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: