Why exit code 141 with grep -q?

Question

Can someone explain why I get exit code 141 from the below?

#!/usr/bin/bash

set -o pipefail

zfs list | grep tank
echo a ${PIPESTATUS[@]}

zfs list | grep -q tank
echo b ${PIPESTATUS[@]}

cat /etc/passwd | grep -q root
echo c ${PIPESTATUS[@]}

I get

...
a 0 0
b 141 0
c 0 0

From my understanding exit code 141 is a failure, but the line above gives zero, so it should be success, I would say.

Answer 1

This is because grep -q exits immediately with a zero status as soon as a match is found. The zfs command is still writing to the pipe, but there is no reader (because grep has exited), so it is sent a SIGPIPE signal from the kernel and it exits with a status of 141.

Another common place where you see this behaviour is with head. e.g.

$ seq 1 10000 | head -1
1

$ echo ${PIPESTATUS[@]}
141 0

In this case, head read the first line and terminated which generated a SIGPIPE signal and seq exited with 141.

See "The Infamous SIGPIPE Signal" from The Linux Programmer's Guide.

Answer 2

I'm not familiar with zfs list, but I guess it complains about its standard output being closed - grep -q exits immediately when a match is found, unlike grep.

Answer 3

Another option would be to not use a pipe, but use a process substitution:

grep -q tank <(zfs list)

Update: I guess is the same thing, as the process run inside parentheses will also receive sigpipe.