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 number is set as an environment variable by docker upon the containers creation. This environment variable is not set in ~/.profile or any other source file that I could load when running my cron job. How can I then access these environment variables from my cron job?

Thanks!

Answer 1

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 2

I would recommend using declare to export your environment and avoid escaping issues. Can be used in CMD or ENTRYPOINT or directly in a wrapper script which might be called by one of them:

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

Grep -v takes care of filtering out read-only variables.

You can later easily load this environment like this:

SHELL=/bin/bash
BASH_ENV=/container.env
* * * * * root /test-cron.sh

Answer 3

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 4

You can run the following command

. <(xargs -0 bash -c 'printf "export %q\n" "$@"' -- < /proc/1/environ)

This is exceptionally works when you have the environment variables with special characters like ' " =

Answer 5

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.

Answer 6

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 7

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)