A simple example of using the coprocessor feature of bash -- in this case, to launch a GUI progress bar via zenity

zentest is for bash GUI

#!/bin/bash

# Start Zenity (might not be installed by default) as a bash coprocess
coproc zenity --progress --pulsate --title='Testing' --text='Starting up...'
z1=${COPROC[1]}

# Update Zenity...
sleep 2s
echo 33 >&$z1
echo '#One third done...' >&$z1
sleep 2s
echo 67 >&$z1
echo '#Two thirds done...' >&$z1
sleep 2s
echo 100 >&$z1      # Close Zenity and therefore the background task.
echo '#Finished!' >&$z1