[Help-bash] Interactive vs non-interactive behavior

[Kazama Seiji]

Hey guys.


I've got such code sample:

timeout 5 tail -fn0 access.log | awk '{print $1}'


It listens to nginx's access.log for 5 secs and outputs the log's first 
column (=client IP).


The sample works the same in interactive shell as well as a standalone 
script.


But as only I add more stages to the pipe it fails in interactive shell 
like in:

timeout 5 tail -fn0 access.log | awk '{print $1}' | wc -l

or

timeout 5 tail -fn0 access.log | awk '{print $1}' | sort | uniq -c | 
sort -k1,1nr -k2,2V


the output is a single line "Terminated". It still works as a standalone 
script though.


I've figured out the enabled monitor=on bash setting for interactive 
mode triggers the behavior. So with "set +m" applied it works in 
interactive shell. Another way to overcome it in interactive shell is 
such trick

( timeout 5 tail -fn0 access.log; true ) | awk '{print $1}' | wc -l


The question is: what is so special to pipes of 3+ stages that it 
presents different behavior vs a 2-stage pipe when monitor=on in bash?
E.g. why this works
set -m
timeout 5 tail -fn0 access.log | awk '{print $1}'

but this doesn't
set -m
timeout 5 tail -fn0 access.log | awk '{print $1}' | wc -l

(I explicitly set monitor=on above for clarity.)