UNIX shells history: Which shell was the earliest and became the most widely used default shell on classic UNIX systems?

Introduction / Context:
Shells are command interpreters that provide the user interface to UNIX. Historically, several shells evolved with different syntax and features. Recognizing their origins and defaults helps with scripting portability and legacy system maintenance.

Given Data / Assumptions:

• The question asks for the earliest and most widely used shell that shipped with UNIX.
• We distinguish between Bourne (sh), C shell (csh), and Korn shell (ksh).
• We focus on historical defaults on System V and earlier releases.

Concept / Approach:
The Bourne shell (sh), created by Stephen Bourne at Bell Labs, was the standard shell on early UNIX and became the baseline for shell scripting. Later, the C shell (csh) added C-like syntax and interactive features, and the Korn shell (ksh) improved on sh with advanced scripting. Nevertheless, sh remained the canonical “earliest and most widely used default.”

Step-by-Step Solution:

Verification / Alternative check:
On many UNIX derivatives, /bin/sh links to the system’s default POSIX shell; historically, this derived from the Bourne shell lineage, confirming its foundational role.

Why Other Options Are Wrong:
C shell: influential but later and not the earliest default. Korn shell: later enhancement; not the original standard. Smith shell: not a historical UNIX shell. None: incorrect because Bourne shell is correct.

Common Pitfalls:
Confusing interactive convenience (csh history, aliases) with historical default status; assuming modern /bin/sh always equals bash—implementations vary.

Final Answer:
Bourne shell