The [[…]] built-in test version was introduced in Peter Korn's korn shell, ksh88 IIRC. As where various other modernized notations like array variables, process

The [[…]] built-in test version was introduced in Peter Korn's korn shell, ksh88 IIRC. As where various other modernized notations like array variables, process substitution or $(command) as a much more readable version of the backquotes version. https://docs.oracle.com/cd/E36784_01/html/E36870/ksh88-1.html https://docs.oracle.com/cd/E36784_01/html/E36870/ksh88-1.htm... While learning the Bourne shell as acstudent, I was rapidly lured by csh and then tcsh, but Tom Christiansen's pamphlet https://everything2.com/title/csh+programming+considered+harmful https://everything2.com/title/csh+programming+considered+har... and (or?) the appearance of Paul Falstad's Z-Shell saved me ;-0