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Advanced use of Singularity

Singularity CLI continued

Build Singularity .sif images

Similar to Docker which uses a Dockerfile to build its images, Singularity uses a file called Singularity.

Important

  • Singularity images use the .sif extension and appear as a single compressed file, versus Docker which uses many cached file layers to create a single image.
  • .sif files also use layers, but these are not apparent.
  • .sif images are cached in the folder where you build them, or designate them.
  • When building from docker:// the Docker image layers are downloaded and cached by Singularity in a /.Singularity folder on your build environment.

build

Create a .sif image using a Docker image as the template

As we've learned from the HPC and HTC groups, building Singularity images is not necessarily the most accessible and reproducible method for managing containers.

Most groups suggest that you build your containers with Docker, and host them on a Docker Registry first.

The market dominance of Docker and its wide acceptance as a container format, has led us to use Singularity with Docker in most cases.

We've already covered how you can pull an existing image from Docker Hub, but we can also build a Singularity image from the Docker Hub using the build command:

$ sudo singularity build --sandbox ubuntu-latest/  docker://ubuntu

Test the new .sif image:

$ singularity shell --writable ubuntu-latest/

Singularity ubuntu-latest.sif:~> apt-get update

Does it work?

$ sudo singularity shell ubuntu-latest.sif

Singularity: Invoking an interactive shell within container...

Singularity ubuntu-latest.sif:~> apt-get update

When I try to install software to the image without sudo it is denied, because root is the owner inside the container. When I use sudo I can install software into the container. The software remains in the sandbox container after closing the container and restart.

In order to make these changes permanant, I need to rebuild the sandbox as a .sif image

$ sudo singularity build ubuntu-latest.sif ubuntu-latest/

Creating Singularity .sif from scratch

The contents of the Singularity file differ from Dockerfile

  • %help - create text for a help menu associated with your container
  • %setup - executed on the host system outside of the container, after the base OS has been installed.
  • %files - copy files from your host system into the container
  • %labels - store metadata in the container
  • %environment - loads environment variables at the time the container is run (not built)
  • %post - set environment variables during the build
  • %runscript - executes a script when the container runs
  • %test - runs a test on the build of the container
Dockerfile FROM ARG COPY RUN ENV LABEL CMD
Singularity Bootstrap: %post %files %post %environment %label %runscript

Writing the Singularity file

SyLabs User-Guide

A Singularity file can be hosted on GitHub and will be auto-detected by Singularity-Hub when you set up your container Collection.

When you are building locally, you can name the Singularity file whatever you wish, but a better practice is to put it in a specified directory and name it Singularity.

Building your own containers requires that you have sudo privileges - therefore you'll need to develop these on your local machine or on a VM that you can gain root access on.

The top of the file, selects the base OS for the container, just like FROM in Docker.

Bootstrap: references another registry (e.g. docker for DockerHub, debootstrap, or library for Sylabs Container Library).

From: selects the tag name.

Using debootstrap with a build that uses a mirror:

BootStrap: debootstrap
OSVersion: jammy
MirrorURL: http://us.archive.ubuntu.com/ubuntu/

Using CentOS-like container:

Bootstrap: yum
OSVersion: 7
MirrorURL: http://mirror.centos.org/centos-7/7/os/x86_64/
Include:yum

Note: to use yum to build a container you should be operating on a RHEL system, or an Ubuntu system with yum installed.

Using a localimage to build:

Bootstrap: localimage
From: /path/to/container/file/or/directory

The container registries which Singularity uses are listed in the Introduction Section 3.1.

  • The Singularity file uses sections to specify the dependencies, environmental settings, and runscripts when it builds.

help

%help section can be as verbose as you want

Bootstrap: docker
From: ubuntu

%help
This is the container help section.

setup

%setup commands are executed on the localhost system outside of the container - these files could include necessary build dependencies. We can copy files to the $SINGULARITY_ROOTFS file system can be done during setup

files

%files include any files that you want to copy from your localhost into the container.

post

%post includes all of the environment variables and dependencies that you want to see installed into the container at build time.

%post
    apt-get -y update
    apt-get -y install fortune cowsay lolcat

environment

%environment includes the environment variables which we want to be run when we start the container

%environment
    export LC_ALL=C
    export PATH=/usr/games:$PATH

runscript

%runscript does what it says, it executes a set of commands when the container is run.

%runscript
    fortune | cowsay | lolcat

labels

%labels are used similar to Dockerfile which allow you to add metadata to the image file in key-value pairs

%labels
Author Your Name
Email email@address.org
Version v2022
CustomLabel statement here

Labels can be read using the inspect command

Example File

Example Singularity file bootstrapping an Ubuntu (22.04) image.

Bootstrap: docker
From: ubuntu:22.04

%post
   apt-get -y update
   apt-get -y install fortune cowsay lolcat

%environment
   export LC_ALL=C
   export PATH=/usr/games:$PATH

%runscript
   fortune | cowsay | lolcat

%labels
   Maintainer Your Name
   Version v2022

Build the container:

singularity build cowsay.sif Singularity

Run the container:

singularity run cowsay.sif

Sandbox

Sandboxing is another approach to building up a container image.


Last update: 2023-03-27