A Singularity image, which can be referred to as a “container,” is a single file that contains a virtual file system. As of Singularity 2.4, we strongly recommend that you build (create and install) an image using build. If you have reason to create an empty image, or use creat for any other reason, the original
create command is replaced with a more specific
image.create. After creating an image you can install an operating system, applications, and save meta-data with it.
Whereas Docker assembles images from layers that are stored on your computer (viewed with the
docker history command), a Singularity image is just one file that can sit on your Desktop, in a folder on your cluster, or anywhere. Having Singularity containers housed within a single image file greatly simplifies management tasks such as sharing, copying, and branching your containers. It also means that standard Linux file system concepts like permissions, ownership, and ACLs apply to the container (e.g. I can give read only access to a colleague, or block access completely with a simple
Creating a new blank Singularity container image
Singularity will create a default container image of 768MiB using the following command:
singularity image.create container.imgCreating empty 768MiB image file: container.img Formatting image with ext3 file system Image is done: container.img
How big is it?
$ du -sh container.img 29M container.img
Create will make an
ext3 filesystem. Let’s create and import a docker base (the pre-2.4 way with two commands), and then compare to just building (one command) from the same base.
singularity create container.img sudo singularity bootstrap container.img docker://ubuntu ... $ du -sh container.img 769M
Prior to 2.4, you would need to provide a
--size to change from the default:
$ singularity create --size 2048 container2.img Initializing Singularity image subsystem Opening image file: container2.img Creating 2048MiB image Binding image to loop Creating file system within image Image is done: container2.img $ ls -lh container*.img -rwxr-xr-x 1 user group 2.1G Apr 15 11:34 container2.img -rwxr-xr-x 1 user group 769M Apr 15 11:11 container.img
Now let’s compare to if we just built, without needing to specify a size.
sudo singularity build container.simg docker://ubuntu ... du -sh container.simg 45M container.simg
Quite a difference! And one command instead of one.
Overwriting an image with a new one
For any commands that If you have already created an image and wish to overwrite it, you can do so with the
$ singularity image.create container.img ERROR: Image file exists, not overwriting. $ singularity image.create --force container.img Creating empty 768MiB image file: container.img Formatting image with ext3 file system Image is done: container.img
@GodLoveD has provided a nice interactive demonstration of creating an image (pre 2.4).