The first Unix
command line interpreter, named after its author
. Many different versions exist, which makes it difficult to write fully portable shell scripts; but every Unix variant is expected to carry a largely compatible Bourne shell in /bin/sh
The Bourne shell has pretty awful syntax (due largely to its crazy rules for the interpretation of whitespace around assignments, and its horrible quoting conventions), but its semantics are elegant, which makes it a good language for shell scripts. Better than the C shell, in any case.
Versions of /bin/sh on different Unix systems vary in features - one example is the support of functions - but there are also sh-compatible shells under a different name. One is the Korn shell, /bin/ksh, which is part of System V and hence, appears in today's officially branded Unixen such as Solaris.
Another is the 'Bourne again' shell, bash, which actually strives for ksh compatibility, but I guess 'Korn again' didn't sound as good.