19 Dockerfile Instructions with Examples | Complete Guide

In this article, We are going to perform Dockerfile Instructions with Examples/Dockerfile Instructions Explained with Examples.

A Dockerfile is a text document that contains all the commands a user can call on the command line to build the Docker image.

Below is workflow to create Docker Container from Dockerfile

Dockerfile –> Docker Image –> Docker Container

Dockerfile Instructions with Examples

• 

#1: FROM –

FROM in Dockerfile Instruction used to specify Docker Image Name and start the build process

Example 1:

#specify a Base Image

FROM ubuntu:latest

Example 2:

#specify a Base Image

FROM node:12

#2: MAINTAINER –

MAINTAINER in Dockerfile Instruction is used to about the person who creates the Docker Image

Example:

MAINTAINER [email protected]

#3: CMD –

CMD in Dockerfile Instruction is used to execute a command in Running container, There should be one CMD in a Dockerfile.

CMD executes the commands when your Docker Image is deployed.

Example 1:

# To run apache2 in foreground

CMD ["/usr/sbin/apache2", "-D", "FOREGROUND"]

Example 2:

FROM ubuntu:latest
CMD /bin/bash

#4: RUN –

RUN in Dockerfile Instruction is used to execute any commands on top of current Docker Image

RUN executes the command when you are building Image.

Example 1:

FROM ubuntu:latest
MAINTAINER [email protected]
RUN apt-get update
RUN apt-get install -y apache2

If you want to run .sh(shell script) file inside Dockerfile

COPY test.sh .
RUN ./test.sh
#OR
RUN /path/to/test.sh

#5: LABEL –

LABEL in Dockerfile Instruction is used to specify metadata information of Docker Image.

Example:

FROM ubuntu:latest
LABEL "author"="FOSS TechNIx"
LABEL "Date"="2020-09-29"

#6: EXPOSE –

EXPOSE in Dockerfile Instruction is used to specify Network port for Docker container

Example 1:

# To Expose port 80 of Docker container

EXPOSE 80

Example 2:

EXPOSE 8080/tcp

#7: ENV –

ENV in Dockerfile Instruction is used to set Environment Variables with key and value.

Example 1:

FROM node:12
ENV workdirectory /usr/node

#8: ADD –

ADD: Copies a file and directory from your host to Docker image, however can also fetch remote URLs, extract TAR/ZIP files, etc. It is used downloading remote resources, extracting TAR/ZIP files.

Syntax:

ADD <source>... <destination>

Example 1:

ADD java/jdk-8u231-linux-x64.tar /opt/jdk/

Example 2:

ADD https://fosstechnix.com/test.tar.xz /home/ubuntu/test/

#9: COPY –

COPY in Dockerfile Instruction used to Copies a file or directory from your host to Docker image, It is used to simply copying files or directories into the build context.

Syntax:

COPY <source>... <destination>

Example 1:

# To Install All dependencies for Node.js App

COPY package*.json ./

RUN npm install 

# To copy all application packages 
COPY . .

Example 2:

COPY index.html /var/www/html

#10: ENTRYPOINT –

ENTRYPOINT in Dockerfile Instruction is used you to configure a container that you can run as an executable.

ENTRYPOINT specifies a commands that will executes when the Docker container starts.

Example 1:

FROM ubuntu:latest

ENTRYPOINT ["ls"]

#11: VOLUME –

VOLUME in Dockerfile Instruction is used to create or mount volume to docker container.

Example 1:

FROM node:12
RUN mkdir /node
WORKDIR /node
RUN echo "Welcome to Node.js" > node
VOLUME /node

#12: USER –  

USER in Dockerfile Instruction is used to set the user name and UID when running container

Example 1:

USER admin

To create new user in Dockerfile and login to user.

Example 2:

RUN adduser -D admin
USER admin

#13: WORKDIR –

WORKDIR in Dockerfile Instruction is used to set the working directory.

Example 1:

# To Create nodejsapp directory

WORKDIR /nodejsapp

#14: ARG –

ARG in Dockerfile Instruction is used to set Environment variables with key and value during the image build .

Example 1:

ARG JAVA_PATH=/opt/jdk/jdk1.8.0_251

ENV JAVA_HOME ${JAVA_PATH}

#15: ONBUILD –

ONBUILD in Dockerfile Instruction is used to specify command that runs when the image in Dockerfile is used as base image for another image. 

Examples 1:

FROM node:12
RUN mkdir -p /usr/node/app
WORKDIR /usr/node/app
ONBUILD COPY package.json /usr/node/app/
ONBUILD RUN npm install
ONBUILD COPY . /usr/node/app
CMD [ "npm", "start" ]

#16: STOPSIGNAL –

STOPSIGNAL in Dockerfile Instruction is used to set the system call signal that will be sent to the container to exit

Example 1:

STOPSIGNAL SIGQUIT

#17: SHELL –

SHELL in Dockerfile Instruction is used to set the default shell.

Example:

SHELL ["/bin/bash", "-c", "echo hello"]

#18: HEALTHCHECK –

HEALTHCHECK in Dockerfile Instruction is used to Check container health by running a command inside the container

Example 1:

FROM ubuntu:latest
HEALTHCHECK --interval=60s --timeout=5s \
  CMD curl -f http://fosstechnix.info/ || exit 1
EXPOSE 80

#19: .dockerignore –

.dockerignore in Dockerfile Instruction is used to prevent copy local modules and other unwanted file being copied into Docker Image.

Create a .dockerignore in same directory and you can add unwanted modules/files into it.

sudo nano .dockerignore

*.yaml
__pycache__/
.git
.aws
.env

Conclusion:

We have covered Dockerfile Instructions with Examples/Dockerfile Instructions Explained with Examples.

Dockerfile Instructions FAQ (Frequently Asked Questions)

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Reference:

Docker Official Guide