A long compile prints diagnostics on standard error. `make | grep -i error` shows nothing useful, because the error lines scroll past on the terminal instead of entering the pipe. Which command line passes both output streams through grep?
LPIC-1 Exam 101-500, objective 103. GNU and Unix commands medium
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Treat it as a good draft rather than as settled fact, and read the source below before you rely on it. It is not used in mock exams here — only questions a person has signed for are.
How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct make 2>&1 | grep -i error
Correct. In a pipeline the shell attaches standard output to the pipe first, so 2>&1 then points standard error at the same pipe and grep sees both streams.
Not correct make | grep -i error 2>&1
Wrong. The 2>&1 applies to grep, not to make, so it merely merges grep's own error output into grep's standard output. make's diagnostics still go to the terminal.
Not correct make 2>&1 >/dev/null | grep -i error
Wrong for this requirement, though it is a real idiom: standard error is duplicated onto the pipe and standard output is then diverted to /dev/null, so grep sees only the error stream and the normal output is thrown away.
Not correct make 1>&2 | grep -i error
Wrong, and it empties the pipe. Standard output is redirected onto standard error, which is still the terminal, so grep receives no input at all.
Why
A plain pipe connects only descriptor 1 of the left command to descriptor 0 of the right one; descriptor 2 is untouched and keeps going wherever it pointed. Merging with 2>&1 must come after standard output has been attached to its final destination, and in a pipeline the shell has already done that before it processes the redirections. Bash offers |& as shorthand for 2>&1 |.
Where this comes from
- Cited
- manual page bash(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 103 and space the ones you get wrong.
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