You may use #!/bin/sh at the start of your Execute shell block as shown below:

Using #!/bin/sh without the default options that Jenkins uses i.e., #!/bin/sh -xe will prevent Jenkins from marking your Execute shell as failure. Doing this will help you run the subsequent command of finding the return status of your script.
-e  Exit immediately if a command exits with a non-zero status.
Ref: http://linuxcommand.org/lc3_man_pages/seth.html
Based on the return status (script_status=$?) of your script, print something that you can search later in your post-build step.
Then, in the Post-build Actions, search that printed text to decide your post-build action as shown below:

Output:
