I don't well what's the name of the situation, I think yesterday I closed a session with a background ipython process, but I am still able to connect to it (the process is running a web server and I can load the page). Can I still get to see the stdout/stderr of the process?

If I run ps -f -Umyname I get (I deleted some unrelated lines, but the PID and PPID are the real ones):

16131     1767  1765  0 11:20 ?        S      0:03 sshd: myname@notty
16131     1946 21019  0 11:25 ?        Sl     1:47 /usr/bin/python -m IPython.kernel -f /home/myname/.ipython/profile_default/security/kernel-c72091
16131     3946 21019  0 14:37 pts/3    Ss+    0:00 /bin/bash
16131     4003  4001  0 14:38 ?        S      0:01 sshd: myname@pts/14
16131     4004  4003  0 14:38 pts/14   Ss     0:00 -bash
16131     6366  4004  0 16:41 pts/14   R+     0:00 ps -f Umyname
16131    21019     1  0 Jul30 ?        Sl     0:52 /usr/bin/python ipython notebook 

I am sure it's not being ran inside a screen. Any idea? I am interest in PID 21019

  • Would like to discover best techical name for this situation, too :) May 5 at 10:40

You can use strace to see the output of a running process.

$ strace -p 21019 -e write=1,2

The above command will attach to the process with PID 21019 and trace write-calls to fd 1 (stdout) and 2 (stderr).

When you're done, you can send a sigint (to strace) to detach (press ctrl + c).

edit: There are also some tools that allow you to attach a running process to a new terminal, e.g. reptyr, screenify, retty. Note that most of these use strace, gdb or similar trace-tools to accomplish this, but they tend to produce prettier output.

  • I got attach: ptrace(PTRACE_ATTACH, ...): Operation not permitted Could not attach to process. If your uid matches the uid of the target process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try again as the root user. For more details, see /etc/sysctl.d/10-ptrace.conf Unfortunately I have not root access in this system, but thanks for the suggestion
    – laika
    Aug 5 '15 at 7:07
  • 2
    strace can not run in docker github.com/moby/moby/issues/20064, To turn off security features use: docker run --security-opt seccomp:unconfined ...
    – Mithril
    Jun 28 '17 at 7:23
  • I had the same sudo issue. Did it afterwards with sudo, and it's hanging on restart_syscall(<... resuming interrupted poll ...> Feb 13 '19 at 9:04
  • Is there an easy way to tell strace to only output ASCII line by line, as if I had started the process in a terminal? Right now I see both ASCII and hex but the ASCII is all garbled / split up over multiple lines.
    – balu
    Apr 12 at 14:06

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