Select lines containing status=500 from access.log.
Count the selected lines without creating a temporary file.
Which command list is correct?
A pipe sends the left command's standard output to the right command's standard input. grep filters, and wc -l counts the resulting lines.
Detailed explanation
grep 'status=500' access.log > wc -lIncorrect. > redirects output to a file named wc rather than invoking wc -l.
grep 'status=500' access.log < wc -lIncorrect. < supplies input to grep and does not connect its output to wc.
grep 'status=500' access.log | wc -lCorrect. grep's matching lines flow directly into wc -l.
wc -l access.log | grep 'status=500'Incorrect. It filters the numeric output of wc rather than counting grep matches.
Try it yourself
An example you can run in a temporary verification environment.
printf 'status=200
status=500
status=500
' | grep 'status=500' | wc -lExpected result
2（前の空白幅は環境により異なる）Key points
- | connects output to input
- grep selects lines
- wc -l counts result lines
Notes
- Environment: Bash 5.2 / GNU grep・coreutils
- Command output formatting can vary slightly by distribution or tool version.
- Run the example in a temporary directory or process when possible.
Foundation review
Read the scope first
Check whether the command acts on the current shell, a new process, an existing process, or a file.
Verify the observable result
Use the supplied command and compare the output with the expected result.