 @@ void emcmotCommandHandler_locked(void *arg, long servo_period)
 	    rtapi_print_msg(RTAPI_MSG_DBG, "ERROR: %d",
 		emcmotStatus->commandStatus);
 	}
-	rtapi_print_msg(RTAPI_MSG_DBG, "\n");
 	/* synch tail count */
 	rtapi_seqlock_write_end(&emcmotStatus->tail, &emcmotStatus->head);
 	rtapi_seqlock_write_end(&emcmotConfig->tail, &emcmotConfig->head);
diff --git a/src/emc/motion/motion.c b/src/emc/motion/motion.c
index 9370b8803be..4a096507d64 100644
--- a/src/emc/motion/motion.c
+++ b/src/emc/motion/motion.c
@@ -224,6 +224,9 @@ void reportError(const char *fmt, ...)
         char fmt_tmp[EMCMOT_ERROR_LEN];
         size_t fmt_len = strlen(fmt);
         //Manually check string length and truncate if it is to long
+        if(fmt_len > 0 && fmt[fmt_len-1] == '\n'){
+            fmt_len--;
+        }
         if(fmt_len < EMCMOT_ERROR_LEN-1){
             memcpy(fmt_tmp, fmt, fmt_len);
             fmt_tmp[fmt_len+0] = '\n';
diff --git a/src/emc/task/emctaskmain.cc b/src/emc/task/emctaskmain.cc
index 72afc74b7bb..049c640f1af 100644
--- a/src/emc/task/emctaskmain.cc
+++ b/src/emc/task/emctaskmain.cc
@@ -222,6 +222,11 @@ static int emcOperatorErrorV(bool echo, const char *fmt, va_list ap)
     // force a NULL at the end for safety
     error_msg.error[LINELEN - 1] = 0;
 
+    // an error is one line: the GUI and the echo below end it
+    size_t n = strlen(error_msg.error);
+    while (n > 0 && error_msg.error[n - 1] == '\n')
+	error_msg.error[--n] = 0;
+
     // write it
     if (echo) {
 	log_info("{}\n", error_msg.error);
diff --git a/src/hal/drivers/hal_ppmc.c b/src/hal/drivers/hal_ppmc.c
index 83e65900f2e..9e8a553198b 100644
--- a/src/hal/drivers/hal_ppmc.c
+++ b/src/hal/drivers/hal_ppmc.c
@@ -566,8 +566,7 @@ int rtapi_app_main(void)
 	    slot = &(bus->slot_data[slotnum]);
 	    /* rv1 is used to flag errors that fail one bus */
 	    rv1 = 0;
-	    rtapi_print_msg(RTAPI_MSG_INFO, "PPMC: slot %d: ", slotnum);
-	
+
 	    /* check slot */
 	    idcode = SelRead(slot->slot_base+SLOT_ID_OFFSET, slot->port_addr);
 	    if ((idcode == 0)||(idcode == 0xFF)||((idcode&0x0f) == 0x0f)) {
@@ -575,13 +574,13 @@ int rtapi_app_main(void)
 	    //   being left on the bus by a non-implemented device slot
 		slot->id = 0;
 		slot->ver = 0;
-		rtapi_print_msg(RTAPI_MSG_INFO, "nothing detected at addr %x reads %x\n",
-				slotnum,idcode);
+		rtapi_print_msg(RTAPI_MSG_INFO, "PPMC: slot %d: nothing detected at addr %x reads %x\n",
+				slotnum,slotnum,idcode);
 		ClrTimeout(slot->port_addr);
 		/* skip to next slot */
 		continue;
 	    }
-	    rtapi_print_msg(RTAPI_MSG_INFO, "ID code: %02X ", idcode);
+	    rtapi_print_msg(RTAPI_MSG_INFO, "PPMC: slot %d: ID code: %02X\n", slotnum, idcode);
 	    slot->id = id = idcode & 0xF0;
 	    slot->ver = ver = idcode & 0x0F;
 	    slot->use_timestamp = 0; /* default is no timestamp */
diff --git a/src/hal/drivers/mesa-hostmot2/hm2_modbus.c b/src/hal/drivers/mesa-hostmot2/hm2_modbus.c
index 01b982cc5e9..a562a3818a2 100644
--- a/src/hal/drivers/mesa-hostmot2/hm2_modbus.c
+++ b/src/hal/drivers/mesa-hostmot2/hm2_modbus.c
@@ -151,6 +151,20 @@ static inline unsigned mtypesize(unsigned mtype) {
 #define MSG_WARN(fmt...)	do { rtapi_print_msg(RTAPI_MSG_WARN, fmt); } while(0)
 #define MSG_PRINT(fmt...)	do { rtapi_print(fmt); } while(0)
 
