Say I've started program and it continues writing some output to stdout. Is there some way to add a pipe so that I can, for example, count the number of lines it has produced with wc -l?
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1It may be possible, but how that would be done is probably dependent on what Unix you are using.Kusalananda– Kusalananda ♦2018-05-25 20:41:42 +00:00Commented May 25, 2018 at 20:41
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2 Answers
You can do this in Linux. Say there's a script s:
#!/bin/bash
i=0
echo my pid: "$$"
while true; do
echo "$i"; ((i++)); sleep 1;
done
Run it:
$ ./s
my pid: 8815
0
1
2
And so it goes... Now you need gdb to attach to the process.
gdb -p 8815
Now you're in gdb. Here do this:
p close(1)
This has closed the STDOUT file descriptor. Now open a new one:
p creat("/tmp/8815.out", 0600)
In another terminal you can already set the reading process:
tail -f /tmp/8815.out
Nothing's coming yet, as you need to detach from gdb in the other terminal. Either Ctl+D, or:
q
Quit anyway? (y or n)
Confirm with y and Enter. And now you should see (where the tail is running) something like this:
173
174
175
If you prefer to use an already existing file or pipe, do this in gdb:
p open("/path/to/your/output", 1)
This illustrates redirecting STDOUT. It's similar with STDERR, only it has number 2.
I used these two answers:
You can use tee when you start the program. If you're using bash, you can do:
program > >(tee -a log.txt)
That will redirect the stdout of program to a process substitution running tee. That essentially writes a copy of the stdout of program to log.txt. tee will continue to send the stdout to wherever it was going before, probably the terminal depending on how program works.
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This is useful but can i do this if I've already started
program?cheft– cheft2018-05-25 20:33:51 +00:00Commented May 25, 2018 at 20:33 -
I'm not sure, but it might be possible with
strace. Try stackoverflow.com/questions/593724/…m0dular– m0dular2018-05-25 20:45:46 +00:00Commented May 25, 2018 at 20:45