mqtt-launcher is a Python program which subscribes to a set of MQTT topics and executes processes on the host it's running on. Launchable processes are configured on a per/wildcard basis, and they can be constrained to run only if a particular text payload is contained in the message.

For example, I can publish a message to my MQTT broker requesting mqtt-launcher create a particular semaphore file for me:

mosquitto_pub -t sys/file -m create

The configuration file must be valid Python and it is loaded once. It contains the topic / process associations.

# topic         payload value           program & arguments
"sys/file"  :   {
                    'create'        :   [ '/usr/bin/touch', '/tmp/file.one' ],
                    'false'         :   [ '/bin/rm', '-f', '/tmp/file.one'    ],
                    'info'          :   [ '/bin/ls', '-l', '/tmp/file.one' ],
                },

Above snippet instructs mqtt-launcher to:

• subscribe to the MQTT topic sys/file
• look up the payload string and launch the associated programs:
  • if the payload is create, then touch a file
  • if the payload is the string false, remove a file
  • if the payload is info, return information on the file

The payload value may be None in which case the eacho of the list elements defining the program and arguments are checked for the magic string @!@ which is replaced by the payload contents. (See example published at dev/2 below.)

mqtt-launcher publishes stdout and stderr of the launched program to the configured topic with /report added to it. So, in the example above, a non-retained message will be published to sys/file/report. (Note that this message contains whatever the command outputs; trailing white space is truncated.)

Here's the obligatory "screenshot".

Publishes					Subscribes
-----------------------		------------------------------------------------------------------
						$ mosquitto_sub -v -t 'dev/#' -t 'sys/file/#' -t 'prog/#' 


mosquitto_pub -t prog/pwd -n
						prog/pwd (null)
						prog/pwd/report /private/tmp

mosquitto_pub -t sys/file -m create
						sys/file create
						sys/file/report (null)	# command has no output

mosquitto_pub -t sys/file -m info
						sys/file info
						sys/file/report -rw-r--r--  1 jpm  wheel  0 Jan 22 16:10 /tmp/file.one

mosquitto_pub -t sys/file -m remove
						sys/file remove
						# report not published: subcommand ('remove') doesn't exist
						# log file says:
						2014-01-22 16:11:30,393 No matching param (remove) for sys/file

mosquitto_pub -t dev/1 -m hi
						dev/1 hi
						dev/1/report total 16231
						drwxrwxr-x+ 157 root  admin     5338 Jan 20 10:48 Applications
						drwxrwxr-x@   8 root  admin      272 Jan 25  2013 Developer
						drwxr-xr-x+  72 root  wheel     2448 Oct 14 10:54 Library
						...
mosquitto_pub -t dev/2 -m 'Hi Jane!'
						dev/2 Hi Jane!
						dev/2/report 111 * Hi Jane! 222 Hi Jane! 333

mqtt-launcher loads a Python configuration from the path contained in the environment variable $MQTTLAUNCHERCONFIG; if unset, the path defaults to launcher.conf. See the provided launcher.conf.example.

mqtt-launcher logs its operation in the file configured as logfile.

• Python
• paho-mqtt

This program was inspired by two related tools:

• Peter van Dijk's mqtt-spawn
• Dennis Schulte's mqtt-exec. (I'm not terribly comfortable running NodeJS programs, so I implemented the idea in Python.)