UNIX reminders: Which command provides a simple automatic reminder service that emails upcoming events listed in a user’s calendar file?

Introduction / Context:
Classic UNIX systems include lightweight utilities for notifications and reminders. One such tool is calendar, which scans a user-maintained file of dates and events and can notify the user (typically by email) when those dates are imminent.

Given Data / Assumptions:

• A per-user calendar file (often named calendar) exists in the home directory.
• The system runs calendar periodically (e.g., via cron) to check upcoming dates.
• Notifications are sent via local mail or display when invoked interactively.

Concept / Approach:
The calendar utility reads the user’s calendar file, matches entries against today’s (and sometimes tomorrow’s) date, and outputs matching lines. System administrators can schedule it to email reminders. Other commands like write and mesg handle live terminal messaging and permission for messages, while mail is a general mail reader and sender rather than an automatic reminder engine.

Step-by-Step Solution:

Verification / Alternative check:
Run calendar - to test formatting; consult system documentation for locale-specific date formats. Check local mail with mail or mailx to confirm delivery.

Why Other Options Are Wrong:
write: sends interactive messages to logged-in users. mesg: toggles permission to receive write wall messages. mail: general-purpose mail tool, not an automatic reminder service by itself.

Common Pitfalls:
Incorrect date formats; forgetting to enable cron or system integration; assuming calendar reads from system-wide calendars without configuration.

Final Answer:
calendar