I am trying to build docker with simple python script the read from a file and print it.
I want that the python script will continue to read from the file as new data may be available in the file.
But when the file does not exist when the scripts start it seems that tail does not work and does not read the file.
Here is simple example:
dockerfile (uncomment to illustrate the problem)
FROM ubuntu:18.10
RUN apt-get update && apt-get install -y --no-install-recommends python3 && rm -rf /var/lib/apt/lists/*
#RUN touch /tmp/file # uncomment will cause the problem
COPY . /app/
CMD ["python3","/app/main.py"]
main.py
import subprocess
argsList = ['tail', '-c-1', '-F', '/tmp/file']
f = subprocess.Popen(argsList, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
line = f.stdout.readline()
while line != b'':
line = f.stdout.readline().decode("utf-8")
print(line)
If I enter to the docker (when there is no /tmp/file) and start to write to that file (using echo "text" >> /tmp/file) I can see the output in the docker screen.
But if I uncomment the touch comment in the dockerfile (that cause /tmp/file to be created) I can not see any output in the docker screen although I use the same command(echo "text" >> /tmp/file).
Why there is such a diffrent and how I can see the output even if the file exists?
-- edit --
I have tried to add exit(1) after the first read line to negate buffer problem.
The result was pretty much the same:
When the line was commant the container exit as expected (with exit code 1)
but when the line was uncommant the container does not exit at all (probably stuck on reading from the pipe).
main.py
import subprocess
argsList = ['tail', '-c-1', '-F', '/tmp/file']
f = subprocess.Popen(argsList, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
line = f.stdout.readline()
exit(1)
-- edit 2 --
The problem happen also if there is only the docker file:
dockerfile
FROM ubuntu:18.10
#RUN touch /tmp/file
CMD "tail" "-c-1" "-F" "/tmp/file"
