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What definition of "shell" are you basing this on? Some (current) Unix systems I use have default shells that have neither command history nor tab completion.

Easy access to command history and tab completion are certainly niceties that came along at some point, but are in no way part of the definition of a shell.

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ok, I'll bite. What current Unix systems ship with a default shell that supports neither command-completion nor command-history?

The one I have to use frequently is the restricted ksh environment in IBM's IVM/VIOS server, which is AIX.

I believe it does log a short set of command history to a file, but it'd not available with, say, the up arrow.

In general, though, I don't think any form of history is required by the POSIX shell standard, and tab-completion definitely isn't.

ksh93 supports both completion and history. I lost interest in looking before uncovering information about versions of ksh more than thirty years old, but neither feature is mentioned in descriptions of differences between ksh93 and predecessors, so I suspect the feature has been there for longer. Posix compliance is neither necessary nor sufficient for a reasonable colloquial definition of "shell."

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