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Was it just to save typing one letter?
I don't think C function names were limited to five characters at the time, as
unlink are probably just as old.
I'm hoping someone who was involved said something on the record about this, but I haven't been able to find it.
This is not the same as asking 'What did Ken Thompson mean when he said, “I'd spell create with an 'e'.' because it is a different question with a different answer.