+#ifdef DEBUG
+// Debug dump of a frame, one message for every 16 bytes
+static void dump_frame(const char *what, unsigned cmdidx, const rtapi_u8 *data, unsigned len)
+{
+	for(unsigned i = 0; i < len; i += 16) {
+		char hex[16 * 3 + 1] = "";
+		unsigned n = 0;
+		for(unsigned j = i; j < len && j < i + 16; j++)
+			n += rtapi_snprintf(hex + n, sizeof(hex) - n, " %02x", data[j]);
+		MSG_DBG("%s cmd=%u [%u]:%s\n", what, cmdidx, i, hex);
+	}
+}
+#endif
+
 // State-machine states
 enum {
 	STATE_START = 0,
@@ -495,9 +509,8 @@ static int send_modbus_pkt(hm2_modbus_inst_t *inst)
 	cc->data[cc->datalen++] = (checksum >> 8) & 0xff;
 
 #ifdef DEBUG
-	MSG_DBG("Sending cmd=%u to '%s' %i bytes", inst->cmdidx, inst->uart, cc->datalen);
-	for(int i = 0; i < cc->datalen; i++) MSG_DBG(" 0x%02x", cc->data[i]);
-	MSG_DBG("\n");
+	MSG_DBG("Sending cmd=%u to '%s' %i bytes\n", inst->cmdidx, inst->uart, cc->datalen);
+	dump_frame("Sending", inst->cmdidx, cc->data, cc->datalen);
 #endif
 
 	rtapi_u8  frames = 1;
@@ -1719,13 +1732,13 @@ static int parse_data_frame(hm2_modbus_inst_t *inst)
 		return -1;
 	}
 
-	MSG_DBG("Return PDU cmd=%u is", inst->cmdidx);
 	for(unsigned i = 0; i < rxcount; i++) {
 		bytes[i] = (data[w] >> b) & 0xFF;
-		MSG_DBG(" %02x", bytes[i]);
 		if((b += 8) >= 32) { b = 0; w++; }
 	}
-	MSG_DBG("\n");
+#ifdef DEBUG
+	dump_frame("Return PDU", inst->cmdidx, bytes, rxcount);
+#endif
 
 	if(bytes[0] != cc->cmd.mbid) {
 		MSG_ERR("%s: error: Modbus device address mismatch: got 0x%02x, expected 0x%02x\n", inst->name, bytes[0], cc->cmd.mbid);
diff --git a/src/hal/utils/halcmd.c b/src/hal/utils/halcmd.c
index 542cb19b127..694801c8d0a 100644
--- a/src/hal/utils/halcmd.c
+++ b/src/hal/utils/halcmd.c
@@ -212,14 +212,17 @@ pid_t hal_systemv_nowait(const char *const argv[]) {
     if ( pid == 0 ) {
 	/* child process */
 	/* print debugging info if "very verbose" (-V) */
+	char cmdline[1024] = "";
         for(n=0; argv[n] != NULL; n++) {
-	    rtapi_print_msg(RTAPI_MSG_DBG, "%s ", argv[n] );
+	    if (n)
+		strncat(cmdline, " ", sizeof(cmdline) - strlen(cmdline) - 1);
+	    strncat(cmdline, argv[n], sizeof(cmdline) - strlen(cmdline) - 1);
 	}
         if (n == 0) {
             halcmd_error("hal_systemv_nowait: empty argv array passed in\n");
             exit(1);
         }
-	rtapi_print_msg(RTAPI_MSG_DBG, "\n" );
+	rtapi_print_msg(RTAPI_MSG_DBG, "%s\n", cmdline);
         /* call execv() to invoke command */
 	execvp(argv[0], (char * const *)argv);
 	/* should never get here */
diff --git a/src/rtapi/rtai_rtapi.c b/src/rtapi/rtai_rtapi.c
index 3eb1bae6101..b019fda81f0 100644
--- a/src/rtapi/rtai_rtapi.c
+++ b/src/rtapi/rtai_rtapi.c
@@ -433,10 +433,19 @@ int rtapi_snprintf(char *buf, unsigned long int size, const char *fmt, ...)
 
