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.