How do I get the output of a program already running?

Question

I have a python program that outputs to stdout and a log file.

However, the person I am making this for wants to know if I can set it up so that he can see the output of the file, while it is running on a different terminal or in the background. For example the program launched at startup, and he wants to ssh into it and see the output it is running.

So is there a way to see the output of a process that is currently running?

Preferably in bash or python, if possible.

If it ouputs into a log file, he could do a tail -f file.log? — fredtantini, Jun 29 '15 at 13:56
I agree with @fredtantini, I use tail to watch a certain file, but I also use watch if I know there are multiple files in that folder that I want to keep an eye on. — Kevdog777, Jun 29 '15 at 14:05
See How can I disown a running process and associate it to a new screen shell? — jofel, Jun 29 '15 at 15:00
Problem with that screen is that requires keyboard input from my understanding, which is something I can't do as the program is meant to go on a headless server. — user3346931, Jun 29 '15 at 15:53

score 3 · Accepted Answer · edited Jun 29 '15 at 22:36

I see two options. One, he can use tail -f to see the log file as it's being written, or two, he can have the program start inside a screen (or similar) session to which he can (re-)attach later.

If he doesn't know the location of the log file, he can use top, ps or a similar tool to find the process ID, then run lsof -p1234 where 1234 is the process id to list the open files of that process.

score 1 · Answer 2 · answered Jun 29 '15 at 17:24

I personally prefer using less to do this same thing.

less your_file

After starting less, the F command (not a command line flag) will begin to actively monitor the end of the file. While watching the end of the file in this mode CTRL-c will stop appending output to less and allow you to page around. Very handy.

How do I get the output of a program already running?

2 Answers2