Getting output from a running process

Question

I have a process running on a Linux machine that I would like to access the output from. It's in it's own container that I'm using docker exec to su into. With ps I can see:

UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 18:45 ?        00:00:00 python ./manage.py runserver

I thought I could do this with

fg 1

but that did not work. How can I capture the output of this already running process?

Answer 1

Use docker logs <yourcontainer> on the host to read its stdout. Add --follow to keep the output going.

Answer 2

You can only fg a job (or process) which you have run on the current terminal and either suspended (e. g. with Ctrl+Z) and/or sent to run in the background (e. g. with bg). Chances are exceedingly unlikely that PID 1 is on the current terminal; it is usually binding its standard input, output, and error streams to /dev/null. You can check with:

$ sudo ls -l /proc/1/fd/{0,1,2}
lrwx------ 1 root root 64 Mar 28 15:13 /proc/1/fd/0 -> /dev/null
lrwx------ 1 root root 64 Mar 28 15:13 /proc/1/fd/1 -> /dev/null
lrwx------ 1 root root 64 Mar 28 15:13 /proc/1/fd/2 -> /dev/null

You can only check on the file descriptors for processes which you own, which is why sudo is needed here. The roles of these descriptors are as follows for any process:

• fd/0 - Standard input
• fd/1 - Standard output
• fd/2 - Standard error