A special case of a Unix filter that reads its entire input before writing any output; the canonical example is a sort utility. Unlike most filters, a sponge can conveniently overwrite the input file with the output data stream. If a file system has file versioning (as ITS did and VMS does now) the sponge/filter distinction loses its usefulness, because directing filter output would just write a new version.
See also slurp.
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Nearby terms: SPML « spod « spoiler « sponge » spoo » spoof » spoofing