How to run a cron job inside a docker container?

Question

I am trying to run a cronjob inside a docker container that invokes a shell script.

Yesterday I have been searching all over the web and stack overflow, but I could

Answer 1

If you're using docker for windows, remember that you have to change your line-ending format from CRLF to LF (i.e. from dos to unix) if you intend on importing your crontab file from windows to your ubuntu container. If not, your cron-job won't work. Here's a working example:

FROM ubuntu:latest

RUN apt-get update && apt-get -y install cron
RUN apt-get update && apt-get install -y dos2unix

# Add crontab file (from your windows host) to the cron directory
ADD cron/hello-cron /etc/cron.d/hello-cron

# Change line ending format to LF
RUN dos2unix /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/hello-cron.log

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

This actually took me hours to figure out, as debugging cron jobs in docker containers is a tedious task. Hope it helps anyone else out there that can't get their code to work!

Answer 2

When you deploy your container on another host, just note that it won't start any processes automatically. You need to make sure that 'cron' service is running inside your container. In our case, I am using Supervisord with other services to start cron service.

[program:misc]
command=/etc/init.d/cron restart
user=root
autostart=true
autorestart=true
stderr_logfile=/var/log/misc-cron.err.log
stdout_logfile=/var/log/misc-cron.out.log
priority=998

Answer 3

this line was the one that helped me run my pre-scheduled task.

ADD mycron/root /etc/cron.d/root

RUN chmod 0644 /etc/cron.d/root

RUN crontab /etc/cron.d/root

RUN touch /var/log/cron.log

CMD ( cron -f -l 8 & ) && apache2-foreground # <-- run cron

--> My project run inside: FROM php:7.2-apache

Answer 4

Define the cronjob in a dedicated container which runs the command via docker exec to your service.

This is higher cohesion and the running script will have access to the environment variables you have defined for your service.

#docker-compose.yml
version: "3.3"
services:
    myservice:
      environment:
        MSG: i'm being cronjobbed, every minute!
      image: alpine
      container_name: myservice
      command: tail -f /dev/null

    cronjobber:
     image: docker:edge
     volumes:
      - /var/run/docker.sock:/var/run/docker.sock
     container_name: cronjobber
     command: >
          sh -c "
          echo '* * * * * docker exec myservice printenv | grep MSG' > /etc/crontabs/root
          && crond -f"

Answer 5

Setup a cron in parallel to a one-time job

Create a script file, say run.sh, with the job that is supposed to run periodically.

#!/bin/bash
timestamp=`date +%Y/%m/%d-%H:%M:%S`
echo "System path is $PATH at $timestamp"

Save and exit.

Use Entrypoint instead of CMD

f you have multiple jobs to kick in during docker containerization, use the entrypoint file to run them all.

Entrypoint file is a script file that comes into action when a docker run command is issued. So, all the steps that we want to run can be put in this script file.

For instance, we have 2 jobs to run:

Run once job: echo “Docker container has been started”

Run periodic job: run.sh

Create entrypoint.sh

#!/bin/bash

# Start the run once job.
echo "Docker container has been started"

# Setup a cron schedule
echo "* * * * * /run.sh >> /var/log/cron.log 2>&1
# This extra line makes it a valid cron" > scheduler.txt

crontab scheduler.txt
cron -f

Let’s understand the crontab that has been set up in the file

* * * * *: Cron schedule; the job must run every minute. You can update the schedule based on your requirement.

/run.sh: The path to the script file which is to be run periodically

/var/log/cron.log: The filename to save the output of the scheduled cron job.

2>&1: The error logs(if any) also will be redirected to the same output file used above.

Note: Do not forget to add an extra new line, as it makes it a valid cron. Scheduler.txt: the complete cron setup will be redirected to a file.

Using System/User specific environment variables in cron

My actual cron job was expecting most of the arguments as the environment variables passed to the docker run command. But, with bash, I was not able to use any of the environment variables that belongs to the system or the docker container.

Then, this came up as a walkaround to this problem:

1. Add the following line in the entrypoint.sh

declare -p | grep -Ev 'BASHOPTS|BASH_VERSINFO|EUID|PPID|SHELLOPTS|UID' > /container.env

2. Update the cron setup and specify-

SHELL=/bin/bash
BASH_ENV=/container.env

At last, your entrypoint.sh should look like

#!/bin/bash

# Start the run once job.
echo "Docker container has been started"

declare -p | grep -Ev 'BASHOPTS|BASH_VERSINFO|EUID|PPID|SHELLOPTS|UID' > /container.env

# Setup a cron schedule
echo "SHELL=/bin/bash
BASH_ENV=/container.env
* * * * * /run.sh >> /var/log/cron.log 2>&1
# This extra line makes it a valid cron" > scheduler.txt

crontab scheduler.txt
cron -f

Last but not the least: Create a Dockerfile

FROM ubuntu:16.04
MAINTAINER Himanshu Gupta

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

# Add files
ADD run.sh /run.sh
ADD entrypoint.sh /entrypoint.sh

RUN chmod +x /run.sh /entrypoint.sh

ENTRYPOINT /entrypoint.sh

That’s it. Build and Run the Docker image!

Answer 6

When running on some trimmed down images that restrict root access, I had to add my user to the sudoers and run as sudo cron

FROM node:8.6.0
RUN apt-get update && apt-get install -y cron sudo

COPY crontab /etc/cron.d/my-cron
RUN chmod 0644 /etc/cron.d/my-cron
RUN touch /var/log/cron.log

# Allow node user to start cron daemon with sudo
RUN echo 'node ALL=NOPASSWD: /usr/sbin/cron' >>/etc/sudoers

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

Maybe that helps someone