Piping both output streams into a filter | LinuC Level 1 101 Exam original practice question

1.03.3Basic

Pass both run-check stdout and stderr to grep and select lines containing FAIL.

Use POSIX syntax.

Which command sequence is appropriate?

Hint 1: Focus on the requirement

Merge the two process streams before connecting them to the downstream filter.

Hint 2: Key terms

Review the short stream and redirection definitions before comparing the choices.

Standard output and error: The separate ordinary-result and diagnostic streams.
Pipeline input: The stream consumed by the command on the right.
Stream merge: Combining multiple descriptors into one stream.
Filter: A command that transforms or selects input data.

Detailed explanation

Incorrectrun-check | grep FAIL

Incorrect. Only stdout enters the pipe; stderr stays on the terminal.

Correctrun-check 2>&1 | grep FAIL

Correct. 2>&1 merges both streams before the pipe sends them to grep.

Incorrectrun-check 2> grep FAIL

Incorrect. This redirects stderr to a file named grep rather than piping it.

Incorrectrun-check | grep FAIL 2>&1

Incorrect. It redirects grep's own stderr and does not pipe run-check stderr.

An example you can run in a temporary verification environment.

sh -c 'echo OK; echo FAIL-error >&2' 2>&1 | grep FAIL

FAIL-error

Environment: POSIX shell / grep / 生成出力
Use temporary files and harmless commands for stream experiments.
Shell redirection syntax and xargs behavior should be checked on the target shell implementation.

Foundation review

A normal pipeline carries standard output only; an explicit merge is needed for diagnostics too.

Emit one line on each descriptor and confirm both reach the filter.

Question IDKL-101-483

EnvironmentPOSIX shell / grep / 生成出力

Last technical review2026-08-13

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