[orig. Multics techspeak, widely propagated
1. [techspeak] The command interpreter used to pass commands to an
operating system; so called because it is the part of the operating system
that interfaces with the outside world.
2. More generally, any interface program that mediates access to a
special resource or server for convenience,
efficiency, or security reasons; for this meaning, the usage is usually
a shell around whatever. This sort
of program is also called a wrapper.
3. A skeleton program, created by hand or by another program (like,
say, a parser generator), which provides the necessary
incantations to set up some task and the control
flow to drive it (the term driver is sometimes used
synonymously). The user is meant to fill in whatever code is needed to get
real work done. This usage is common in the AI and Microsoft Windows
worlds, and confuses Unix hackers.
Historical note: Apparently, the original Multics shell (sense 1) was
so called because it was a shell (sense 3); it ran user programs not by
starting up separate processes, but by dynamically linking the programs
into its own code, calling them as subroutines, and then dynamically
de-linking them on return. The VMS command interpreter still does
something very like this.