Crontab in docker container

Question

Hi I'm trying to run a cron job in a docker container. So I have added that in my Dockerfile

My Dockerfile

FROM nginx:stable

RUN apt-get update -y && apt-get upgrade -y && apt-get install -y \
    vim \
    git \
    curl \
    wget \
    certbot \
    cron

COPY cron/crontab /etc/crontab
RUN chmod 0644 /etc/cron.d/crontab
RUN /etc/init.d/cron start 

My crontab file

# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.

SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user command
*/1 *   *   *   * root echo "test" >>~/readme

But it does not work.

I have to run the command /etc/init.d/cron start manually in my nginx container if I want it to work.

So I have added an entrypoint in my Dockerfile, so this command can be executed when the container start.

# ENTRYPOINT
ADD entrypoint.sh /entrypoint.sh
RUN chmod 777 /entrypoint.sh

My entrypoint.sh

#!/usr/bin/env bash

/etc/init.d/cron start

My docker-compose

entrypoint: /entrypoint.sh

But I have this error:

OCI runtime exec failed: exec failed: container_linux.go:296: starting container process caused "process_linux.go:86: executing setns process caused \"exit status 21\"": unknown

Did I miss something?

PS: I have followed this tutorial

Answer 1

Few days ago i was struggling with similar problem and here are few notes based on what i have learnt first:

• To be able to run a container like a service, somewhere (in cmd or entrypoint) must be a foreground program running. For your case it would be nginx -g daemon off; (from nginx image).

• Also if there is an entrypoint and a cmd, they are launched in the way that the cmd is passed to the entrypoint as the argument (just like ./entrypoint.sh [cmd])

• There is no point of having RUN /etc/init.d/cron start since after the image is built, it will be terminated anyway.

And to the solution: It was using entrypoint.sh similar to this one (in my case django/gunicorn/cron):

#!/bin/bash

set -e # exit on any error

if [ "$1" = './gunicorn.sh' ]; then  # './gunicorn.sh' is default command
  cron                               # this runs cron as daemon
  crontab -u django /app/crontab     # this applies a crontab from a file
  exec su django -c "$@"             # this actually runs the default command as different user (you can just use exec "$@" if you dont need to run it as different user)
fi

exec "$@" # this runs any other cmd without starting cron (e.g. docker run -ti myimage /bin/bash)

Answer 2

What are you using as the base for your Docker image (i.e. what's in the FROM line)?

Many docker base images start with a minimal install of an existing distribution (e.g. alpine, debian, ubuntu, centos, etc) and then you add whatever packages you need to it.

If your base image doesn't include cron by default, install it in your Dockerfile using the distribution's appropriate package tool (e.g. apt-get install cron), and then configure it as you would on any other system - e.g. by adding an entry to a system crontab file like /etc/crontab, or by dropping an executable script in /etc/cron.d.

In some cases, cron may be installed but disabled. You need to modify your container's startup script so that it starts crond. This is not the same as using the RUN command in the Dockerfile (that's for running commands during the container build process, not on startup of each container). The details of how to do that vary according to the base image you used.