This is because open on Linux refers to some other command

$ man open
NAME
       openvt - start a program on a new virtual terminal (VT).
...

The equivalent command on Linux is xdg-open (SO thread on this issue)

$ man xdg-open
NAME
       xdg-open - opens a file or URL in the user's preferred application
...

Definition of done

• Define open command as a bash variable (e.g., $OPEN) and set it based on the operating system (we do something similar in our keptn-on-k3s install script) and evaluate this bash variable instead of open directly in multistage-delivery.sh
• Do the same for expose-keptn.sh