Simple MQTT handler in bash

How to subscribe to an MQTT topic efficiently in bash.

This is hosted on a Raspberry Pi which controls a pool pump via a solid state relay on GPIO 4, and these topics :

• /pool/pump/action 0  # Command, set externally (subscribed)
• /pool/pump/state  1  # Feedback state (published)

''

#!/bin/bash -feu

set -o pipefail

mqtt()

{

action="$1"

shift

"mosquitto_${action}" -h 192.168.1.1 -p 8833 -t "$@"

}

handler()

{

new_state=$(echo "$1" | sed 's/[^01]//g')

[[ -z "$new_state" ]] && new_state="0"

if /usr/local/bin/gpio_out.sh 4 "$new_state"; then

echo "pump state change OK, now $new_state"

else

echo "ERROR: pump state change fail towards $new_state"

fi

# Ping back

real_state=$(cat /sys/class/gpio/gpio4/value)

mqtt pub '/pool/pump/state' -m "$real_state" -r

}

export -f mqtt

export -f handler

action={1-}

cfg=$(basename $0 .sh).service

if [[ $action = 'stop' ]]; then

systemctl stop $cfg

elif [[ $action = 'start' ]]; then

systemctl restart $cfg

elif [[ $action = 'install' ]]; then

cat > /etc/systemd/system/$cfg << EOF

[Install]

WantedBy=multi-user.target

[Service]

Restart=always

RestartSec=3

ExecStart=$(realpath $0)

#Type=simple

#Restart=on-failure

EOF

systemctl daemon-reload &&

systemctl restart $cfg &&

systemctl enable $cfg

else # here is the deal

# Run only once

LOCK_FILE="/var/lock/$(basename $0 .sh).lock"

exec 9> "$LOCK_FILE" && flock -n 9 || exit 5

trap "rm -f '$LOCK_FILE'; logger 'Stopped $0 watcher on $(date)'" EXIT

logger "Started $0 watcher on $(date)"

# Wait for orders

mqtt sub '/pool/pump/action' |\

xargs -n 1 -I {} /bin/bash -c 'handler "$@"' _ {}

fi

''

This small script embeds its own ''systemd'' config, just run it with ''install' as the sole parameter.