KSH

Korn shell, commonly abbreviated as ksh, is a Unix shell and scripting language developed by David Korn at Bell Labs in the 1980s. It was designed as a backward‑compatible superset of the Bourne shell (sh) that adds many programming features, making it easier to write complex scripts while preserving sh scripts.

Korn shell has undergone several major revisions. The initial version, ksh88, appeared in the late 1980s and introduced many enhancements over the Bourne shell. A subsequent update, ksh93, released in the early 1990s, expanded the feature set further and improved portability. Over the years, various implementations and patches have appeared, and ksh has remained in use on many Unix‑like systems, often alongside other shells.

Key features of ksh include command substitution with both $(...) and backticks, arithmetic evaluation using the (( ... )) construct, and powerful scripting constructs such as functions and control structures. It supports arrays (and, in ksh93, associative arrays), enhanced variable manipulation via builtins like typeset, and features such as job control, coprocesses, and a built‑in menu-driven select construct. It also provides pattern matching, file name generation, and history/editing facilities for interactive use.

Ksh has influenced other shells, most notably Bash, and several compatible implementations exist, including pdksh, mksh, and yash. It remains available on many Unix‑like systems as a separate package or as an alternative to other shells, and is used in environments where ksh semantics or compatibility with legacy scripts are required.