Cronjobs-in-Docker

This repository shows how to construct a simple alpine docker container and running/scheduling cronjobs within that container, it is important to note that the script.sh file and the python script (should there be one) have to be located at the same directory level as the dockerfile in order to work. Note that both the Dockerfile and the script.sh produce individual log files separately from each other. Once the container is up and running these log files can be found in /var/log/ The code here produces a total of two logs as output, the Dockerfile produces a cron.log while the script produces a script.log

CLI command to build and run docker container

docker build -t cron-job .
docker run -d --name cron-container cron-job

To check logs to this container docker logs -f cron-container

To stop and remove container

docker stop cron-container
docker rm cron-container

How to check both logs

1. Viewing the Dockerfile container log

To check the log that contains the cron job's output

Inside the container:

docker exec -it cron-container /bin/sh
cat /var/log/cron.log

Or if you want to follow the log as new entries are added

docker exec -it cron-container /bin/sh
tail -f /var/log/cron.log

Copying to the Host:

docker cp cron-container:/var/log/cron.log /path/to/destination/on/host/
cat /path/to/destination/on/host/cron.log

2. Viewing the Script specific log (script.sh)

To check the log that contains the specific output from the script

Inside the container:

docker exec -it cron-container /bin/sh
cat /var/log/script.log

Or if you want to follow the log as new entries are added

docker exec -it cron-container /bin/sh
tail -f /var/log/script.log

Running a Python Script

1. Running Python code directly via cron in Docker

Dockerfile

# Use a lightweight Linux distribution as a base
FROM python:3.11-alpine

# Install cron and any other dependencies
RUN apk update && apk add --no-cache cronie

# Install Python packages (if necessary)
RUN pip install --no-cache-dir external-package-you-want-to-install

# Copy the Python script into the container
COPY script.py /usr/local/bin/script.py

# Add the cron job directly to the root's crontab
RUN echo "* * * * * python3 /usr/local/bin/script.py >> /var/log/cron.log 2>&1" > /etc/crontabs/root

# Create the log file & make sure it's accessible
RUN touch /var/log/cron.log

# Start the cron service when the container starts
CMD ["crond", "-f", "-s"]

Any output produced by the python file will be directly piped and appended over to the /var/log/cron.log file here

Python Script

# script.py
import datetime

with open("/var/log/script.log", "a") as log_file:
    log_file.write(f"This Python script ran at {datetime.datetime.now()}\n")

2. Running Python script via a Shell script in Docker

Dockerfile

# Use a lightweight Linux distribution as a base
FROM python:3.11-alpine

# Install cron and any other dependencies
RUN apk update && apk add --no-cache bash cronie

# Install Python packages (if necessary)
RUN pip install --no-cache-dir external-package-you-want-to-install

# Copy the Python script and shell script into the container
COPY script.py /usr/local/bin/script.py
COPY script.sh /usr/local/bin/script.sh

# Make sure the shell script is executable
RUN chmod +x /usr/local/bin/script.sh

# Add the cron job that runs the shell script, since the shell script below already directs output into script.log we can discard output here
RUN echo "* * * * * /usr/local/bin/script.sh >> /dev/null 2>&1" > /etc/crontabs/root

# Create the log file & make sure it's accessible
RUN touch /var/log/cron.log

# Start the cron service when the container starts
CMD ["crond", "-f", "-s"]

Shell script

#!/bin/bash

# Run python script and then append logs to /var/log/script.log 
python3 /usr/local/bin/script.py >> /var/log/script.log 2>&1

Python script is the same as the first example

Log errors and discard regular output

RUN echo "* * * * * /usr/local/bin/script.sh > /dev/null 2>> /var/log/script-errors.log" > /etc/crontabs/root

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