How can a process intercept stdout and stderr of another process on Linux?

Question

I have some scripts that ought to have stopped running but hang around forever. Is there some way I can figure out what they\'re writing to STDOUT and STDERR in a readable

Answer 1

Since I'm not allowed to edit Jauco's answer, I'll give the full answer that worked for me (Russell's page relies on un-guaranteed behaviour that, if you close file descriptor 1 for STDOUT, the next creat call will open FD 1.

So, run a simple endless script like this:

import time

while True:
    print 'test'
    time.sleep(1)

Save it to test.py, run with

$ python test.py

Get the PID:

$ ps auxw | grep test.py

Now, attach gdb:

$ gdb -p (pid)

and do the fd magic:

(gdb) call creat("/tmp/stdout", 0600)
$1 = 3
(gdb) call dup2(3, 1)
$2 = 1

Now you can tail /tmp/stdout and see the output that used to go to STDOUT.