If the command text box contents has a first line that starts with #!, then Jenkins will just execute that directly (instead of through its own shell invocation. This means you can get rid of the -x parameter to the shell by prefacing your text box contents with:
From Sami Tikka
If the first line of a shell build step starts with hashbang (#!), then Jenkins will simply drop the script as is into a tmp file and execute it. Otherwise Jenkins copies the script into a tmp file and executes it using /bin/sh -xe /tmp/somefilename And /bin/sh, is just the shell used by Jenkins by default. You can change the default under Manage Jenkins -> Configure System.