diff --git a/commandline.h b/commandline.h
index 39764b4..0cf655c 100644
--- a/commandline.h
+++ b/commandline.h
@@ -19,13 +19,81 @@
 
 #include "adb.h"
 
+// Callback used to handle the standard streams (stdout and stderr) sent by the
+// device's upon receiving a command.
+//
+class StandardStreamsCallbackInterface {
+  public:
+    StandardStreamsCallbackInterface() {
+    }
+    // Handles the stdout output from devices supporting the Shell protocol.
+    virtual void OnStdout(const char* buffer, int length) = 0;
+
+    // Handles the stderr output from devices supporting the Shell protocol.
+    virtual void OnStderr(const char* buffer, int length) = 0;
+
+    // Indicates the communication is finished and returns the appropriate error
+    // code.
+    //
+    // |status| has the status code returning by the underlying communication
+    // channels
+    virtual int Done(int status) = 0;
+
+  protected:
+    static void OnStream(std::string* string, FILE* stream, const char* buffer, int length) {
+        if (string != nullptr) {
+            string->append(buffer, length);
+        } else {
+            fwrite(buffer, 1, length, stream);
+            fflush(stream);
+        }
+    }
+
+  private:
+    DISALLOW_COPY_AND_ASSIGN(StandardStreamsCallbackInterface);
+};
+
+// Default implementation that redirects the streams to the equilavent host
+// stream or to a string
+// passed to the constructor.
+class DefaultStandardStreamsCallback : public StandardStreamsCallbackInterface {
+  public:
+    // If |stdout_str| is non-null, OnStdout will append to it.
+    // If |stderr_str| is non-null, OnStderr will append to it.
+    DefaultStandardStreamsCallback(std::string* stdout_str, std::string* stderr_str)
+        : stdout_str_(stdout_str), stderr_str_(stderr_str) {
+    }
+
+    void OnStdout(const char* buffer, int length) {
+        OnStream(stdout_str_, stdout, buffer, length);
+    }
+
+    void OnStderr(const char* buffer, int length) {
+        OnStream(stderr_str_, stderr, buffer, length);
+    }
+
+    int Done(int status) {
+        return status;
+    }
+
+  private:
+    std::string* stdout_str_;
+    std::string* stderr_str_;
+
+    DISALLOW_COPY_AND_ASSIGN(DefaultStandardStreamsCallback);
+};
+
+// Singleton.
+extern DefaultStandardStreamsCallback DEFAULT_STANDARD_STREAMS_CALLBACK;
+
 int adb_commandline(int argc, const char** argv);
 int usage();
 
 // Connects to the device "shell" service with |command| and prints the
 // resulting output.
+// if |callback| is non-null, stdout/stderr output will be handled by it.
 int send_shell_command(TransportType transport_type, const char* serial, const std::string& command,
-                       bool disable_shell_protocol, std::string* output = nullptr,
-                       std::string* err = nullptr);
+                       bool disable_shell_protocol, StandardStreamsCallbackInterface* callback =
+                                                        &DEFAULT_STANDARD_STREAMS_CALLBACK);
 
 #endif  // COMMANDLINE_H