Capture and view output simultaneously: Which command copies standard output to a file while also echoing it to the terminal at the same time?

Introduction / Context:Sometimes you need to keep a record of command output while still monitoring it on screen. The tee utility solves this by duplicating standard output—writing to both a file and the terminal—making it invaluable for logging and troubleshooting pipelines.

Given Data / Assumptions:

• The requirement is to save output to a file and display it simultaneously.
• We are using standard POSIX command-line tools.
• No special formatting or filtering is required.

Concept / Approach:

The syntax is cmd | tee file. By default, tee overwrites the file; use tee -a file to append. tee acts as a T-junction in a pipeline: it passes input through to standard output and also writes it to the named file(s).

Step-by-Step Solution:

Verification / Alternative check:

Run ls -lR / | tee listing.txt. Observe output on screen and verify listing.txt contains the same data. Re-run with -a to append additional listings without truncating previous content.

Why Other Options Are Wrong:

• more and cat: Display output but do not simultaneously save to a file.
• grep: Filters lines by pattern; does not split output to a file and screen.
• None of the above: Incorrect because tee matches the requirement exactly.

Common Pitfalls:

Forgetting -a when you intend to append; accidentally overwriting existing logs; misunderstanding that tee only captures what is piped into it (stdout), not stderr unless you redirect stderr to stdout (for example, 2>&1).

Final Answer:

tee