This came accross multiple project :

How can I run a command periodically ? Obviously, on Linux you can use a crontab.

But when docker arrives :

How can I run a crontab without explicitly redo a Dockerfile ? And there was too many options.

Current situation

• A website is built with ExpressJS on node so we have a process which run the web server.

• A script named my-awesome-command in the bin directory is executing some process.

• We are in development environment using this docker-compose file

  version: '2'
  services:
      myapp:
          build: .
          volumes:
              - .:/app
              - /app/node_modules
          ports:
              - 3000:3000
  

  And Dockerfile for the app :

  FROM node:8
  
  ADD . app
  WORKDIR app
  RUN npm install
  EXPOSE 3000
  
  ENTRYPOINT ["npm", "start"]
  

  Sample working repository : Running express web server

Give it a try

I want to run a job. So in the Dockerfile I will add some line to register this cron.

FROM node:8

ADD . app
WORKDIR app
RUN npm install
EXPOSE 3000

RUN apt-get update && apt-get install -y cron
RUN echo '* * * * * cd /app && .bin/my-awesome-command > /app/logs/crontab.log 2>&1' > /etc/crontab
RUN crontab /etc/crontab

ENTRYPOINT ["npm", "start"]

Sample working repository : Installing crontab in Dockerfile

Now try running docker-compose up -d --build, the webpage will be available at http://localhost:3000/, but my-awesome-command should write some things to crontab.log, where is this crappy file ?!

Take a look at the output of the docker build :

Step 7/9 : RUN echo '* * * * * cd /app && .bin/my-awesome-command > /app/logs/crontab.log 2>&1' > /etc/crontab
 ---> Running in eb9ec8fd5200
 ---> bdcebf265736
Removing intermediate container eb9ec8fd5200
Step 8/9 : RUN crontab /etc/crontab
 ---> Running in c20b3d7ea6df
 ---> a1926b68ecb2
Removing intermediate container c20b3d7ea6df

I’m not an expert of Docker but I’ve found that the fact that crontab /etc/crontab is executed inside an intermediate container totally fucked our crontab … How can I execute this job ?

The solution

The solution is to build another image in docker-compose with the same configuration as myapp except for one thing. This will not execute the web server this will execute the crontab !

We need to do 2 things :

Adding a service for docker-compose

Just copy the myapp service to another service mycron :

version: '2'
services:
  myapp:
    build: .
    volumes:
      - .:/app
      - /app/node_modules
    ports:
      - 3000:3000
  mycron:
    build: .
    volumes:
      - .:/app
      - /app/node_modules
#    we don't need ports with this because it will execute the crontab
#    ports:
#      - 3000:3000

Now the real solution is overwriting the entrypoint of the Dockerfile. In myapp it’s npm start but in mycron this will be a script.

So, add entrypoint: sh /app/crontab.sh under mycron definition.

Set up a script

Let’s create the crontab.sh file.

What will be used for ?

1. Create the log file
2. Registering the command to be executed by the crontab
3. Start the crontab
4. (optionnal) display logs

#!/usr/bin/env bash

# Ensure the log file exists
touch /app/logs/crontab.log

# Ensure permission on the command
chmod a+x /app/bin/my-awesome-command

# Added a cronjob in a new crontab
echo "* * * * * /usr/local/bin/node /app/bin/my-awesome-command >> /app/logs/crontab.log 2>&1" > /etc/crontab

# Registering the new crontab
crontab /etc/crontab

# Starting the cron
/usr/sbin/service cron start

# Displaying logs
tail -f /app/logs/crontab.log

Sample working repository : Using an entrypoint script

Now build then start, you should have this output :