WilliamLam.com

  • About
    • About
    • Privacy
  • VMware Cloud
  • Tanzu
    • Application Modernization
    • Tanzu services
    • Tanzu Community Edition
    • Tanzu Kubernetes Grid
    • vSphere with Tanzu
  • Home Lab
  • Nested Virtualization
  • Apple

Quick Tip - Creating a multiline Dockerfile using heredoc w/variable substitution

04.26.2017 by William Lam // 1 Comment

I was helping out a fellow colleague yesterday who was having some troubles handling a multiline echo statement within his Dockerfile. There are multiple ways in which you can create multiline Dockerfiles, the web is full of examples from using multiple echo statements (pretty ugly) to using heredocs which is easier to read and manage. The challenge was that he also wanted to substitute some variables into his multiline statement which apparently there were no examples online, at least neither of us could find.

Taking a closer look, I found that we can just leverage Bash's ANSI-C Quoting syntax $'string' to do what we want, which was actually something new to me as well. You can then pass in the variable like you normally would between the strings and that would give you the readability of heredocs and still be able to use Docker variables. I am sure there are other methods with more extensive escapes with single-ticks, but I also prefer a solution that is easy to read and use in case others need to manage it.

Here is a quick sample Dockerfile which demonstrates how this works:

FROM photon:1.0

ARG BASEURL="https://vmware.bintray.com/powershell"

RUN echo $'[powershell]\n\
name=VMware Photon Linux 1.0(x86_64)\n\
baseurl='$BASEURL$'\n\
gpgcheck=0\n\
enabled=1\n\
skip_if_unavailable=True\n '\
>> /etc/yum.repos.d/powershell.repo

CMD ["/bin/bash"]

Basically the echo statement has $'SOME-STRING'$VARIABLE$'SOME-STRING'

If we build and run this Docker image, we can see that we have properly substituted the BASEURL variable into our file as seen in the screenshot below.

docker build -t sample .
docker run --rm -it sample cat /etc/yum.repos.d/powershell.repo


I personally prefer to keep such logic within a separate script which the Dockerfile can reference, but I was also sympathetic to that fact that my colleague wanted to keep things simple and just have everything within the Dockerfile. I figure I would share this in case other comes across this problem as well as benefiting myself as I will probably forget in a months time 🙂

Categories // Automation, Docker Tags // Docker, dockerfile, heredoc

Search

Author

William Lam is a Senior Staff Solution Architect working in the VMware Cloud team within the Cloud Infrastructure Business Group (CIBG) at VMware. He focuses on Cloud Native, Automation, Integration and Operation for the VMware Cloud based Software Defined Datacenters (SDDC) across Private, Hybrid and Public Cloud

Connect

  • Email
  • GitHub
  • LinkedIn
  • RSS
  • Twitter
  • Vimeo

Recent

  • vSphere Code Capture is your ChatGPT for vSphere Automation 11/30/2023
  • ESXi 8.0 Update 2 not detecting Apple NVMe on Apple Mac Mini 2018 11/28/2023
  • Is there a vSphere REST API for ... 11/21/2023
  • Custom vSAN HCL JSON for VMware Cloud Foundation (VCF) 5.1 and vSAN ESA using Nested ESXi 11/20/2023
  • Heads Up - Performance Impact with VMware Workstation on Windows 11 with Intel Hybrid CPUs 11/19/2023

Advertisment

Privacy & Cookies: This site uses cookies. By continuing to use this website, you agree to their use.
To find out more, including how to control cookies, see here: Cookie Policy

Copyright WilliamLam.com © 2023

 

Loading Comments...