Example of how to use the COPROC feature of Bash by showing a progress bar with zenity and updating it.

bash_coproc_example.sh

#!/bin/bash

on_exit() {
	echo "#The script has quit unexpectidly on line $1" >&$z
	echo "The script has quit unexpectidly on line $1" >&2
	exit
}

on_error() {
	echo "Error on line $1"
	exit
}

coproc zenity --display=:0 --progress --title 'COPROC Example' --text 'Running...'
z=${COPROC[1]}

# Trap when an error occurs, such as '$z: Bad file descriptor' when the user closes the progress dialog
trap 'on_exit $LINENO' EXIT
trap 'on_error $LINENO' ERR

for i in $(seq 1 100); do
	echo $i >&$z
	echo "#Running... $i%" >&$z  # The text will not change if you do not include the hash sign. 
	sleep 0.1
	if [ $i == 50 ]; then
		exit 42
	fi
done