Cannot watch a running log file

Question

I have a simple python program, sample.py:

import time
n = 1
while True:
    print 'Testing', n
    n += 1
    time.sleep (1)

When I run the program in bash as a background job:

$ python sample.py &> sample.log &

Now watch the log file:

$ watch tail -n5 sample.log

I don't see anything unless the program is stopped. Also, if view the sample.log file in any text editor, its empty.

So, how can I view the changes in the log file in real-time?

Try just using `tail -f -n5 sample.log`, the `-f` flag means to follow the file as it changes. — Matt Clark, Jun 02 '17 at 16:37
[Does this help](https://stackoverflow.com/a/107717/5067311)? — Andras Deak -- Слава Україні, Jun 02 '17 at 16:38
Unfortunately, that does not work. It seems that I can see the whole sample.log file once the program terminated. It shouldn't be the behavior though. Thanks anyway! — Debdut, Jun 02 '17 at 16:47

score 2 · Answer 1 · answered Jun 02 '17 at 16:53

2

Output gets buffered; flush the buffer after each print, and you'll see your watch command produce the expected output.

import time
import sys

n = 1
while True:
    print 'Testing', n
    sys.stdout.flush()
    n += 1
    time.sleep (1)

answered Jun 02 '17 at 16:53

Prune

1 Answers1