librespot supports the --onevent flag to pass the path to a script to be run on every event that is dispatched.

Information about the event such as it's name and possibly others are passed as environment variables from librespot:

#!/bin/bash

echo Spotify Event:
echo PLAYER_EVENT: $PLAYER_EVENT
echo TRACK_ID:     $TRACK_ID
echo OLD_TRACK_ID: $OLD_TRACK_ID

To use this, simply add the following to your config in /etc/raspotify/conf:

# script name passed to the --onevent flag, make sure raspotify has permissons call this script or you'll get an error
LIBRESPOT_ONEVENT="/usr/bin/yourscripthere.sh"

# you can even pass parameters as environment variables, you'll have access to those in your script
MY_SECRET_PASSWORD="muhahaha"

Reload raspotify with sudo systemctl restart raspotify.service and now your script will be called on an every event.

Available Events and Variables

See https://github.com/librespot-org/librespot/wiki/Events

Example: Turn on Sound System over MQTT

The following example turns on a Tasmota Smart Switch when a client connects to raspotify.

#!/bin/bash

# Sends 'ON' to the MQTT Topic of my Tasmota smart switch telling it to turn on my sound system
# depends on 'mosquitto-clients' for 'mosquitto_pub' command
#
# the following is set in '/etc/raspotify/conf' to run this script and pass the relevant variables:
#
# LIBRESPOT_ONEVENT="/usr/bin/soundsystem-on.sh" # copy script here so librespot can find it
# MQTT_BROKER="your broker ip/url here"
# MQTT_USER="your mqtt user here"
# MQTT_PASS="your password"
# MQTT_TOPIC="cmnd/<tasmota name>/Power"

# 'started' for librespot <=0.4.2; 'session_connected' for librespot > 0.4.2
if [ $PLAYER_EVENT = "started" ] || [ $PLAYER_EVENT = "session_connected" ]; then
  mosquitto_pub -h $MQTT_BROKER -u $MQTT_USER -P $MQTT_PASS -t $MQTT_TOPIC -m "ON"
  echo "Sent 'ON' to $MQTT_TOPIC to turn on sound system"
fi