bash set -o pipefail — catch errors in pipelines

Quick Answer

set -o pipefail

# Now this will fail (grep fails because the file is missing)
cat missing-file.txt | grep "pattern" | wc -l
# Without pipefail: exit code 0 (wc succeeded)
# With pipefail:    exit code 1 (cat failed)

Problem without pipefail

Without pipefail, only the exit code of the last command in a pipeline matters:

false | true
echo $?   # prints 0 — the "false" failure is silently swallowed

Silent failures in pipelines are a common source of subtle bugs in shell scripts.

Other causes & fixes

Combine with set -e and set -u

#!/usr/bin/env bash
set -euo pipefail

# Script now exits on:
# - any command failure (-e)
# - unset variable reference (-u)
# - pipeline failure (-o pipefail)

Get individual pipeline exit codes

cat file.txt | grep pattern | sort
echo "${PIPESTATUS[@]}"   # e.g. "0 1 0" — grep failed, others ok

Allow specific pipeline failures

# Use || true to ignore expected failures
grep "pattern" file.txt | head -5 || true