Tasmota has a huge catalog of commands that can be run via the serial console, MQTT, or HTTP. These commands are great for scripting purposes. The easiest way I've found to send a command is with Curl.

curl http://192.168.1.142/cm?cmnd=POWER+TOGGLE

The API will answer in JSON which is easily digestible with JQ.

curl -s http://192.168.1.142/cm?cmnd=STATUS+8 | jq

I bought an Arduino Relay Shield to use on my Wemos D1 (ESP8266). After some poking around here is the mapping for which pins enable which relays:

To enable the relays on this shield you pull the appropriate pin HIGH. Other relay boards I've seen require you to pull the pin LOW, so don't get confused.

Here is the Tasmota template I used for the relay shield.

{"NAME":"Relay Shield","GPIO":[0,0,0,0,24,0,0,0,22,21,23,0,0],"FLAG":0,"BASE":18}

I've been using the Wemos D1 mini for several home automation projects. Unforunately the pin designations on the board don't match what Tasmota uses. I was able to track down the Wemos D1 mini pin mappings to create a cheat sheet.

Bolded pins can be used for any project without limitation, other pins can be used but may have some limitations.

I have two Tasmota enabled devices that I want to turn on/off at the same time. In my case I have a wall switch that powers the main lights, and a wall socket that powers some corner lights and a radio. Using Tasmota's rule system this is not too complicated. We need tell each Tasmota device that whenever the state of it's relay changes (for whatever reason: button, timer, etc.) it needs to tell the other Tasmota device to set it's relay to the matching state. We can use the WebSend feature built in to Tasmota to send commands between two devices.

On device #1

RULE1 on POWER1#State do WebSend [192.168.1.33] POWER1 %value% ENDON

On device #2

            RULE1 on POWER1#State do WebSend [192.168.1.34] POWER1 %value% ENDON

This can also be done using MQTT pretty simply, but I wanted to see if I could do it with vanilla Tasmota. Doing it direct with WebSend is pretty simple and saves having to run an MQTT broker if you just have two simple devices.