My understanding is that the name dd was essentially a joke. A DD statement in JCL defined a logical name (seen by the program) to refer to a file (`dataset').

My understanding is that the name dd was essentially a joke. A DD statement in JCL defined a logical name (seen by the program) to refer to a file (`dataset'). It had many strange operands, and often its behavior was completely unintuitive. For example, here is a job that deletes the file MYFILE. //MYJOB JOB //FOOBAR DD DSNAME=MYFILE,DISP=(,DELETE) //STEP1 EXEC PGM=IEFBR14 IEFBR14 is a program that does nothing at all! Although the Unix dd command wasn't patterned on the JCL command, I suspect that the multiplicity of possible options led its designers to choose the key=value option syntax that looked vaguely OS/360ish. By the way, the - flag for options first appeared in MIT's CTSS, which was the direct ancestor (at least at the user level) of Multics.