diff --git a/src/main/java/io/hyperfoil/tools/qdup/shell/AbstractShell.java b/src/main/java/io/hyperfoil/tools/qdup/shell/AbstractShell.java
index facf76ff..219f2ce4 100644
--- a/src/main/java/io/hyperfoil/tools/qdup/shell/AbstractShell.java
+++ b/src/main/java/io/hyperfoil/tools/qdup/shell/AbstractShell.java
@@ -600,22 +600,28 @@ public void addShObserver(String name, Consumer<String> consumer) {
         addShObserver(name,(output,promptName)->consumer.accept(output));
     }
     public void addShObserver(String name, BiConsumer<String,String> consumer) {
-        shObservers.put(name, consumer);
+        synchronized (AbstractShell.class) {
+            shObservers.put(name, consumer);
+        }
     }
     public void removeShObserver(String name) {
-        shObservers.remove(name);
+        synchronized (AbstractShell.class) {
+            shObservers.remove(name);
+        }
     }
 
     private void shObservers(String output,String promptName) {
-        shObservers.forEach((name,consumer)->{
-            int v = c.getAndIncrement();
-            logger.info("in: consumer.accept: {}x", v);
-            if (v == 15) {
-                logger.info("{}={}", name, output);
-            }
-            consumer.accept(output,promptName);
-            logger.info("out: consumer.accept: {}x", v);
-        });
+        synchronized (AbstractShell.class) {
+            shObservers.forEach((name, consumer) -> {
+                int v = c.getAndIncrement();
+                logger.info("in: consumer.accept: {}x", v);
+                if (v == 15) {
+                    logger.info("{}={}", name, output);
+                }
+                consumer.accept(output, promptName);
+                logger.info("out: consumer.accept: {}x", v);
+            });
+        }
     }
     public int permits() {
         return shellLock.availablePermits();