 #define BUFFERLEN 1024
 
+/* Every rtapi_print_msg() message is one line: end one that lacks the
+   newline. rtapi_print() (RTAPI_MSG_ALL) prints like printf. */
 void default_rtapi_msg_handler(msg_level_t level, const char *fmt, va_list ap) {
     char buf[BUFFERLEN];
-    rtapi_vsnprintf(buf, BUFFERLEN, fmt, ap);
-    rt_printk(buf);
+    size_t n;
+    /* leave room for the newline */
+    rtapi_vsnprintf(buf, sizeof(buf) - 1, fmt, ap);
+    n = strlen(buf);
+    if (level != RTAPI_MSG_ALL && n > 0 && buf[n - 1] != '\n') {
+        buf[n] = '\n';
+        buf[n + 1] = '\0';
+    }
+    rt_printk("%s", buf);
 }
 static rtapi_msg_handler_t rtapi_msg_handler = default_rtapi_msg_handler;
 
diff --git a/src/rtapi/rtai_ulapi.c b/src/rtapi/rtai_ulapi.c
index cc73ead56b2..47f2eccbc5f 100644
--- a/src/rtapi/rtai_ulapi.c
+++ b/src/rtapi/rtai_ulapi.c
@@ -269,9 +269,18 @@ void rtapi_print_msg(msg_level_t level, const char *fmt, ...)
     va_list args;
 
     if ((level <= msg_level) && (msg_level != RTAPI_MSG_NONE)) {
+	/* one message is one line: end one that lacks the newline */
+	char *msg;
+	int n;
 	va_start(args, fmt);
-	vfprintf(stderr, fmt, args);
+	n = vasprintf(&msg, fmt, args);
 	va_end(args);
+	if (n < 0)
+	    return;
+	fputs(msg, stderr);
+	if (level != RTAPI_MSG_ALL && n > 0 && msg[n - 1] != '\n')
+	    fputc('\n', stderr);
+	free(msg);
     }
 }
 
diff --git a/src/rtapi/uspace_common.h b/src/rtapi/uspace_common.h
index 4032e7eb2fa..c3f22399b00 100644
--- a/src/rtapi/uspace_common.h
+++ b/src/rtapi/uspace_common.h
@@ -230,6 +230,15 @@ int rtapi_shmem_delete(int handle, int module_id)
 
 void default_rtapi_msg_handler(msg_level_t level, const char *fmt, va_list ap);
 
+/* Every rtapi_print_msg() message is one line, and the default handlers
+   end one that lacks the newline. rtapi_print() (RTAPI_MSG_ALL) prints
+   like printf. */
+static inline int rtapi_msg_needs_newline(msg_level_t level, const char *msg)
+{
+    size_t n = strlen(msg);
+    return level != RTAPI_MSG_ALL && n > 0 && msg[n - 1] != '\n';
+}
+
 static rtapi_msg_handler_t rtapi_msg_handler = default_rtapi_msg_handler;
 
 rtapi_msg_handler_t rtapi_get_msg_handler(void) {
diff --git a/src/rtapi/uspace_rtapi_app.cc b/src/rtapi/uspace_rtapi_app.cc
index 2b4bcb0f9a6..93e3e2014b8 100644
--- a/src/rtapi/uspace_rtapi_app.cc
+++ b/src/rtapi/uspace_rtapi_app.cc
@@ -22,6 +22,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <string>
 
 std::atomic_int WithRoot::level;
 uid_t WithRoot::ruid, WithRoot::euid;
@@ -243,13 +244,13 @@ int RtapiApp::find_rt_cpu_number() {
     parse_cpu_list(isolated_file, &cpuset);
 
     //Print list
-    rtapi_print_msg(RTAPI_MSG_INFO, "cpuset isolated ");
+    std::string list;
     for (int i = 0; i < CPU_SETSIZE; i++) {
         if (CPU_ISSET(i, &cpuset)) {
-            rtapi_print_msg(RTAPI_MSG_INFO, "%i ", i);
+            list += " " + std::to_string(i);
         }
     }
-    rtapi_print_msg(RTAPI_MSG_INFO, "\n");
+    rtapi_print_msg(RTAPI_MSG_INFO, "cpuset isolated%s", list.c_str());
 
