The Korn shell (ksh) is a shell originally written at AT&T, with more advanced programming features than the then-existing Bourne and C shells, and a syntax compatible with the Bourne shell. Many programming features in bash and zsh mimic ksh's. Several versions of ksh exist, including:
- ksh88, the original, which was always proprietary software.
- pdksh (public domain ksh), a free clone of ksh88 (with a few incompatibilities). Not developed since 1999; superseded by…
- mksh (MirBSD Korn Shell), an actively developed descendant of pdksh.
- ksh93, a major new version of the original Korn shell, superseding ksh88; initially proprietary, then released as free software.
Further resources are available on the #ksh on Freenode channel homepage collection.