CODEWITHSUNDEEP
dockerdocker-composeenvironment-variablesdockerfile
M.Alsioufi
M.Alsioufi

Reputation: 695

Set today's date as environment variable

I need to set today's date as environment variable in my docker container.

What I am trying to do is to get today's date so I can use it to call some API with Logstash's http_poller plugin.

The solution I am thinking about is to get today's date using the command date +%Y%m%d

How can I set the result of this command as an environment variable at run time?

Upvotes: 6

Views: 20117

Answers (5)

AV_Jeroen
AV_Jeroen

Reputation: 101

Besides the solution as described by acran, to use ARG to bring outside information in. There is a second solution.

You can write the date to a file, and read the content of that file before executing your command. Here is an example Dockerfile

FROM alpine:latest

RUN (date +'%Y-%m-%d %R') > date.txt
CMD x=$(cat date.txt) && echo "The build time is: $x"

Upvotes: 0

qwerty
qwerty

Reputation: 1449

My solution was to first do export command and then running service startup like this

CMD export INSTALL_DATE=$(date +%s) && python3 /product/server.py

Upvotes: 0

Sylwit
Sylwit

Reputation: 1577

So according to your answer to my comment, if you want to have the variable set at runtime you need to inject it when you start your container

✗ docker run -e TS=$(date +%Y%m%d) python:3.7-alpine env
TS=20201230
PYTHON_VERSION=3.7.9
HOME=/root

Here is the link to the docker documentation : https://docs.docker.com/engine/reference/run/#env-environment-variables

But you are confusing the Dockerfile with the container. A Dockerfile is a definition for building an image. This image is static. Then you start multiples containers from this base image.

So if your start 3 containers with the same image, according to your question you expect 3 different dates in your containers. So you see why you can't set a date or a command in a Dockerfile. The previous snippet does what you ask.

Upvotes: 1

acran
acran

Reputation: 9018

ARG should be what you are looking for:

FROM base

# to be able to use in Dockerfile
ARG now

# to store the value as environment variable in the image
ENV build_date=$now

Now you can build this with

# pass value explicitly
docker build --build-arg now="$(date +%Y%m%d)" .

# pass value from environment
export now="$(date +%Y%m%d)"
docker build --build-arg now .

This still requires to run date on the host since doing this inside the Dockerfile is not possible unfortunately:

  • The only way to execute arbitrary commands in the build is within a RUN statement; but
  • The only way to persist a variable into the environment of an image/container is with an ENV statement which can only use environment variables from outside the build

You could use a custom ENTRYPOINT tough and inject the date to the environment from a file:

FROM base

RUN date +%Y%m%d > /build-timestamp
COPY entrypoint.sh /entrypoint.sh
ENTRYPOINT /entrypoint.sh

entrypoint.sh:

#!/bin/bash

export BUILD_TIMESTAMP="$(cat /build-timestamp)"
exec "$@"

Upvotes: 9

anemyte
anemyte

Reputation: 20286

According to the documentation ENV only supports <key>=<value> entries, I see no way to put it there with a command. I suggest you drop the value into some file then read it when needed. Or, if you have a launch script, using RUN you can add export VAR_NAME=<value> to the launch script so that the variable is automatically added to the environment on launch.

Upvotes: 0

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