Dd: /dee-dee/ [UNIX: From IBM {JCL}] Vt. Equivalent To {cat} Or {BLT}.

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:dd: /dee-dee/ [UNIX: from IBM {JCL}] vt. Equivalent to
{cat} or {BLT}. Originally the name of a UNIX copy command
with special options suitable for block-oriented devices; it was
often used in heavy-handed system maintenance, as in "Let's
`dd' the root partition onto a tape, then use the boot PROM to
load it back on to a new disk". The UNIX `dd(1)' was
designed with a weird, distinctly non-UNIXy keyword option syntax
reminiscent of IBM System/360 JCL (which had an elaborate DD
`Dataset Definition' specification for I/O devices); though the
command filled a need, the interface design was clearly a prank.
The jargon usage is now very rare outside UNIX sites and now nearly
obsolete even there, as `dd(1)' has been {deprecated} for a
long time (though it has no exact replacement). The term has been
displaced by {BLT} or simple English `copy'.
-- The AI Hackers Dictionary