How to write docker cron logs to stdout/stderr

Question

I have docker where cron is ran using CMD ["cron","-f"] command. This writes the output to the cron log file.

Is there any way to redirect these logs to the console.

My Base Image looks like

FROM ubuntu:latest
RUN ls

RUN apt-get update
RUN apt install -y software-properties-common
RUN add-apt-repository -y ppa:openjdk-r/ppa
RUN apt-get update
RUN apt-get install -y openjdk-12-jre

RUN apt-get -y install cron wget unzip



RUN wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | apt-key add - \
    && echo "deb http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google-chrome.list \
    && apt-get update -qqy \
    && apt-get -qqy install google-chrome-stable \
    && rm /etc/apt/sources.list.d/google-chrome.list \
    && rm -rf /var/lib/apt/lists/* /var/cache/apt/* \
    && sed -i 's/"$HERE\/chrome"/"$HERE\/chrome" --no-sandbox/g' /opt/google/chrome/google-chrome


ARG CHROME_DRIVER_VERSION=76.0.3809.68
RUN wget --no-verbose -O /tmp/chromedriver_linux64.zip https://chromedriver.storage.googleapis.com/$CHROME_DRIVER_VERSION/chromedriver_linux64.zip \
    && rm -rf /opt/chromedriver \
    && unzip /tmp/chromedriver_linux64.zip -d /opt \
    && rm /tmp/chromedriver_linux64.zip \
    && mv /opt/chromedriver /opt/chromedriver-$CHROME_DRIVER_VERSION \
    && chmod 755 /opt/chromedriver-$CHROME_DRIVER_VERSION \
    && ln -fs /opt/chromedriver-$CHROME_DRIVER_VERSION /usr/bin/chromedriver

COPY tests-cron /etc/cron.d/tests-cron
RUN chmod 0644 /etc/cron.d/tests-cron
WORKDIR /etc/cron.d
RUN ls
RUN crontab /etc/cron.d/tests-cron

RUN touch /var/log/cron.log



WORKDIR /app
RUN mkdir pricetest

COPY --from=maven-container /app/ pricetest/
RUN cp /usr/bin/chromedriver ./pricetest

WORKDIR /app/pricetest
ENV SHELL=/bin/bash

CMD ["cron", "-f"]

PS: I am still work in progress on this project. So I havent optimized the docker build.

And the cronfile ( tests-cron ) I am contains

* * * * * root echo "Hello world"

Base image used: ubuntu:latest

Answer 1

Yes, there is a way to redirect logs to console, All you need to change the entry point of your Dockerfile. Here is the working example base on alpine which runs a cronjob every minute with output.

Alpine image

CMD flags

-f  Foreground
-L FILE Log to FILE

Dockerfile

FROM alpine:latest
RUN echo "* * * * * echo hello" | crontab - 
CMD ["crond","-f", "-L", "/dev/stdout"]

Another thing, Please always share your base image so it's easy to understand the context of the issue.

Ubuntu image

The workaround in ubuntu is to replace the CMD a bit, but I will not recommend this approach as the cron as cron will not be the parent process of container. You can read more about this approach here and here

If you want to optimization then use alpine the future image of docker container and here is the difference in the size of these two image.

Alpine is just 5MB where Ubuntu is 91MB

Here is the working example of cron with output using base image Ubuntu

FROM ubuntu:latest
RUN apt-get update &&  apt-get -y install cron
RUN touch /var/log/cron.log
RUN (crontab -l ; echo "* * * * * echo "Hello world" >> /var/log/cron.log") | crontab
CMD cron && tail -f /var/log/cron.log

Answer 2

There is another way to get cron to log to stdout under Debian/Ubuntu, without writing to a log file. Find the details here