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What are Dockerfile Directives

In our previous tutorial, you learned how to build images with Dockerfile. This tutorial will help you to understand the basic Dockerfile directives and there uses.

FROM

The from directive is used to set base image for the subsequent instructions. A Dockerfile must have FROM directive with valid image name as the first instruction.

Examples:

FROM ubuntu
FROM tecadmin/ubuntu-ssh:16.04

LABEL

Using label you can organize images in a proper way. this is useful to set maintainer address, vender name, version of image, release date etc. The line must be begain with keywork “LABEL”.

LABEL maintainer="[email protected]"
LABEL vendor="TecAdmin"
LABEL com.example.version="0.0.1"

You can add multiple lable in single line with space seprated, or you can define in multiple lines as following.

LABEL maintainer="[email protected]" vendor="TecAdmin" \
      com.example.version="0.0.1"

RUN

Using RUN directing ,you can run any command to image during build time. For example you can install required packages during the build of image.

RUN apt-get update 
RUN apt-get install -y apache2 automake build-essential curl

As a more formatted syntax, you can use as following.

RUN apt-get update && apt-get install -y \
    automake \
    build-essential \
    curl \

COPY

The COPY directive used for coping files and directories from host system to the image during build. For example the first commands will copy all the files from hosts html/ directory /var/www/html image directory. Second command will copy all files with extension .conf to /etc/apache2/sites-available/ directory.

COPY html/* /var/www/html/
COPY *.conf /etc/apache2/sites-available/

WORKDIR

The WORKDIR directive used to sets the working directory for any RUN, CMD, ENTRYPOINT, COPY and ADD commands during build.

WORKDIR /opt

CMD

The CMD directive is used to run the service or software contained by your image, along with any arguments during the launching the container. CMD uses follwoing basic syntax

CMD ["executable","param1","param2"]
CMD ["executable","param1","param2"]

For example, to start Apache service during launch of container, Use the following command.

CMD ["apachectl", "-D", "FOREGROUND"]

EXPOSE

The EXPOSE directive indicates the ports on which a container will listen for the connections. After that you can bind host system port with container and use them.

EXPOSE 80
EXPOSE 443

ENV

The ENV directive is used to set environment variable for specific service of container.

ENV PATH=$PATH:/usr/local/pgsql/bin/ \
    PG_MAJOR=9.6.0

VOLUME

The VOLUME directive creates a mount point with the specified name and marks it as holding externally mounted volumes from native host or other containers.

VOLUME ["/data"]
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