If the editor has not been explicitly /set, the environment variable EDITOR should be used.

Mark writes --
Every Unix tool I've ever seen that uses EDITOR has assumed that the
command defined by that environment variable will not return until the
editing operation is complete. I think jshell can assume that too.

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Research shows that if a tool allows visual editors that the VISUAL environment should be checked first. Many tools define their specific editor XXXXXEDITOR. So, environment variables will be tested in this order: JSHELLEDITOR, VISUAL, EDITOR.