How do I create a shell script on a Macintosh?

If not, you can use an application like BBEdit, save the file as text-only with or without line breaks, then open up the Terminal application and type in the name of the script, as in sh myscript.sh (assuming you saved the file as 'myscript.sh').

The final step is for you to actually make the script executable, which you can do by using the chmod command, as in: chmod +x myscript.sh.

Once you've done that - particularly if you have the script in one of the directories in your PATH - you can execute the script by simply typing its name at the command line.