How to view the logs of a container? - docker

Question

I'm having a Dockerfile
FROM centos:7
ENV container docker
RUN yum -y update && yum -y install net-tools && yum -y install initscripts && yum -y install epel-release && yum -y install nginx && yum clean all
RUN echo "daemon off;" >> /etc/nginx/nginx.conf
# expose Nginx ports http/https
EXPOSE 80 443
RUN curl https://www.sheldonbrown.com/web_sample1.html > /usr/share/nginx/index.html
RUN mv /usr/share/nginx/index.html /usr/share/nginx/html/index.html -f
RUN mkdir -p /local/nginx
RUN touch /local/nginx/start.sh
RUN chmod 755 /local/nginx/start.sh
RUN echo "#!/bin/bash" >> /local/nginx/start.sh
RUN echo "nginx" >> /local/nginx/start.sh
RUN echo "tail -f /var/log/nginx/access.log" >> /local/nginx/start.sh
ENTRYPOINT ["/bin/bash", "-c", "/local/nginx/start.sh"]
I'm building it with docker build -t "my_nginx" .
And then running it with docker run -i -t --rm -p 8888:80 --name nginx "my_nginx"
https://localhost:8888/ shows the page but no logging is shown.
If I press Ctrl-C, nginx is stopped but the tail on the logging is shown.
If I press Ctrl-C again, the container is gone.
Question: How can I let nginx run AND show the tail on the logging (which is preferably also visible using the "docker logs"-command)

Answer 1

The easiest way to accomplish this is just to use the Docker Hub nginx image, which deals with this for you. A Dockerfile that could be as little as
FROM nginx:1.19
# Specifically use ADD because it will fetch a URL
ADD https://www.sheldonbrown.com/web_sample1.html /usr/share/nginx/html/index.html
If you look at its Dockerfile it actually uses symbolic links to cause Nginx's "normal" logs to go to the container stdout
# forward request and error logs to docker log collector
RUN ln -sf /dev/stdout /var/log/nginx/access.log \
&& ln -sf /dev/stderr /var/log/nginx/error.log
That gets around most of the mechanics in your Dockerfile: you can just run nginx as the main container command without trying to use a second process to cat logs. You can basically trim out the entire last half, and get
FROM centos:7
ENV container docker
RUN yum -y install epel-release \
&& yum -y install net-tools initscripts nginx \
&& yum clean all
RUN echo "daemon off;" >> /etc/nginx/nginx.conf \
&& ln -s /dev/stdout /var/log/nginx/access.log \
&& ln -s /dev/stderr /var/log/nginx/error.log
RUN curl -o /usr/share/nginx/html/index.html https://www.sheldonbrown.com/web_sample1.html
EXPOSE 80 443
CMD ["nginx"]
Yet another possibility here (with any of these images) is to mount your own volume over the container's /var/log/nginx directory. That gives you your own host-visible directory of logs that you can inspect at your convenience.
mkdir logs
docker run -v $PWD/logs:/var/log/nginx -d ...
less logs/access.log
(In the shell script you construct in the Dockerfile, you use the Nginx daemon off directive to run as a foreground process, which means the nginx command will never exit on its own. That in turn means the script never advances to the tail line, which is why you don't get logs out.)

Answer 2

You should remove the tail command from the Dockerfile, run the container with docker run -it -d --rm -p 8888:80 --name nginx "my_nginx" and then use docker logs -f nginx.