# A pipe made a failed check look like a pass

Problem 47. Published Nov 7. Last checked Nov 7. Tags: shell scripts, quality checks, silent failures.
Page: https://bluhook.com/log/pipe-made-failed-check-look-like-pass
Exact text to search: bash pipe hides exit code, && still runs after a failed check, set -o pipefail

## What happened

We run a quality check on every client email before it sends. The send only runs if the check passes: check && send.

One day we added "| tail -4" after the check so the output was shorter. The check failed, but the line still sent the email. A pipe returns the exit code of its last command, and tail succeeded.

So one client email went out before it had passed its check. The same text passed the check a few minutes later, so nothing wrong was said, but it should not have gone. The email also showed as one block of text, because the body's blank lines were never turned into paragraphs.

## What we did

1. Removed the pipe. The check now writes its output to a file (check > out.txt 2>&1 && send), and we read the file after.
2. Where a pipe is needed, set -o pipefail goes first, so any failed step fails the whole line.
3. The send step now turns blank lines into paragraphs, so a body can't arrive as one block.
4. The rule went into the instructions every sender on the team loads.

## How we know it worked

We ran a step we knew would fail, three ways. With the pipe, the next step still ran. With output to a file, it did not. With set -o pipefail first, it did not. A second person on the team read the new rule back from the live instructions.

## What you can take from it

Never pipe a gate: run "check > out.txt 2>&1 && send", or put "set -o pipefail" first.