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dockerfile multiline to file

Outputing a multiline string from Dockerfile

I motsly use a Dockerfile by sourcing from a base ditribution: CentOS or Debian.
But I also have a local mirror and would like to use it for packages installation.

Espacially on CentOS it is about many lines to write to the /etc/yum.repos.d/CentOS-Base.repo file.

Easiest way: one RUN per line

The first method that comes in mind is to issue one RUN per line to write.
Here you are:

RUN echo "[base]                                                                           "   >      /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - Base                                                   "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=           "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "[updates]                                                                        "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - Updates                                                "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=        "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "[extras]                                                                         "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - Extras                                                 "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=         "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "[centosplus]                                                                     "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - Plus                                                   "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=     "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "[contrib]                                                                        "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - Contrib                                                "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=        "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "[epel-mada]                                                                      "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "name=CentOS-$releasever - EPEL Mada                                              "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "baseurl=             "   >>     /etc/yum.repos.d/CentOS-Base.repo  
RUN echo "gpgcheck=0                                                                       "   >>     /etc/yum.repos.d/CentOS-Base.repo

This has one big drawback: It creates one layer just for a line to write and that make it very slow. Obviously, this is a NOGO solution.

More subtle way: one multiline RUN

I found this solution from the Docker Github issue discussion about multiline.
Just see how it is:

RUN echo  $'[base]                                                            \n\
name=CentOS-$releasever - Base                                                \n\
baseurl=        \n\
gpgcheck=0                                                                    \n\
[updates]                                                                     \n\
name=CentOS-$releasever - Updates                                             \n\
baseurl=     \n\
gpgcheck=0                                                                    \n\
[extras]                                                                      \n\
name=CentOS-$releasever - Extras                                              \n\
baseurl=      \n\
gpgcheck=0                                                                    \n\
[centosplus]                                                                  \n\
name=CentOS-$releasever - Plus                                                \n\
baseurl=  \n\
gpgcheck=0                                                                    \n\
[contrib]                                                                     \n\
name=CentOS-$releasever - Contrib                                             \n\
baseurl=     \n\
gpgcheck=0                                                                    \n\
[epel-mada]                                                                   \n\
name=CentOS-$releasever - EPEL Mada                                           \n\
baseurl=          \n\
gpgcheck=0                                                                    \n'\
> /etc/yum.repos.d/CentOS-Base.repo

Note the "$" on the first line.
This has the advantage of issuing only one RUN for all these lines, but be carefull to escape BASH special characters. One minor drawback I found  is it screws syntax highlighting & indentation. But it's minor.

COPY a file

The last method I will cover here (becarefull, there are many other methods) is to have a CentOS-Base.repo file ready and jus copy it this way:

COPY CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo

The only drawback I see for this is if you change its content and rebuild your image, you have to rebuild it without using the cache ( --no-cache). But it's fast and simple.

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