Installation

git clone https://github.com/jpmens/mqtt-watchdir.git
cd mqtt-watchdir
python3 -mvenv watchdir
source watchdir/bin/activate
pip install -e .

Configuration

Set the following optional environment variables before invoking mqtt-watchdir.py:

• MQTTHOST (default "localhost") is the name/address of the MQTT broker.

• MQTTPORT (default 1883) is the TCP port number of the broker.

• MQTTUSERNAME is the username to connect to the broker.

• MQTTPASSWORD is the password to connect to the broker.

• MQTTWATCHDIR (default: ".") is the path to the directory to watch.

• MQTTQOS (default: 0) is the MQTT Quality of Service (QoS) to use on publish. Allowed values are 0, 1, or 2.

• MQTTRETAIN (default: 0) specifies whether the “retain” flag should be set on publish. Set to 1 if you want messages to be retained.

• MQTTPREFIX (default: "watch") is the prefix to be prepended (with a slash) to the MQTT topic name. The topic name is formed from this prefix plus the path name of the file that is being modified (i.e. watched). You can set this to an empty string ("") to avoid prefixing the topic name.

• MQTTFILTER (default None) allows modifying payload (see below). Takes path to a Python file (e.g. "/path/to/example-filter.py".

• MQTTFIXEDTOPIC (default None) sets a MQTT topic to which all messages are published. If set, the MQTTPREFIX setting is overruled and ignored.

• Set WATCHDEBUG (default: 0) to 1 to show debugging information.

Testing

Launch mosquitto_sub:

mosquitto_sub -v -t 'watch/#'

Launch this program and, in another terminal, try something like this:

echo Hello World > message
echo JP > myname
rm myname

whereupon, on the first window, you should see:

watch/message Hello World
watch/myname JP
watch/myname (null)

Filters

Without a filter (the default), mqtt-watchdir reads the content of a newly created or modified file and uses that as the MQTT payload. By creating and enabling a so-called filter, mqtt-watchdir can pass the payload into said filter (a Python function) to have a payload translated.

Consider the included example-filter.py:

def mfilter(filename, topic, payload):
    '''Return a tuple [pub, newpayload] indicating whether this event
       should be published (True or False) and a new payload string
       or None'''

    print "Filter for topic %s" % topic

    if filename.endswith('.jpg'):
        return False, None

    if payload is not None:
        return True, payload.replace("\n", "-").replace(" ", "+")
    return True, None

The mfilter function is passed the fully qualified path to the file, the (possibly prefixed) MQTT topic name and the payload. In this simple example, spaces and newlines in the payload are replaced by dashes and plusses.

The function must return a tuple with two elements:

1. The first specifies whether the payload will be published (True) or not (False)

2. The second is a string with a possibly modified payload or None. If the returned payload is None, the original payload is not modified.

Possible uses of filters include

• Limiting payload lengths

• Conversion to JSON

• Ignore certain file types (e.g. binary data)

• Process content of files, say, YAML or JSON, and extract elements returning as string

Requirements

• watchdog, a Python library to monitor file-system events.

• Paho-MQTT’s Python module

Related utilities & Credits

• Roger Light (of Mosquitto fame) created mqttfs, a FUSE driver (in C) which works similarly.

• Roger Light (yes, the same busy gentleman) also made treewatch, a program to watch a set of directories and execute a program when there is a change in the files within the directories.

• Thanks to Karl Palsson for the setup.py and version.py magic.