Executing cron jobs in docker containers: a step-by-step guide

Question

I'm trying to run a cronjob inside a docker container that calls a shell script.

I've been searching online and on stack overflow yesterday but I can't find a solution that really works.

How can I do this?

Answer 1

Accepted Answer Can be dangerous in a production environment.

When you use CMD cron && tail -f /var/log/cron.log, the cron process basically forks to execute cron in the background and the main process exits And let you execute tailf in the foreground. The background cron process may stop or fail without you noticing, your container will still run silently, and your orchestration tool will not restart it.

Using basic shell redirection, you may want to do the following:

* * * * * root echo hello > /proc/1/fd/1 2>/proc/1/fd/2

Your CMD will be: CMD ["cron", "-f"]

However: If you want to run the task as a non-root user .

Answer 2

You can copy crontab into an image so that containers launched from that image run the job.

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IMPORTANT: Like docker-cron issue 3< 中所述< /a>: Use LF instead of CRLF for cron files.

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SeeRunning cron jobs using Docker” /github.com/julienboulay" rel="noreferrer">Julien Boulay in his <代码>Ekito/docker -cron：

# must be ended with a new line "LF" (Unix) and not "CRLF" (Windows)
* * * * * echo "Hello world" >> /var/log/cron.log 2>&1
# An empty line is required at the end of this file for a valid cron file.

If you're wondering what 2>&1 is, Ayman Hourieh explains.

FROM ubuntu:latest
MAINTAINER docker@ekito.fr

RUN apt-get update && apt-get -y install cron

# Copy hello-cron file to the cron.d directory
COPY hello-cron /etc/cron.d/hello-cron
 
# Give execution rights on the cron job
RUN chmod 0644 /etc/cron.d/hello-cron

# Apply cron job
RUN crontab /etc/cron.d/hello-cron
 
# Create the log file to be able to run tail
RUN touch /var/log/cron.log
 
# Run the command on container startup
CMD cron && tail -f /var/log/cron.log

But: If cron dies, containercontinues to run.

(See Gaafar's comments and How to make apt-get install less noisy? :
apt-get -y install -qq --force-yes cron also works)

As Nathan Lloyd commented on :

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Alternatively, ensure that your job itself redirects directly to stdout/stderr rather than a log file, as hugoShaka answer :

* * * * * root echo hello > /proc/1/fd/1 2>/proc/1/fd/2

Replace the last Dockerfile line with

CMD ["cron", "-f"]

However: If you want to run the task as a non-root user .

See also (about cron -f, the cron "frontend") "docker ubuntu cron -f does not work "

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Build and run it:

sudo docker build --rm -t ekito/cron-example .
sudo docker run -t -i ekito/cron-example

Hello world
Hello world

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Eric added in comments:

See tail -f output > not shown " at the end of "docker CMD

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See "Running Cron in Docker" for more information (April 2021) by Jason Kulatunga, Because his comments are as follows

View Jason's image AnalogJ/docker-cron Based on:

• Dockerfile installs cronie/crond, depending on the distribution.

• Entry point initialization/etc/environmentThen call

  cron -f -l 2