How can I use a several line command in a Dockerfile in order to create a file within the resulting Image

Question

I'm following installation instructions for RedhawkSDR, which rely on having a Centos7 OS. Since my machine uses Ubuntu 22.04, I'm creating a Docker container to run Centos7 then installing RedhawkSDR in that.

One of the RedhawkSDR installation instructions is to create a file with the following command:

cat<<EOF|sed 's@LDIR@'`pwd`'@g'|sudo tee /etc/yum.repos.d/redhawk.repo
[redhawk]
name=REDHAWK Repository
baseurl=file://LDIR/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhawk
EOF

How do I get a Dockerfile to execute this command when creating an image?

(Also, although I can see that this command creates the file /etc/yum.repos.d/redhawk.repo, which consists of the lines from [redhawk] to gpgkey=...., I have no idea how to parse this command and understand exactly why it does that...)

Answer 1

Using the text editor of your choice, create the file on your local system. Remove the word sudo from it; give it an additional first line #!/bin/sh. Make it executable using chmod +x create-redhawk-repo.

Now it is an ordinary shell script, and in your Dockerfile you can just RUN it.

COPY create-redhawk-repo ./
RUN ./create-redhawk-repo

But! If you look at what the script actually does, it just writes a file into /etc/yum.repos.d with a LDIR placeholder replaced with some other directory. The filesystem layout inside a Docker image is fixed, and there's no particular reason to use environment variables or build arguments to hold filesystem paths most of the time. You could use a fixed path in the file

[redhawk]
name=REDHAWK Repository
baseurl=file:///redhawk-yum/
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhawk

and in your Dockerfile, just COPY that file in as-is, and make sure the downloaded package archive is in that directory. Adapting the installation instructions:

ARG redhawk_version=3.0.1
RUN wget https://github.com/RedhawkSDR/redhawk/releases/download/$redhawk_version/\
redhawk-yum-$redhawk_version-el7-x86_64.tar.gz \
 && tar xzf redhawk-yum-$redhawk_version-el7-x86_64.tar.gz \
 && rm redhawk-yum-$redhawk_version-el7-x86_64.tar.gz \
 && mv redhawk-yum-$redhawk_version-el7-x86_64 redhawk-yum \
 && rpm -i redhawk-yum/redhawk-release*.rpm
COPY redhawk.repo /etc/yum.repos.d/

Remember that, in a Dockerfile, you are root unless you've switched to another USER (and in that case you can use USER root to switch back); you do not need generally sudo in Docker at all, and can just delete sudo where it appears in these instructions.

Answer 2

How do I get a Dockerfile to execute this command when creating an image?

Just use printf and run this command as single line:

FROM image_name:image_tag
ARG LDIR="/default/folder/if/argument/not/set"

# if container has sudo command and default user is not root
# you should choose this variant
RUN printf '[redhawk]\nname=REDHAWK Repository\nbaseurl=file://%s/\nenabled=1\ngpgcheck=1\ngpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhawk\n' "$LDIR" | sudo tee /etc/yum.repos.d/redhawk.repo

# if default container user is root this command without piping may be used
RUN printf '[redhawk]\nname=REDHAWK Repository\nbaseurl=file://%s/\nenabled=1\ngpgcheck=1\ngpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhawk\n' "$LDIR" > /etc/yum.repos.d/redhawk.repo

Where LDIR is an argument and docker build process should be run like:

docker build ./ --build-arg LDIR=`pwd`