launch a CAT command unix into Dockerfile

Question

I would like to launch this vagrant command cat(run perfectly!) to provisionning my container with a Dockerfile :

# Configure Virtualenvwrapper.
RUN cat <<EOF >> /home/docker/.bashrc
# Virtualenvwrapper configuration.
export WORKON_HOME=\$HOME/.virtualenvs
export PROJECT_HOME=\$HOME/Devel
source /usr/local/bin/virtualenvwrapper.sh
EOF

But I have this error return when I launch my building image docker :

 ---> 40f9ed8e187d
Removing intermediate container 85f6c8536520
Step 69 : RUN cat <<EOF >> /home/docker/.bashrc
 ---> Running in dcbb3d441f79
 ---> 78acd9c2e5d5
Removing intermediate container dcbb3d441f79
Step 70 : EXPORT
Unknown instruction: EXPORT

What is the trick for run a cat command unix into image with Dockerfile ?

Answer 1

Update [07/29/2021]: As of dockerfile/dockerfile:1.3.0-labs, the Here-Document syntax is now supported with the Docker BuildKit backend enabled.

You need to use Docker Buildkit by setting DOCKER_BUILDKIT=1 in your environment, set the syntax parser directive to use dockerfile/dockerfile:1.3.0-labs, and swap the position of the here delimeter with cat. The rest is used like normal.

Dockerfile Example:

# syntax = docker/dockerfile:1.3-labs

...

RUN <<EOF cat >> /home/docker/.bashrc
# Virtualenvwrapper configuration.
export WORKON_HOME=\$HOME/.virtualenvs
export PROJECT_HOME=\$HOME/Devel
source /usr/local/bin/virtualenvwrapper.sh
EOF

---

Prior to Docker BuildKit labs channel dockerfile/dockerfile:1.3.0-labs release, instead of using cat, try using echo instead!

Turn this shell cat example...

#!/usr/bin/env sh
cat <<EOF > /tmp/example.txt
line 1
line 2
line 3
EOF

... into this Dockerfile echo example!

RUN echo -e '\
line 1\n\
line 2\n\
line 3\
' > /tmp/example.txt

Note the pair of single quotes (') in the echo example.

Also note the -e flag for echo to support the escaped newlines (\n).

Caution: Unfortunately, the -e flag may or may not be required depending on the version of echo your image has installed. For example, the npm:16 image's echo does not require -e and actually will print the -e along with the single-quoted lines. On the other hand, the ubuntu:20.04 image's echo does require -e.

The same example could be written on one line as: RUN echo -e 'line 1\nline 2\nline 3' >> /tmp/example.txt, but I find the above example more readable.

To answer the OP's question, use this:

# Configure Virtualenvwrapper.
RUN echo -e '\
# Virtualenvwrapper configuration.\n\
export WORKON_HOME=\$HOME/.virtualenvs\n\
export PROJECT_HOME=\$HOME/Devel\n\
source /usr/local/bin/virtualenvwrapper.sh\
' >> /home/docker/.bashrc

---

Caution: The escape character can be redefined by the escape directive. If your Dockerfile has a different escape character set, you'll need to modify the examples accordingly.

Answer 2

Based on this comment to an issue posted on Github, this works:

RUN echo 'All of your\n\
multiline that you ever wanted\n\
into a dockerfile\n'\
>> /etc/example.conf