     int top = -1;
     for (int i = 0; i < CPU_SETSIZE; i++) {
diff --git a/src/rtapi/uspace_rtapi_main.cc b/src/rtapi/uspace_rtapi_main.cc
index 9946f3d16d1..337bce75d90 100644
--- a/src/rtapi/uspace_rtapi_main.cc
+++ b/src/rtapi/uspace_rtapi_main.cc
@@ -1708,9 +1708,23 @@ void default_rtapi_msg_handler(msg_level_t level, const char *fmt, va_list ap) {
         message_t m;
         m.level = level;
         vsnprintf(m.msg, sizeof(m.msg), fmt, ap);
+        if (rtapi_msg_needs_newline(level, m.msg)) {
+            size_t n = strlen(m.msg);
+            if (n == sizeof(m.msg) - 1)
+                n--;	/* truncated: the newline replaces the last character */
+            m.msg[n] = '\n';
+            m.msg[n + 1] = '\0';
+        }
         rtapi_msg_queue.push(m);
     } else {
-        vfprintf(level == RTAPI_MSG_ALL ? stdout : stderr, fmt, ap);
+        FILE *f = level == RTAPI_MSG_ALL ? stdout : stderr;
+        char *msg;
+        if (vasprintf(&msg, fmt, ap) < 0)
+            return;
+        fputs(msg, f);
+        if (rtapi_msg_needs_newline(level, msg))
+            fputc('\n', f);
+        free(msg);
     }
 }
 
diff --git a/src/rtapi/uspace_ulapi.c b/src/rtapi/uspace_ulapi.c
index 0bec4b70257..65509783d3b 100644
--- a/src/rtapi/uspace_ulapi.c
+++ b/src/rtapi/uspace_ulapi.c
@@ -75,11 +75,13 @@ void rtapi_delay(long ns) {
 }
 
