How can I toggle the touchpad depending on whether a mouse is connected?

Okay, I've made an udev rule for it, and it is, like @terdon said, a much cleaner way

So, thanks to this guide, I created a "touchpad_toggle.rules" file in /etc/udev/rules.d/ (requires root access) and filled it with two lines:

SUBSYSTEM=="input", KERNEL=="mouse[0-9]*", ACTION=="add", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/username/.Xauthority", RUN+="/home/username/on.sh"
SUBSYSTEM=="input", KERNEL=="mouse[0-9]*", ACTION=="remove", ENV{DISPLAY}=":0", ENV{XAUTHORITY}="/home/username/.Xauthority", RUN+="/home/username/off.sh"

Don't forget to replace "username" with your username!

The contents of these on and off shell scripts are just castrated version of the script in my question. Example - off.sh:

#!/bin/bash

declare -i TID
TID=`xinput list | grep -Eo 'Touchpad\s*id\=[0-9]{1,2}' | grep -Eo '[0-9]{1,2}'`
xinput disable $TID

You'll have to use xinput enable $TID in the on.sh

And don't forget to add the script in my question (or the one @terdon suggested, but without a loop) to session autostart like he told you in his answer.

That is it, but I have to add one thing:

If you have a Synaptics touchpad (I have Elantech, so it's not suitable for me), you can replace your scripts (paths to which you write after RUN+=) with a simple command /usr/bin/synclient TouchpadOff=0 and 1 respectively

The basic script you need is simply:

#!/usr/bin/env bash

## Get the touchpad id. The -P means perl regular expressions (for \K)
## the -i makes it case insensitive (better portability) and the -o
## means print only the matched portion. The \K discards anything matched
## before it so this command will print the numeric id only.
TID=$(xinput list | grep -iPo 'touchpad.*id=\K\d+')

## Run every second
while :
do
   ## Disable the touchpad if there is a mouse connected
   ## and enable it if there is none.
    xinput list | grep -iq mouse &&  xinput disable "$TID" || xinput enable "$TID" 
    ## wait one second to avoind spamming your CPU
    sleep 1
done

The script above will toggle the trackpad depending on whether a mouse is connected. When launched, it will run for ever and will check for a mouse every second, disabling or enabling the touchpad accordingly.

Now, save the script as ~/touchpad.sh, make it executable (chmod +x ~/touchpad.sh) and add it to your GUI session startup programs. You have not specified which desktop environment you are using but since you mentioned lxtask, I will assume you are using LXDE. In any case, here are instructions for both LXDE and Unity:

1. Add the script to LXDE's autostart files

   echo "@$HOME/touchpad.sh" >> ~/.config/lxsession/PROFILE/autostart file
   

   Make sure you replace "PROFILE" with the actual name of your LXDE profile, you can find out what it is by running ls ~/.config/lxsession/.

2. Add the script to Unity's autostart files

   Open Startup Applications (search in the dashboard for "Startup")

   

   Click on "Add" and then paste the path to your script in the command field: