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In the shell, when I pipe jack_cpu_load through sed, or cut, no matter what options I use it stops printing just before the lines I want to see.

jack_cpu_load | sed -n 8p will print:

Jack: JackClient::kActivateClient name = jack_cpu_load ref = 4

The very next line should read something like jack DSP load 0.294772 which is what I'm looking for, but when I run jack_cpu_load | sed -n 9p which should print that line, there is nothing. Just a cursor, until I hit Ctrl+C and kill it.

Unfortunately there is very little documentation on this command and I'm just a user, a musician no less, trying to hack together something that will let me see the dsp load at a glance in my status bar.

Terminal Output:

tony@hydra ~ $ jack_cpu_load
Jack: JackClient::SetupDriverSync driver sem in flush mode
Jack: JackPosixSemaphore::Connect name = jack_sem.1000_default_jack_cpu_load
Jack: JackPosixSemaphore::Connect sem_getvalue 0
Jack: Clock source : system clock via clock_gettime
Jack: JackLibClient::Open name = jack_cpu_load refnum = 4
Jack: JackClient::Activate
Jack: JackClient::ClientNotify ref = 4 name = jack_cpu_load notify = 2
Jack: JackClient::kActivateClient name = jack_cpu_load ref = 4 
jack DSP load 0.163633
jack DSP load 0.159914
jack DSP load 0.159449
jack DSP load 0.164087
jack DSP load 0.159971
^CJack: jack_client_close

For this:

tony@hydra ~ $ jack_cpu_load 2>&1 | sed -n 8p
Jack: JackClient::kActivateClient name = jack_cpu_load ref = 4

And for this:

tony@hydra ~ $ jack_cpu_load 2>&1 | sed -n 9p

There's nothing.

The output for strace -f jack_cpu_load http://justpaste.it/e7st

user57649
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  • Maybe the output doesn't all go to stdout. Try `jack_cpu_load 2>&1 | sed -n 8p`. Does that help? If not can you fully copy what happens in your terminal please – Marki Jan 26 '14 at 12:09
  • I keep thinking that's what's going on but no, 2>&1 doesn't change anything. I added terminal output to the OP. – user57649 Jan 26 '14 at 16:33
  • Maybe it starts something in the background which outputs the other stuff... Here probably only using `strace` will shed light on what's going on. (`strace -f jack.....`) – Marki Jan 26 '14 at 18:27
  • I added a link to a file with the output. I see three pids, but beyond that I'm out of my depth. Thanks for the help. – user57649 Jan 26 '14 at 18:58
  • Currently I don't understand either. You see they all write to fd1 (stdout). You see it more clearly when you run `strace -f -t -e trace=write jack_cpu_load`. So no idea why you can't catch anything beyond the 8th line. What happens when you redirect to a file? (`jack_cpu_load > bla.txt`, wait for a while, then Ctrl-C, then inspect the file) – Marki Jan 26 '14 at 19:30
  • It's the same output that shows on the terminal. I'm going to ask the developer about it. I guess I should have started there, but I thought the problem was something obvious that I'm just not aware of, thanks! – user57649 Jan 26 '14 at 19:41
  • Then it can only be a buffering issue. Try `stdbuf -i0 -o0 -e0 jack_cpu_load | sed -n 9p` – Marki Jan 26 '14 at 19:44
  • That's it! Thanks. Now I have to get back to trying to understand how to use sed... – user57649 Jan 26 '14 at 20:07
  • Feel free to accept my answer ;) Oh and feel free to create another question about sed or more generally about what you want to do with the output. – Marki Jan 26 '14 at 20:24

1 Answers1

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Maybe the output doesn't all go to stdout. Try jack_cpu_load 2>&1 | sed -n 8p

Or it is a buffering issue. Try stdbuf -i0 -o0 -e0 jack_cpu_load | sed -n 9p

Marki
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