Documented in Volume 1 of the UNIX Programmers Manual.
% rmdir direc-name will remove the empty directory named direc-name. If it is non-empty, see the example below for rm.
% rm -r direc1 removes subdirectory direc1 in a recursive fashion, (i.e. the directory and all of its files and subdirectories) whether it is empty or not.
Useful for file space housekeeping.
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