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dd /dee-dee/ vt.
[Unix: from IBM JCL] 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'.
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