How to write commands with multiple lines in Dockerfile while preserving the new lines?

Question

I want to write the following RUN command in the Dockerfile. But, docker is not preserving the new lines.

RUN echo "[repo] \
name            = YUM Repository \
baseurl         = https://example.com/packages/ \
enabled         = 1 \
gpgcheck        = 0" > /etc/yum.repos.d/Repo.repoxyz

I know that \ at the end of each line escapes the new line. But, is there any way that I can write multiple lines preserving the new line?

Answer 1

You can use what is called "ANSI-C quoting" with $'...'. It was originally a ksh93 feature but it is now available in bash, zsh, mksh, FreeBSD sh and in busybox's ash (but only when it is compiled with ENABLE_ASH_BASH_COMPAT).

As RUN uses /bin/sh as shell by default you are required to switch to something like bash first by using the SHELL instruction.

Start your command with $', end it with ' and use \n\ for newlines, like this:

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

RUN echo $'[repo] \n\
name            = YUM Repository \n\
baseurl         = https://example.com/packages/ \n\
enabled         = 1 \n\
gpgcheck        = 0' > /etc/yum.repos.d/Repo.repoxyz

Answer 2

I used printf. Writing all the text in one line using \n.

Executing:

RUN printf 'example \ntext \nhere' >> example.txt

inserts:

example
text
here

in example.txt

Answer 3

You can use:

RUN echo -e "\
[repo] \n\
name            = YUM Repository \n\
baseurl         = https://example.com/packages/ \n\
enabled         = 1 \n\
gpgcheck        = 0\
" > /etc/yum.repos.d/Repo.repoxyz

This way you will have a quick way to check what the file contents are. You just need to be aware that you need to end every line with \ and insert the \n when needed.

Answer 4

I ended up using a combination of the examples listed above since the new line \n did not work with echo.

RUN printf 'example \n\
text \n\
here' >> example.txt

It produces the following, as expected:

example
text
here

Answer 5

May be it's help you ( https://github.com/jen-soft/pydocker )

[ Dockerfile.py ]

from pydocker import DockerFile  # sudo pip install -U pydocker

d = DockerFile(base_img='debian:8.2', name='jen-soft/custom-debian:8.2')

d.RUN_bash_script('/opt/set_repo.sh', r'''
cat >/etc/apt/sources.list <<EOL
deb     http://security.debian.org/ jessie/updates main
deb-src http://security.debian.org/ jessie/updates main
EOL
apt-get clean && apt-get update
''')

d.EXPOSE = 80
d.WORKDIR = '/opt'
d.CMD = ["python", "--version"]

# d.generate_files()
d.build_img()

# sudo wget -qO- https://get.docker.com/ | sh

python Dockerfile.py
docker images

Answer 6

As of Docker 18.09 and Dockerfile syntax 1.4, Dockerfiles support heredocs (ie what you're looking for here) natively!

1. Enable BuildKit, eg by setting DOCKER_BUILDKIT=1.
2. Add this line to the top of your Dockerfile:

   # syntax=docker/dockerfile:1.3-labs
   

3. Rewrite your heredoc like so:

   COPY <<EOF /etc/yum.repos.d/Repo.repoxyz
   [repo]
   name            = YUM Repository
   baseurl         = https://example.com/packages/
   enabled         = 1
   gpgcheck        = 0
   EOF
   

You can also use this to run multiple bash commands in a single RUN block, etc. More details: Docker blog post, Dockerfile syntax docs.