 void default_rtapi_msg_handler(msg_level_t level, const char *fmt, va_list ap) {
-    if(level == RTAPI_MSG_ALL) {
-	vfprintf(stdout, fmt, ap);
-        fflush(stdout);
-    } else {
-	vfprintf(stderr, fmt, ap);
-        fflush(stderr);
-    }
+    FILE *f = level == RTAPI_MSG_ALL ? stdout : stderr;
+    char *msg;
+    if (vasprintf(&msg, fmt, ap) < 0)
+        return;
+    fputs(msg, f);
+    if (rtapi_msg_needs_newline(level, msg))
+        fputc('\n', f);
+    fflush(f);
+    free(msg);
 }
diff --git a/tests/rtapi-msg-newline/.gitignore b/tests/rtapi-msg-newline/.gitignore
new file mode 100644
index 00000000000..284bd9713dd
--- /dev/null
+++ b/tests/rtapi-msg-newline/.gitignore
@@ -0,0 +1 @@
+halrun.out
diff --git a/tests/rtapi-msg-newline/README b/tests/rtapi-msg-newline/README
new file mode 100644
index 00000000000..d79375e4caf
--- /dev/null
+++ b/tests/rtapi-msg-newline/README
@@ -0,0 +1,10 @@
+Every rtapi_print_msg() message is a line of its own, also when its
+text lacks the newline (#4689).
+
+msgline prints two errors without a newline from its setup, which runs
+in the main thread of rtapi_app, and an error, a warning and an info
+message without a newline from its function, whose messages rtapi_app
+queues and prints from another thread. Each must be a line of its own
+in the output, and an error that ends in a newline must not get a
+second one. rtapi_print() prints like printf: a line printed with it
+in two parts must stay one line.
diff --git a/tests/rtapi-msg-newline/expected b/tests/rtapi-msg-newline/expected
new file mode 100644
index 00000000000..fd4e5f72093
--- /dev/null
+++ b/tests/rtapi-msg-newline/expected
@@ -0,0 +1,7 @@
+msgline: error at load
+msgline: error at load
+msgline: error in the thread
+msgline: warning in the thread
+msgline: info in the thread
+msgline: error with its newline
+msgline: print in parts
diff --git a/tests/rtapi-msg-newline/halrun.err b/tests/rtapi-msg-newline/halrun.err
new file mode 100644
index 00000000000..ac064432a8d
--- /dev/null
+++ b/tests/rtapi-msg-newline/halrun.err
@@ -0,0 +1,14 @@
+Note: realtime scheduling unavailable (sched_setscheduler SCHED_FIFO: Operation not permitted).
+  Process capabilities: cap_sys_nice=no cap_ipc_lock=no.
+  Falling back to POSIX non-realtime.
+  Fix: 'sudo make setcap' (preferred) or 'sudo make setuid' on rtapi_app.
+  Override (testing only): set LINUXCNC_FORCE_REALTIME=1.
+Note: Using POSIX non-realtime
+No isolated CPU's found, expect some latency or set RTAPI_CPU_NUMBER to select CPU
+msgline: error at load
+msgline: error at load
+msgline: error in the thread
+msgline: warning in the thread
+msgline: info in the thread
+msgline: error with its newline
+THREADS: hal_exit 7 0 0
diff --git a/tests/rtapi-msg-newline/msgline.comp b/tests/rtapi-msg-newline/msgline.comp
new file mode 100644
index 00000000000..976bf82dd60
--- /dev/null
+++ b/tests/rtapi-msg-newline/msgline.comp
@@ -0,0 +1,31 @@
+component msgline "Test component for tests/rtapi-msg-newline: prints messages that lack a newline";
+
+pin io bool print "set to print the messages from the realtime thread once; cleared by the function";
+
+function _;
+option extra_setup;
+option period no;
+license "GPL";
+author "Luca Toniolo";
+;;
+EXTRA_SETUP() {
+    (void)__comp_inst;
+    (void)prefix;
+    (void)extra_arg;
+    rtapi_print_msg(RTAPI_MSG_ERR, "msgline: error at load");
+    rtapi_print_msg(RTAPI_MSG_ERR, "msgline: error at load");
+    return 0;
+}
+
+FUNCTION(_) {
+    if (!print)
+        return;
+    print_set(0);
+    rtapi_print_msg(RTAPI_MSG_ERR, "msgline: error in the thread");
+    rtapi_print_msg(RTAPI_MSG_WARN, "msgline: warning in the thread");
+    rtapi_print_msg(RTAPI_MSG_INFO, "msgline: info in the thread");
+    rtapi_print_msg(RTAPI_MSG_ERR, "msgline: error with its newline\n");
+    /* rtapi_print() prints like printf */
+    rtapi_print("msgline: print ");
+    rtapi_print("in parts\n");
+}
diff --git a/tests/rtapi-msg-newline/msgline.hal b/tests/rtapi-msg-newline/msgline.hal
new file mode 100644
index 00000000000..b985977d858
--- /dev/null
+++ b/tests/rtapi-msg-newline/msgline.hal
@@ -0,0 +1,6 @@
+loadrt threads name1=t period1=1000000
+loadrt msgline
+addf msgline.0 t
+debug 3
+start
+loadusr -w ./print.py
diff --git a/tests/rtapi-msg-newline/print.py b/tests/rtapi-msg-newline/print.py
new file mode 100755
index 00000000000..3cd8457a458
--- /dev/null
+++ b/tests/rtapi-msg-newline/print.py
@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+# Make msgline print from the realtime thread and wait until it has.
+# A wait that takes more than 10 s of wall time means the machine is
+# dying; the test then aborts.
+
+import hal
+import sys
+import time
+
+hal.set_p("msgline.0.print", "1")
+deadline = time.monotonic() + 10
+while hal.get_p("msgline.0.print"):
+    if time.monotonic() > deadline:
+        sys.exit("FAIL: the thread did not run msgline.0 in 10 s")
+    time.sleep(0.01)
diff --git a/tests/rtapi-msg-newline/skip b/tests/rtapi-msg-newline/skip
new file mode 100755
index 00000000000..14f65ebbd51
--- /dev/null
+++ b/tests/rtapi-msg-newline/skip
@@ -0,0 +1,4 @@
+#!/bin/sh
+# Builds a realtime component with halcompile, which needs the build
+# tree.  Skip when testing installed packages.
+[ -z "$SYSTEM_BUILD" ]
diff --git a/tests/rtapi-msg-newline/test.sh b/tests/rtapi-msg-newline/test.sh
new file mode 100755
index 00000000000..ba8d80bd391
--- /dev/null
+++ b/tests/rtapi-msg-newline/test.sh
@@ -0,0 +1,7 @@
+#!/bin/bash
+set -e
+halcompile --install msgline.comp >/dev/null
+timeout -k 5 60 halrun -f msgline.hal > halrun.out 2> halrun.err
+# the messages of msgline, each on a line of its own, then rtapi_print()
+grep '^msgline:' halrun.err
+grep '^msgline:' halrun.out