update doc with CTRL_FILE

doc/src/SUMMARY.md

@@ -15,7 +15,8 @@
 - [Request Methods](./core_request_methods.md)
 - [Nats timeouts](./core_nats_timeouts.md)
 - [Startup folder](./core_startup_folder.md)
-- [{{ctrl_DATA}} variable](./core_messaging_ctrl_DATA.md)
+- [{{CTRL_DATA}} variable](./core_messaging_CTRL_DATA.md)
+- [{{CTRL_FILE}}](./core_messaging_CTRL_FILE.md)
 - [Errors](./core_errors.md)
 
 # Example standard messages

doc/src/core_messaging_CTRL_FILE.md

@@ -0,0 +1,23 @@
+# Message methodArgs variables
+
+## {{CTRL_FILE}}
+
+Read a local text file, and embed the content of the file into the methodArgs.
+
+```yaml
+---
+- toNodes:
+    - btdev1
+  #jetstreamToNode: btdev1
+  method: cliCommand
+  methodArgs:
+    - /bin/bash
+    - -c
+    - |
+      echo {{CTRL_FILE:/some_directory/source_file.yaml}}>/other_directory/destination_file.yaml
+  methodTimeout: 3
+  replyMethod: console
+  ACKTimeout: 0
+```
+
+The above example will before sending the message read the content of the file `/some_directory/source_file.yaml`. When the message is received at it's destination node and the cliCommand is executed the content will be written to `/other_directory/destination_file.yaml`.

doc/src/core_messaging_methodargs_variables.md

@@ -0,0 +1 @@
+# Message methodArgs variables