where to find a singularity definition file in sandbox

Question

I am trying to learn singularity so I pulled from their library this example:

singularity pull library://sylabsed/examples/lolcow

After pulling I wanted to check what are the directories inside this image and if there is any definition file that was used to build the image, therefore I went to develop mode by building a sandbox:

sudo singularity build --sandbox lowcow/ library://sylabsed/examples/lolcow

Once I inspected into the directory lowcow/ I have not found any singularity .def definition file.

Could someone please explain to me if it's possible to find the definition file in the sandbox ?

Answer 1

Yes, you can find the definition file at /.singularity.d/Singularity. You do not have to build a sandbox image for this. You can do this directly from the downloaded image.

singularity exec library://sylabsed/examples/lolcow cat /.singularity.d/Singularity

This is even possible with Docker images:

$ singularity exec docker://alpine:3.11 cat /.singularity.d/Singularity
bootstrap: docker
from: alpine:3.11