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📄 Building images

Now that we’ve seen some examples, we’re almost ready to set up a real instance. Before doing so, however, we’ll quickly explore definition files and how they are used. A definition file consists of a series of instructions or commands aimed at configuring the image for end users. This process includes setting up environment variables and installing software.

The Singularity Workflow (development process) can be summarized in the following steps:

  1. Create a sandbox container.
  2. Enter the container using the --writable option.
  3. Perform necessary actions to get your software running including executing commands, setting environment variables, and documenting them in a definition file.
  4. Build the container from the definition file and do a test-run. If any issues arise, return to step 1.

The definition file (def for short) is divided into 2 parts, a header and sections.

  • Header: Describes the core operating system to build within the container.
  • Sections: These are group of commands to describe particular actions in the final image. They can include setup, copy files, set environment variables, download files, testing, and many more.

The following is a simple definition file that will serve as a demo for this tutorial. These are the contents of the demo.def file:

Bootstrap: library
From: ubuntu:22.04

%post
echo “Display any message during build!”

And here is the build command:

[josue@localhost ~]$ sudo singularity build demo.sif demo.def
INFO:    Starting build...
INFO:    Downloading library image
28.4MiB / 28.4MiB [=====================================] 100 % 3.4 MiB/s 0s
INFO:    Verifying bootstrap image /root/.singularity/cache/library/sha256.7a63c...98b
INFO:    Creating SIF file...
INFO:    Build complete: demo.sif

The resulting container image is named demo.sif.