Implement set -o pipefail

feat: I want set -o pipefail. #272 (closed) asked for both errexit and pipefail, but so far only errexit is implemented.

BREAKING CHANGE: it is possible to add pipefail without breaking existing scripts by defaulting it to off. In the perfect world both errexit and pipefail would have been always on without any way to disable them (just like in all programming languages that have exceptions except for Visual Basic), but that might break existing scripts.

perf: impact

performance none

usability increase

maintainability increase

code: input

set -eo pipefail
echo 1
false | true
echo 2

expect: output

1

reason: I want my script to fail fast by default if an external command fails.

context:

Executing an external command can fail for a million different reasons. Here are some examples:

1. The external command is not installed
2. It encounters an internal bug and fails fast and loud
3. It receives bad input due to a bug in my script
4. An admin kills it
5. The OOM Killer kills it https://linuxwheel.com/oom-killer-explained/
6. It tries to talk to another process possibly on a different machine and that fails for any reason
7. It uses a temporary file for internal buffering and /tmp is full
8. It tries to read or write a file and that fails for any reason

A robust script can not afford to ignore the possibility that any of the external commands it executes may fail. Often the right choice of action is to fail fast because the code that follows requires that the preceding commands executed successfully.

Unix-y pipelines have an issue:

foo | bar

If foo exits successfully bar sees EOF. If foo fails bar also sees EOF. bar has no way to know whether foo failed or not. In many cases bar will exit successfully even if foo fails. The script has to detect that and fail. This is why you often see set -o errexit -o pipefail in bash.

Unfortunately set -o errexit is very broken in bash which is why I'm looking for a different shell.