grep, tee, and sleep not playing nice together

Question

(echo foo && sleep 1 && echo bar) | grep -P . | tee /dev/null

The above code is the smallest case of my problem I could come up with. The expected behavior is that foo would be echo'd, a second would pass, then bar would be echo'd.

What actually happens is that a second passes then foo and bar are echo'd at the same time. If you remove either the grep or the tee command (or both, obviously) the correct behavior happens. But with them together it does not.

I assume this is some kind of buffering issue, but I don't know how to get around it. The actual script this is taking place in for me is running for quite a while and I'm not seeing any log messages till the very end. Halp! :(

possible duplicate of [bash: force exec'd process to have unbuffered stdout](http://stackoverflow.com/questions/3332045/bash-force-execd-process-to-have-unbuffered-stdout) — Etan Reisner, Dec 04 '14 at 02:36
By 'correct' and 'expected', you mean 'desired'. The behavior you see is both correct and expected. — William Pursell, Dec 04 '14 at 02:43

score 4 · Answer 1 · answered Dec 04 '14 at 02:40

4

Etan's comment prompted me to look through grep's man page and I found the --line-buffered flag. Adding that fixed the problem

(echo foo && sleep 1 && echo bar) | grep --line-buffered -P . | tee /dev/null

answered Dec 04 '14 at 02:40

Mediocre Gopher

2,274
1
22
39

grep, tee, and sleep not playing nice together

1 Answers1