Node-RED Exec nodes explained
Node-RED by default comes with the Exec node. This node allows you to run a command as if you're on the command line. In this article we discuss how that can be useful and why exec is not included in FlowForge.
Node-RED is written in Javascript, as are the custom nodes in the
Flows Library. At times it would
be great to use programs written in other languages. In this guide we'll explain
the exec node.
# Exec Node
Node-RED by default comes with the exec node. This node allows you to run a
command as if you're on the command line. The exec node has one input, and three
outputs. Let's create a basic example and work from there. Let's connect the
inject to the exec input, and connect the three dots to debug nodes. I've set
the command in the exec node to date.
When triggered, you'll see two outputs in the debug pane. One from Standard out;
the date and timestamp of the execution and the Exit code will output too.
Standard error didn't output anything.
Let's explain what happened: the date command was executed by the NodeJS without
any input. The command returned the date as a string, and execution of the commmand
completed. There weren't any errors, so the error code is set to zero. Non-zero
exit codes imply didn't go as intended. Output is written to the standard output,
and not standard error. This allows us to handle errors differently from succesful
exections and the output in standard output.
# Exec vs Spawn mode
Let's change the exec node to execute ping google.com. The ping command
provides the round-trip time to google.com in milliseconds, each second. When
we deploy, the program is stuck. That's because the output is generated each
second and won't complete until there's a failure in the command. Not what you'd
want.
When there's continous output you'll need to select spawn mode in the settings
windown of the exec node:
This will produce a message per line to the standard out output for further processing.
# FlowForge
On FlowForge Cloud, the Node-RED exec nodes are disabled. The Stacks, that is the containers that Node-RED runs in, don't include any accessible executatables so there would be no benefit to running exec commands. All the data is exposed through Javascript and Node.JS already.