MQTT Publisher

A multi-threaded MQTT publisher for Linux that reads metrics from an INI config file and publishes them to a local MQTT broker at specified intervals.

Supports three source types: files, shell commands, and remote MQTT broker subscriptions with automatic republishing.

• Author: (c) 2026, Chapvic
• License: GPLv3
• Version: v1.0

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Features

• Multi-threaded: each metric runs in its own thread with an independent publish interval.
• Three source types: read from files, execute shell commands, or subscribe to remote MQTT brokers.
• Key extraction: extract a value from source content using a literal key (key=T searches for T=<value>) or a POSIX ERE regex (key=/^pattern$/).
• Remote metrics: subscribe to remote broker topics via $[user:pass@]host:port/topic and republish to the local broker. Event-driven (no interval) or timer-republish (with interval).
• Shared remote connections: multiple metrics on the same host:port share a single mosquitto instance.
• Auto-reconnect: both local and remote brokers automatically reconnect with bounded delay (1–60 seconds).
• QoS support: per-metric QoS 0, 1, or 2 (default 0).
• Authentication: optional username/password for both local and remote brokers.
• Graceful shutdown: SIGINT / SIGTERM triggers a clean shutdown — all threads are joined, connections are closed, resources are freed.
• Config validation: duplicate sections, duplicate topics, missing required fields, invalid ports, buffer overflows — all caught at startup.
• Debug mode: verbose logging of every published value, connection events, and source read errors.

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Requirements

• GCC or any C compiler with C11 support
• pthread library (part of glibc on Linux)
• libmosquitto — the Mosquitto MQTT client library

Install on Debian / Ubuntu:

sudo apt install build-essential libmosquitto-dev

Install on RHEL / Fedora:

sudo dnf install gcc mosquitto-devel

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Building

make

Or manually:

gcc -O2 -Wall -o mqtt_pub mqtt_pub.c -lmosquitto -lpthread

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Installation

sudo make install

With parameters:

sudo make install HOST=192.168.1.10 PORT=1883 MQTT_USER=sensor MQTT_PASS=secret

Config handling during install

The install target follows this priority for the configuration file:

Priority	Condition	Behavior
1	mqtt_pub.ini exists in project dir	Copied to destination. HOST, PORT, MQTT_USER, MQTT_PASS overrides applied on top via sed.
2	No local config, no config at destination	Generated dynamically with default values and any provided overrides.
3	No local config, config exists at destination	Only override parameters (HOST, PORT, MQTT_USER, MQTT_PASS) are updated. Existing metrics are preserved.

Install parameters

Parameter	Default	Description
HOST	127.0.0.1	Broker hostname or IP (applied to config)
PORT	1883	Broker port (applied to config)
MQTT_USER	(empty)	Username for auth (applied to config)
MQTT_PASS	(empty)	Password for auth (applied to config)
CONF	/etc	Destination directory for config file

What install does

1. Stops the mqtt_pub service if running.
2. Installs binary to /usr/local/bin/mqtt_pub.
3. Installs systemd unit to /etc/systemd/system/mqtt_pub.service.
4. Handles config file (see priority table above).
5. Creates/updates env file at /etc/default/mqtt_pub with PARAMS=-f <config_path>.
6. Saves install.log with the CONF path for future updates.
7. Reloads systemd, enables and starts the service.

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Uninstallation

sudo make uninstall

Removes the binary, systemd unit, env file, and config file. The install.log is preserved.

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Usage

Usage: mqtt_pub [-f <config_file>] [-h]

Option	Description
-f <config_file>	Full path to an INI config file. Defaults to mqtt_pub.ini in the current directory.
-h	Print help and exit.

Typical invocation

./mqtt_pub -f /etc/mqtt_pub.ini

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Configuration File

See mqtt_pub.ini.example for a complete example, and docs/MQTT_Publisher.md for full documentation.

Quick reference

[MQTT]
host=localhost
port=1883
user=sensor
password=secret
debug=0

[cpu_temp]
topic=sensors/rpi/cpu_temp
interval=5
source=/sys/class/thermal/thermal_zone0/temp
key=-
qos=0

[remote_temp]
topic=sensors/weather/temp
source=$10.0.0.5:1883/sensors/weather/temp
key=-
qos=0

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Makefile targets

Target	Description
make	Build mqtt_pub
make install	Install binary, service, config, env file
make uninstall	Remove all installed files
make clean	Remove compiled binary
make distclean	Remove binary and install.log

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License

GPLv3 — see the source file header for details.