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How far back does Linux's backwards compatibility extend?

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closed as not constructive by Breakthrough, bwDraco, Nifle, MaQleod, RedGrittyBrick Oct 21 '11 at 21:00

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Are you asking about source (API) compatibility or binary (ABI) compatibility? – bwDraco Oct 21 '11 at 19:47
Hello World. -- – Hello71 Oct 21 '11 at 21:16

According to this, dc is older than the C programming language.

This article indicates that roff, which evolved into troff dates back to about 1970 -- but it says it was added to Unix, implying that other programs already existed. It mentions the text editor, probably ed.

One could argue that /bin/true is a reimplementation of IEFBR from IBM's OS/360.

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