Get a variable from a running program

Question

I have a script that runs for days, and inside it there's a counter. The counter gets written to a file periodically, is it possible to find out the value that the counter is set from either another python script, linux command, or even java?

Example of simple python counter:

import time
import random

a = 0
while True:
    a +=1
    time.sleep(random.random())

I'm interested in the value of a at a given time.

Answer 1

Yes, you could use any IPC method e.g., you could send the counter over a socket:

self.request.sendall(json.dumps(dict(counter=a)).encode('ascii'))

---

If you want to get the value from an already running process that you can't modify then you could try to attach a debugger:

$ sudo gdb python <pid of running process>

To enable python-specific helper commands, add to your ~/.gdbinit:

add-auto-load-safe-path /path/to/python-gdb.py

The example gdb session could look like:

>>> thread apply all py-list

Thread 1 (Thread 0x7f68ff397700 (LWP 9807)):
   2    import random
   3    
   4    a = 0
   5    while True:
   6        a +=1
  >7        time.sleep(random.random())
>>> py-print a
global 'a' = 83

From another Python script, you could run gdb in the batch mode:

#!/usr/bin/env python
import shlex
import subprocess

cmd = shlex.split("sudo gdb --batch -ex 'py-print a' python") + [str(pid)]
output = subprocess.check_output(cmd, stderr=subprocess.DEVNULL,
                                 cwd=path_to_python_gdb)
a = int(output.rsplit(b'\n', 2)[-2].rpartition(b' ')[2])