How to run a cron job inside a docker container?

You can copy your crontab into an image, in order for the container launched from said image to run the job.

See "Run a cron job with Docker" from Julien Boulay in his Ekito/docker-cron:

Let’s create a new file called "hello-cron" to describe our job.

* * * * * 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 are wondering what is 2>&1, Ayman Hourieh explains.

The following Dockerfile describes all the steps to build your image

FROM ubuntu:latest
MAINTAINER [email protected]

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

(see Gaafar's comment and How do I make apt-get install less noisy?:
apt-get -y install -qq --force-yes cron can work too)

As noted by Nathan Lloyd in the comments:

Quick note about a gotcha:
If you're adding a script file and telling cron to run it, remember to
RUN chmod 0744 /the_script
Cron fails silently if you forget.

OR, make sure your job itself redirect directly to stdout/stderr instead of a log file, as described in hugoShaka's answer:

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

Replace the last Dockerfile line with

CMD ["cron", "-f"]

See also (about cron -f, which is to say cron "foreground") "docker ubuntu cron -f is not working"

Build and run it:

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

Be patient, wait for 2 minutes and your commandline should display:

Hello world
Hello world

Eric adds in the comments:

Do note that tail may not display the correct file if it is created during image build.
If that is the case, you need to create or touch the file during container runtime in order for tail to pick up the correct file.

See "Output of tail -f at the end of a docker CMD is not showing".

See more in "Running Cron in Docker" (Apr. 2021) from Jason Kulatunga, as he commented below

See Jason's image AnalogJ/docker-cron based on:

• Dockerfile installing cronie/crond, depending on distribution.

• an entrypoint initializing /etc/environment and then calling

  cron -f -l 2
  

The accepted answer may be dangerous in a production environment.

In docker you should only execute one process per container because if you don't, the process that forked and went background is not monitored and may stop without you knowing it.

When you use CMD cron && tail -f /var/log/cron.log the cron process basically fork in order to execute cron in background, the main process exits and let you execute tailf in foreground. The background cron process could stop or fail you won't notice, your container will still run silently and your orchestration tool will not restart it.

You can avoid such a thing by redirecting directly the cron's commands output into your docker stdout and stderr which are located respectively in /proc/1/fd/1 and /proc/1/fd/2.

Using basic shell redirects you may want to do something like this :

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

And your CMD will be : CMD ["cron", "-f"]

For those who wants to use a simple and lightweight image:

FROM alpine:3.6

# copy crontabs for root user
COPY config/cronjobs /etc/crontabs/root

# start crond with log level 8 in foreground, output to stderr
CMD ["crond", "-f", "-d", "8"]

Where cronjobs is the file that contains your cronjobs, in this form:

* * * * * echo "hello stackoverflow" >> /test_file 2>&1
# remember to end this file with an empty new line

What @VonC has suggested is nice but I prefer doing all cron job configuration in one line. This would avoid cross platform issues like cronjob location and you don't need a separate cron file.

FROM ubuntu:latest

# Install cron
RUN apt-get -y install cron

# Create the log file to be able to run tail
RUN touch /var/log/cron.log

# Setup cron job
RUN (crontab -l ; echo "* * * * * echo "Hello world" >> /var/log/cron.log") | crontab

# Run the command on container startup
CMD cron && tail -f /var/log/cron.log

After running your docker container, you can make sure if cron service is working by:

# To check if the job is scheduled
docker exec -ti <your-container-id> bash -c "crontab -l"
# To check if the cron service is running
docker exec -ti <your-container-id> bash -c "pgrep cron"

If you prefer to have ENTRYPOINT instead of CMD, then you can substitute the CMD above with

ENTRYPOINT cron start && tail -f /var/log/cron.log