Initial commit

Dockerfile

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+FROM ubuntu
+
+MAINTAINER "Julien Dubreuil"
+
+RUN apt-get update && apt-get install -y cron
+RUN rm -rf /var/lib/apt/lists/*
+
+COPY crontab /tmp/crontab
+COPY run-crond.sh /run-crond.sh
+
+RUN chmod -v +x /run-crond.sh
+
+RUN touch /var/log/cron.log
+
+CMD ["/run-crond.sh"]

LICENSE.txt

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+Copyright 2017 Julien Dubreuil
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# Creating a container for cron jobs
+
+This is an example of how to create a docker container to run scheduled commands
+using the crontab.
+
+You can run it using
+
+    $ docker build -t docker-cron-example .
+
+    $ docker build -it --name cron-example docker-cron-example
+
+Few minutes after having started the container you should see something similar to this:
+
+```
+Hello
+Hello
+Hello
+```
+
+## Using environment variables
+
+Once the simple cron job is working, the main goal was to access environment variables in the command. For the example a simple "hello $world" is displayed but you may want to use variables inside a script. Unfortunately, the command executed by cron had not access to the container's environment variables. Thanks to this useful [stackoverflow thread](http://stackoverflow.com/questions/26822067/running-cron-python-jobs-within-docker) who gave me the solution who show me how to inject variables to the crontab file.
+
+ The following command included in [run-crond.sh](run-crond.sh) loads all the environment variables, gets the one starting with ENV_, combine them at the top of the /tmp/crontab file and finally move the result to /etc/cron.d/crontab
+
+    env | egrep '^ENV_' | cat - /tmp/crontab > /etc/cron.d/crontab
+
+Add env variable to the docker run command to make use of them:
+
+    $ docker run -it -e ENV_NAME=f00 docker-cron-example
+
+This should output you the following log.
+
+```
+Hello f00
+Hello f00
+Hello f00
+```
+
+## Accessing to the logs
+
+You can access to the crontab output by running docker logs on your container
+
+    $ docker run -d -e ENV_NAME=f00 docker-cron-example
+    $ docker logs <container_name>
+    hello f00
+    hello f00    
+
+## The Dockerfile
+
+```
+FROM ubuntu
+
+MAINTAINER "Julien Dubreuil"
+
+RUN apt-get update && apt-get install -y \
+       cron \ 
+&& rm -rf /var/lib/apt/lists/*
+
+COPY crontab /tmp/crontab
+
+COPY run-crond.sh /run-crond.sh
+RUN chmod -v +x /run-crond.sh
+
+RUN touch /var/log/cron.log
+
+CMD ["/run-crond.sh"]
+
+```
+
+1. The FROM command specify our base image. It used a ubuntu image as our base image but you can use a smaller one like Alpine and so one.
+
+2. We install the cron package using the RUN command and in addition, we clean up the apt cache by removing /var/lib/apt/lists in orddr to reduce the image size.
+
+3. Next, we copy the crontab file to the local directory of the container in order to make it available as declared in the run-crond.sh file.
+
+3. Then, we copy the crontab run script to the container and make it executable.
+
+4. We Create a default log file for the cron job.
+
+5. Finally, we specify the command that container will execute on startup.
+
+## Copyright and License
+
+MIT License, see [LICENSE](License.txt) for details.
+
+Copyright (c) 2017 Julien Dubreuil

crontab

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+* * * * * root echo "Hello ${ENV_NAME}" >> /var/log/cron.log 2>&1
+# Don't forget the mandatory empty line at the end of the file to make the cron file valid.

run-crond.sh

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+#!/bin/bash
+# Docker's environment variable are not passed to cron. We need to inject them
+# inside the result file. The following command will add all variables starting
+# with ENV_ to the top of the result file.
+env | egrep '^ENV_' | cat - /tmp/crontab > /etc/cron.d/crontab
+
+cron && tail -f /var/log/cron.log