Why doesn't my docker actually log anything?

logging docker

11,458

Without a tty attached, python is likely buffering stdout. There are lots of ways to solve this, including adding this change to your script:

print(datetime.now(), flush=True)

Or without changing your script, you can run stdbuf to turn off output buffering:

CMD ["stdbuf", "-oL", "python", "test.py"]

11,458

Wayne Werner

I'm a husband to my beautiful wife, father to our children, Computer Science graduate from University of Central Arkansas. I love my wife, our children, computers, playing guitar (especially singing/playing for my wife and/or kids), bicycling (including taking my kids for rides in my bike trailer), woodworking, airbrushing, digital and traditional artistry, playing games with my family (traditional and digital), my poor Chevette that I had to sell, throwing knives, firearms, knot tying, rope making, whip making, and really just learning new stuff in general. If I don't know about it I probably want to learn about it, if only so I can make informed decisions about it. My three favorite programming languages are Python, Lisp, and Assembly (though I'm not sure about the order of those last two languages...). I think the CANSPAM act is one of the dumbest pieces of legislation in the history of the universe I love Python and HTML+Javascript. I'm #SOreadytohelp (I really want a t-shirt, what can I say?)

Updated on September 18, 2022

Comments

Wayne Werner over 1 year
I have the following python file:
```
 import time
 from datetime import datetime


 while True:
     time.sleep(1)
     print(datetime.now())
```
I've added it to a Dockerfile:
```
FROM python:3.7


WORKDIR /test
COPY test.py /test/
CMD ["python", "test.py"]
```
If I build this and run with
```
sudo docker run -it --name test myimage
```
everything is awesome. Every second or so it prints the current time.

On the other hand, if I run it with:
```
sudo docker run -d --name test myimage
```
It runs off into the background, but if I do...
```
sudo docker logs test
```
I get nothing! Not a darn thing.

Any clue why this is happening, or how I could figure it out? Both when I'm using -d and not when I run docker inspect it gives me "Type": "json-file", "Config": {}

I'm runing Docker 18.06.1-ce, build e68fc7a, on Ubuntu 18.04.
- BMitch over 5 years
  
  Check if it's IO buffering: stackoverflow.com/q/107705/596285
- Wayne Werner over 5 years
  
  @BMitch that was it, thanks! I spent a couple of hours last night fighting this darn thing.
Wayne Werner over 5 years

Because I was using another command (i.e. waitress-serve in my case), so I added ENV PYTHONUNBUFFERED definitely to my Dockerfile (it could be any non-empty string, FWIW)