Problem matching new terminal window ID with xdotool in Bash

I have a script that kicks off a new terminal window that I want to move to a specific position on the left terminal using xdotool. After kicking off the terminal I run xdotool search --class gnome-term and I use the returned Window ID in the xdotool windowsize and windowmove commands to size and position the new terminal window. xdotool windowsize $WIN_ID 1978 407 xdotool windowmove &WIN_ID -26 702 Since I have multiple terminals running I need to identify which WIN_ID is associated with the last terminal window just created. In the script I execute the below command before launching the new terminal to get a list of existing WIN_IDs. for i in xdotool search --class gnome-term; do id_list="$id_list $i"; done Sample content of $id_list: 65011713 65011722 65103242 65019818 65029314 65027106 After launcing the new terminal, to identify the new WIN_ID, I run this test. for WID in xdotool search --class gnome-term; do if [[ ! $WID =~ $id_list ]]; then echo "New Window ID = $WID"; fi; done When I run this command, instead of getting just one (the new) Window ID, I get all the Window IDs associated with the gnome-terminal class. Why is the below command not returning only the WID that isn't in id_list? if [[ ! $WID =~ $id_list ]]; then ... Script: ... ... # Generate a list of Window IDs for existing gnome-terminal windows for i in xdotool search --class gnome-term; do id_list="$id_list $i"; done # Kick off a new terminal window gnome-terminal # Get the Window ID of the new terminal window New_WID=for WID in xdotool search --class gnome-term; do if [[ ! $WID =~ $id_list ]]; then echo $WID"; fi; done ... ... Thanks for time and feedback! Tex