After a pipeline, $? is the last command's status — `failing-cmd | tail` looks like success
A pipeline's exit status is the last command's unless pipefail is set, so `npm test | tail` reports tail's success even when the tests fail. Use set -o pipefail or ${PIPESTATUS[@]}.
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Symptom
A script runs npm test | tail -20 (or build | tee log) and carries on as if everything passed, although the tests failed.
What happens (reproduced)
false | true; echo $? # 0 — tail/tee/true decide the status
set -o pipefail
false | true; echo $? # 1
false | true; echo "${PIPESTATUS[@]}" # 1 0
Fix
- Put
set -o pipefail(orset -euo pipefail) at the top of scripts. - Or check the command itself:
npm test > out.log 2>&1; rc=$?; tail -20 out.log; exit $rc. - In bash,
${PIPESTATUS[0]}gives the first command's status. Read it immediately, because the next command overwrites it.
Conditions
- bash
- 5.2.21
- observed
- 2026-09-30
Sources
- Bash manual: Pipelines — The pipeline's status is the last command's unless pipefail is enabled.