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The Dockerfile ARG Instruction

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Razvan Ludosanu

Razvan Ludosanu

Founder, learnbackend.dev

Updated: 7/11/2024

Published: 7/11/2024

About Terminus

The short answer

In Docker, the ARG property allows you to define arbitrary variables that will be passed to the Dockerfile at build-time:

ARG variable[=default]

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Where:

  • variable is the name of the user-defined variable.
  • default is the default value of the variable if not specified.

Once defined, these variables can be used within the Dockerfile using the following syntax:

${variable}

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And their value can be set using the --build-arg flag of the docker build command as follows:

$ docker build --build-arg variable=value .

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For example, let's consider this minimal Dockerfile for building a Node.js application:

ARG NODE_VERSION=14
ARG APP_PORT=3000

FROM node:${NODE_VERSION}

WORKDIR /usr/src/app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE ${APP_PORT}

CMD ["node", "app.js"]

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The following command will set the NODE_VERSION variable to 20 and the APP_PORT variable to 3000, and build the Dockerfile while tagging it node-server:

$ docker build --build-arg NODE_VERSION=20 --build-arg APP_PORT=3000 -t node-server .

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Easily retrieve this syntax using Warp's Agent Mode

If you’re using Warp as your terminal, you can easily retrieve this syntax using the Warp Agent Mode feature:

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Entering docker build args in the Agent Mode question input will prompt a human-readable step by step guide including code snippets.

Written by

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Razvan Ludosanu

Razvan Ludosanu

Founder, learnbackend.dev

Filed Under

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