docker permissions issue writing to txt file with simple python script

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why do I need permission to write to a txt file? How do I solve this problem

main.py

text_file = open("sample.txt", "w")
n = text_file.write('some words dzfsadfadfs')
text_file.close()

Dockerfile

FROM rayproject/ray
ADD main.py .
CMD ["python", "main.py"]

error

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    text_file = open("sample.txt", "w")
PermissionError: [Errno 13] Permission denied: 'sample.txt'
docker python
1 Answers

The problem is that the base image you're using (rayproject/ray) is configured to run as a non-root user (ray), but sets the working directory to /. There is a writeable directory /home/ray available; the easiest solution is probably adding an appropriate WORKDIR directive to your Dockerfile:

FROM rayproject/ray
WORKDIR /home/ray
ADD main.py .
CMD ["python", "main.py"]

Update

I figured this out by first spawning a shell and checking what uid the process was running as:

$ docker run -it --rm testimage bash
(base) ray@bca6b7788820:~$ id
uid=1000(ray) gid=100(users) groups=100(users),27(sudo)

I checked the passwd file for information about that user:

(base) ray@bca6b7788820:~$ grep ray /etc/passwd
ray:x:1000:100::/home/ray:/bin/bash

And verified that directory was writeable:

(base) ray@bca6b7788820:~$ cd /home/ray
(base) ray@bca6b7788820:~$ touch foo

In general you can also figure things like this out by inspecting the corresponding Dockerfile, but for this particular base image that ended up being a bit of a pain because you need to track down a chain of dependencies:

And if you look at that last Dockerfile, you find:

USER $RAY_UID
ENV HOME=/home/ray
user_1473560
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