Exit from BASH Infinite loop in a pipeline

Question

I encountered a somewhat strange behavior of BASH infinite loops which outputs are pipelined to another processes. Namely, I run these two commands:

(while true; do echo xxx; done) | head -n 1
(while true; do date; done) | head -n 1

The first one exits instantly while the second one does not (and I assume it would run forever without being killed). I also tried an implicit infinite loop:

yes | head -n 1

and it also exits by itself. An appropriate line of output is immediately printed on the screen in each case. I am just curious what determines if such a commmand will finish.

Answer 1

When head exits, the standard output of the parenthesized expression is closed. If an external command, like date, is used, the loop hangs. If an internal command of bash is used, like echo, the loop exits. For proof, use

(while true; do /bin/echo xxx; done) | head -n 1

and it will hang. If you use

(while true; do date; echo $? 1>&2; sleep 1; done) | head -n 1

you will see that on the second round, the date command returns an error exit code, i.e. something other but zero. Bash obviously does this not take as serious as when an internal command gets into problems. I wonder if this is intended or rather a bug in bash.

To make sure the loop is exited, this seems to work:

(set -e; while true; do date ; done) | head -n 1