A Singularity image, which can be referred to as a “container,” is a single file that contains a virtual file system. 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 chmod command).
Creating a new blank Singularity container image
Singularity will create a default container image of 768MiB using the following command:
$ singularity create container.img Initializing Singularity image subsystem Opening image file: container.img Creating 768MiB image Binding image to loop Creating file system within image Image is done: container.img
How big is it?
$ du -sh container.img 29M container.img
We can now use
ls to list details about the image.
$ ls -l container.img -rwxr-xr-x 1 user group 805306400 Apr 15 11:11 container.img
Note the permissions of the image make it executable. Singularity images can be executed directly.
Creating an image of a different size
You can change the maximum size of an image you create using the
--size option. Note that
--size is not a global option. It is an option to the
create sub-command and must therefore follow it:
$ 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
Overwriting an image with a new one
If you have already created an image and wish to overwrite it, you can do so with the
--force option. This option must also follow the
$ singularity create --size 512 --force container2.img Initializing Singularity image subsystem Opening image file: container2.img Creating 512MiB 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 513M Apr 15 11:39 container2.img -rwxr-xr-x 1 user group 769M Apr 15 11:11 container.img
Increasing the size of an existing image
You can increase the size of an image after it has been instantiated by using the
expand Singularity sub-command as follows:
$ ls -lh container.img -rwxr-xr-x 1 user group 769M Apr 15 11:11 container.img $ singularity expand container.img Initializing Singularity image subsystem Opening image file: container.img Expanding image by 768MiB Binding image to loop Checking file system e2fsck 1.42.13 (17-May-2015) Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Pass 3: Checking directory connectivity Pass 4: Checking reference counts Pass 5: Checking group summary information /dev/loop0: 11/49152 files (0.0% non-contiguous), 7387/196608 blocks Resizing file system resize2fs 1.42.13 (17-May-2015) Resizing the filesystem on /dev/loop0 to 393216 (4k) blocks. The filesystem on /dev/loop0 is now 393216 (4k) blocks long. Image is done: container.img $ ls -lh container.img -rwxr-xr-x 1 user group 1.6G Apr 15 12:21 container.img
Similar to the create sub-command, you can override the default size increase (which is 768MiB) by using the
Mounting an image
Once an image has been created and an OS has been added with the
bootstrap commands, you can use the
shell command to start an interactive shell within the container. But this is not possible when an image does not yet contain a functional OS or shell. For debugging, development, or simply inspecting an image that lacks a functional shell you can use the
mount command like so:
$ mkdir /tmp/container $ singularity mount container.img /tmp/container/ container.img is mounted at: /tmp/container/ Spawning a new shell in this namespace, to unmount, exit shell Singularity: \w> df Filesystem 1K-blocks Used Available Use% Mounted on /dev/loop0 1531760 1172 1451948 1% /tmp/container Singularity: \w> cd /tmp/container Singularity: \w> ls -a . .. lost+found
Singularity mounts images in private name-spaces so that the mount is only visible and accessible from within the freshly spawned shell. When you are finished, you can simply exit the shell and the file system will be automatically unmounted.
Files can be copied from the image to the host when it is mounted in this way, but they cannot be copied from the host into the image. This is because the image is mounted in read-only mode by default and the mount point is owned by the root user. To copy files into a mounted image, first become root and then mount the image with the
--writable option to the
$ sudo -i # singularity mount --writable /home/user/container.img /tmp/container
Copying, sharing, branching, and distributing your image
A primary goal of Singularity is mobility. The single file image format makes mobility easy.
Because Singularity images are single files, they are easily copied and managed. You can copy the image to create a branch, share the image and distribute the image as easily as copying any other file you control!
If you want an automated solution for building and hosting your image, you can use our container registry Singularity Hub. Singulairty Hub can automatically build bootstrap specification files from a Github repository each time that you push. It provides a simple cloud solution for storing and sharing your image.