In the classic Unix 'mail' command interface, which single-letter command saves the current message into a separate file (mailbox or named file)?

Introduction / Context:
Traditional Unix mail programs provide single-letter commands to manage messages from within the interactive prompt. Knowing how to save a message to a separate mailbox or file is essential for archiving, forwarding through scripts, and organizing important correspondence from the terminal.

Given Data / Assumptions:

• Using the classic mail (or mailx) command-line interface.
• You have a message selected (by default the current one).
• You want to save it to a specified filename or mailbox.

Concept / Approach:

The s command (save) writes the current message to a named file or mailbox, including headers. Syntax typically is s filename. The similar command w (write) often writes the message body only (omitting some headers) depending on implementation, while d deletes, and r replies.

Step-by-Step Solution:

Verification / Alternative check:

After saving, check the target file with ls -l saved.txt and view its contents using less saved.txt. You should see the message with headers intact.

Why Other Options Are Wrong:

• r: reply to sender; does not save.
• w: writes the message (often body only); the question emphasizes saving to a separate file in standard practice, which is s.
• d: delete message.
• None of the above: incorrect because s is the conventional save command.

Common Pitfalls:

• Confusing s and w differences across variants (mail vs mailx vs Heirloom); when in doubt, check man mail.
• Forgetting to specify a filename causes save to default mailbox behavior.

Final Answer:

s.