How can I access Docker set Environment Variables From a Cron Job

Question

I\'ve recently tried running a cron job from within a linked docker container and run into an issue. My main docker container is linked to a postgres container and its port numb

Answer 1

The environment is set, but not available to the cron job. To fix that, you can do these two simple things

1) Save the env to a file in your ENTRYPOINT or CMD

CMD env > /tmp/.MyApp.env && /bin/MyApp

2) Then read that env into your cron command like this:

0 5 * * * . /tmp/.MyApp.env; /bin/MyApp

Answer 2

You should export your environment variable before you run cronjobs.

Other solutions are fine but they will fail when there are any special characters in your environment variable.

I have found the solution:

eval $(printenv | awk -F= '{print "export " "\""$1"\"""=""\""$2"\"" }' >> /etc/profile)

Answer 3

I ran into this same problem. I have a docker container that runs cron to execute some shell scripts periodically. I too had a hard time finding out why my scripts would run fine when I manually executed them inside the container. I tried all the tricks of creating a shell script that would run first to set the environment, but they never worked for me (most likely I did something wrong). But I continued looking and found this and it does work.

1. Setup a start or entry point shell script for your cron container
2. Make this the first line to execute printenv | grep -v "no_proxy" >> /etc/environment

The trick here is the /etc/environment file. When the container is built that file is empty, I think on purpose. I found a reference to this file in the man pages for cron(8). After looking at all the versions of cron they all elude to an /etc/? file that you can use to feed environment variables to child processes.

Also, note that I created my docker container to run cron in the foreground, cron -f. This helped me avoid other tricks with tail running to keep the container up.

Here is my entrypoint.sh file for reference and my container is a debian:jessie base image.

printenv | grep -v "no_proxy" >> /etc/environment

cron -f

Also, this trick worked even with environment variables that are set during, docker run commands.

Answer 4

To escape any weird characters that could break your script, and according to reasoning from Mark's answer, add this line to your entrypoint.sh:

env | sed -r "s/'/\\\'/gm" | sed -r "s/^([^=]+=)(.*)\$/\1'\2'/gm" \ > /etc/environment

This way, if you have any variable like affinity:container==My container's friend, it will be converted to affinity:container='=My container\'s friend, and so on.

Answer 5

One can append the system environment variables to the top of a crontab file by using wrapper shell script to run the cron daemon. The following example is from CentOs 7,

In the Dockerfile

COPY my_cron /tmp/my_cron
COPY bin/run-crond.sh run-crond.sh
RUN chmod -v +x /run-crond.sh
CMD ["/run-crond.sh"]

run_cron.sh:

#!/bin/bash

# prepend application environment variables to crontab
env | egrep '^MY_VAR' | cat - /tmp/my_cron > /etc/cron.d/my_cron

# Run cron deamon
# -m off : sending mail is off 
# tail makes the output to cron.log viewable with the $(docker logs container_id) command
/usr/sbin/crond -m off  && tail -f /var/log/cron.log

This is based on a great blog post somewhere, but I lost the link.

Answer 6

inspired by @Kannan Kumarasamy answer:

  for variable_value in $(cat /proc/1/environ | sed 's/\x00/\n/g'); do
    export $variable_value
  done

I can't state for sure, what process with pid1 is and that its stable during lifetime of OS. but as it is the first process to be run, inside a container i guess we can take it for granted it is a process with desired env vars set. take all of this with pich of salt unless some linux/docker docs proves this is completely ok.