How to run a cron job inside a docker container?
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 not really find a solution that works.
How can I do this?
EDIT:
I've created a (commented) github repository with a working docker cron container that invokes a shell script at given interval.
Accepted Answer
You can copy your crontab into an image, in order for the container launched from said image to run the job.
See "Run a cron job with Docker" from Julien Boulay in his Ekito/docker-cron
:
Let’s create a new file called "
hello-cron
" to describe our job.
* * * * * echo "Hello world" >> /var/log/cron.log 2>&1
# An empty line is required at the end of this file for a valid cron file.
The following Dockerfile describes all the steps to build your image
FROM ubuntu:latest
MAINTAINER [email protected]
RUN apt-get update && apt-get -y install cron
# Copy hello-cron file to the cron.d directory
COPY hello-cron /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/cron.log
# Run the command on container startup
CMD cron && tail -f /var/log/cron.log
(see Gaafar's comment and How do I make apt-get
install less noisy?:
apt-get -y install -qq --force-yes cron
can work too)
As noted by Nathan Lloyd in the comments:
Quick note about a gotcha:
If you're adding a script file and telling cron to run it, remember to
RUN chmod 0744 /the_script
Cron fails silently if you forget.
OR, make sure your job itself redirect directly to stdout/stderr instead of a log file, as described in hugoShaka's answer:
* * * * * root echo hello > /proc/1/fd/1 2>/proc/1/fd/2
Replace the last Dockerfile line with
CMD ["cron", "-f"]
See also (about cron -f
, which is to say cron "foreground") "docker ubuntu cron -f
is not working"
Build and run it:
sudo docker build --rm -t ekito/cron-example .
sudo docker run -t -i ekito/cron-example
Be patient, wait for 2 minutes and your commandline should display:
Hello world
Hello world
Eric adds in the comments:
Do note that
tail
may not display the correct file if it is created during image build.
If that is the case, you need to create or touch the file during container runtime in order for tail to pick up the correct file.
See "Output of tail -f
at the end of a docker CMD
is not showing".
Popular Answer
The adopted solution may be dangerous in a production environment.
In docker you should only execute one process per container because if you don't, the process that forked and went background is not monitored and may stop without you knowing it.
When you use CMD cron && tail -f /var/log/cron.log
the cron process basically fork in order to execute cron
in background, the main process exits and let you execute tailf
in foreground. The background cron process could stop or fail you won't notice, your container will still run silently and your orchestration tool will not restart it.
You can avoid such a thing by redirecting directly the cron's commands output into your docker
stdout
andstderr
which are located respectively in/proc/1/fd/1
and/proc/1/fd/2
.
Using basic shell redirects you may want to do something like this :
* * * * * root echo hello > /proc/1/fd/1 2>/proc/1/fd/2
And your CMD will be : CMD ["cron", "-f"]
Read more... Read less...
For those who wants to use a simple and lightweight image:
FROM alpine:3.6
# copy crontabs for root user
COPY config/cronjobs /etc/crontabs/root
# start crond with log level 8 in foreground, output to stderr
CMD ["crond", "-f", "-d", "8"]
Where cronjobs is the file that contains your cronjobs, in this form:
* * * * * echo "hello stackoverflow" >> /test_file 2>&1
# remember to end this file with an empty new line
What @VonC has suggested is nice but I prefer doing all cron job configuration in one line. This would avoid cross platform issues like cronjob location and you don't need a separate cron file.
FROM ubuntu:latest
# Install cron
RUN apt-get -y install cron
# Create the log file to be able to run tail
RUN touch /var/log/cron.log
# Setup cron job
RUN (crontab -l ; echo "* * * * * echo "Hello world" >> /var/log/cron.log") | crontab
# Run the command on container startup
CMD cron && tail -f /var/log/cron.log
After running your docker container, you can make sure if cron service is working by:
# To check if the job is scheduled
docker exec -ti <your-container-id> bash -c "crontab -l"
# To check if the cron service is running
docker exec -ti <your-container-id> bash -c "pgrep cron"
If you prefer to have ENTRYPOINT instead of CMD, then you can substitute the CMD above with
ENTRYPOINT cron start && tail -f /var/log/cron.log
There is another way to do it, is to use Tasker, a task runner that has cron (a scheduler) support.
Why ? Sometimes to run a cron job, you have to mix, your base image (python, java, nodejs, ruby) with the crond. That means another image to maintain. Tasker avoid that by decoupling the crond and you container. You can just focus on the image that you want to execute your commands, and configure Tasker to use it.
Here an docker-compose.yml
file, that will run some tasks for you
version: "2"
services:
tasker:
image: strm/tasker
volumes:
- "/var/run/docker.sock:/var/run/docker.sock"
environment:
configuration: |
logging:
level:
ROOT: WARN
org.springframework.web: WARN
sh.strm: DEBUG
schedule:
- every: minute
task: hello
- every: minute
task: helloFromPython
- every: minute
task: helloFromNode
tasks:
docker:
- name: hello
image: debian:jessie
script:
- echo Hello world from Tasker
- name: helloFromPython
image: python:3-slim
script:
- python -c 'print("Hello world from python")'
- name: helloFromNode
image: node:8
script:
- node -e 'console.log("Hello from node")'
There are 3 tasks there, all of them will run every minute (every: minute
), and each of them will execute the script
code, inside the image defined in image
section.
Just run docker-compose up
, and see it working. Here is the Tasker repo with the full documentation:
VonC's answer is pretty thorough. In addition I'd like to add one thing that helped me. If you just want to run a cron job without tailing a file, you'd be tempted to just remove the && tail -f /var/log/cron.log
from the cron command.
However this will cause the Docker container to exit shortly after running because when the cron command completes, Docker thinks the last command has exited and hence kills the container. This can be avoided by running cron in the foreground via cron -f
.
Though this aims to run jobs beside a running process in a container via Docker's exec
interface, this may be of interest for you.
I've written a daemon that observes containers and schedules jobs, defined in their metadata, on them. Example:
version: '2'
services:
wordpress:
image: wordpress
mysql:
image: mariadb
volumes:
- ./database_dumps:/dumps
labels:
deck-chores.dump.command: sh -c "mysqldump --all-databases > /dumps/dump-$$(date -Idate)"
deck-chores.dump.interval: daily
'Classic', cron-like configuration is also possible.
Here are the docs, here's the image repository.