UNIX utilities: which program traditionally copies files to or from another UNIX time-sharing system over a single communication link?

Introduction:
Before always-on IP networking was common, UNIX systems often exchanged files, emails, and news via dial-up or direct serial links. A classic utility automated these transfers with scheduled jobs and store-and-forward routing. This question asks you to identify the program historically used to copy files between UNIX systems over a single link.

Given Data / Assumptions:

• Environment: classic UNIX to UNIX communication.
• Transport: single serial or dial-up link, possibly scheduled.
• Goal: copy files between hosts reliably.

Concept / Approach:
UUCP (Unix-to-Unix Copy Program) provided command-line tools and daemons for file transfer, remote command execution, and email/news forwarding across point-to-point links, building store-and-forward networks (e.g., UUCPNET). TFTP is a simple UDP/IP file protocol used on IP networks, not over arbitrary single serial links. VMTP is a historical transport protocol (Versatile Message Transaction Protocol), not a file-copy utility. UART is a hardware component (Universal Asynchronous Receiver/Transmitter), not software for file transfers.

Step-by-Step Solution:

Verification / Alternative check:
Historical references to Usenet and early email systems show UUCP scripts dialing neighbors nightly to exchange content, confirming UUCP’s role in single-link, store-and-forward operations.

Why Other Options Are Wrong:

• TFTP: requires IP; lacks authentication/robustness for dial-up serial links.
• VMTP: a protocol, not a UNIX file-copy tool.
• UART: hardware interface, not a program.
• None of the above: incorrect because UUCP is correct.

Common Pitfalls:
Confusing modern IP-based tools (SCP, rsync) with historical pre-IP mechanisms; mistaking hardware components for software utilities.

Final Answer:
UUCP.