How to Run Cron Inside Docker Containers

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How to Run Cron Inside Docker Containers – Guide

As system administrators, we are inevitably faced with the need to schedule tasks. We can achieve this using cron services on Linux systems. Additionally, we can enable cron scheduling services on container systems. In this article, we will explain the best way to enable cron services in Docker containers. Context up a cron job inside a Docker container might not seem new to many of us. But depending on the base image we use to build the Docker image, we may face different issues.

It’s much better to run Docker cron jobs from the host system, not from inside Docker containers. Your Docker containers should only have one concern and should not be loaded with cron jobs.

How to Run Cron inside Docker Containers

Create a script file

First, create a script file as per your need, what commands or tasks you should run and save it in the “.sh” file. Here, I created a script.sh file:

!/bin/bashtouch/var/log/cron.logcrontab/etc/cron.d/container_cronjob

Create a cron job file

Now create a cron file and write the cron job details inside the file such as what time you want your job to run. Below is the sample cron file, in this the script is running every minute.

* * * * * echo “Test cron was run on $(date)” > /proc/1/fd/1 2>/proc/1/fd/2

Create a docker file

Finally, create a docker file and build your docker image. Below is an example of the Dockerfile.

# Pulling the ubuntu image with a specific tag from docker hub.FROM ubuntu:18.04# Adding the name of the maintainer (optional).MAINTAINER Nivesh_goyal# Updating the packages and installing the cron and vim editor if you later want to edit your build script inside your container. RUN apt-get update && apt-get install cron -y && apt-get install vim -y# Crontab file copied to cron.d.COPY directory ./files/cronjob /etc/cron.d/container_cronjob# script copied into container.COPY ./files/script.sh /script.sh# Giving executable permission to the script file.RUN chmod +x /script.sh# Executing commands to launch a container.CMD [“/bin/bash”, “-c”, “/script.sh && chmod 644 /etc/cron.d/container_cronjob && cron && tail -f /var/log/cron.log”]

Create a Docker image

Now run the following command to compile the Dockerfile. You can change the tag as per your need:

# docker build –tag cronjob:testing

Run the docker image inside the container

now turn up a container using the image above, run the following command:

# docker run -itd –name container_name cronjob:testing

If you want to check whether your cron is running or not, run the command below from outside the container:

# docker logs –tail 100 -f container_name

Final note

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