How can I run a non-blocking bash command and block on its output?

Question

I'm trying to write a script that will run a program, wait for a certain output from the program, then continue execution (and leave the program running.) My current code doesn't seem to ever output anything, sed never returns true. This echos "Peerflix started" but that's it.

exec 3< <(peerflix $1 -p 8888)
echo "Peerflix started."
sed '/server$/q' <&3
echo 'Matched'

Idan Arye · Accepted Answer · 2014-04-30 06:21:41Z

1

Use pipes!

Use mkfifo to created a pipe and stream the program's output to it in a non-blocking command. Then use your blocking sed to read from that pipe.

Something like(did not test - I don't have peerflix):

mkfifo myfifo
peerflix $1 -p 8888 > myfifo &
echo "Peerflix started."
sed '/server$/q' myfifo
echo 'Matched'
rm myfifo

edited Apr 30, 2014 at 6:21

answered Apr 30, 2014 at 0:48

Idan Arye

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2 Comments

iruvar Over a year ago

This will block at peerflix $1 -p 8888 > myfifo unless you explicitly run it in the background. The OP's exec 3< <(peerflix $1 -p 8888) process substitution is however, non-blocking

Ari Lotter Over a year ago

weirdly enough, it's not working at all. I've realized that I need to be checking stderr instead of stdout, but my solution of gist.github.com/anonymous/81d10eb2cc228a7c90fd isn't working at all. (external link cause comments don't have newlines)

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