OCI Runtime Support


The Open Containers Initiative is an independent organization whose mandate is to develop open standards relating to containerization. There are three OCI specifications covering the OCI container image format, distribution methods for containers, and the behaviour of compliant container runtimes.

The OCI specifications inherited from the historic behaviour of Docker, and have been refined over time. The majority of container runtimes, and tools that work with containers on Linux follow the OCI standards.

SingularityCE was initially developed to address difficulties with using Docker in shared HPC compute environments. A development goal is to allow users to work with Docker/OCI containers where Docker or other OCI runtimes cannot easily be deployed, for various reasons.

OCI Spec Support

OCI Image Spec - SingularityCE can convert container images that satisfy the OCI Image Specification into its own SIF format, or a simple sandbox directory. Most of the configuration that a container image can specify is supported by the SingularityCE runtime, but there are some limitations, and workarounds required for certain container images.

OCI Distribution Spec - SingularityCE is able to pull images from registries that satisfy the OCI Distribution Specification.

OCI Runtime Spec - By default, SingularityCE does not follow the OCI Runtime Specification closely. Instead, it uses its own runtime that is better matched to the requirements and limitations of multi-user shared compute environments. The singularity oci commands were added to provide a mode of operation in which SingularityCE does implement the OCI runtime specification and container lifecycle. These commands are primarily of interest to tooling that might use SingularityCE as a container runtime, rather than end users.

Future Development

As newer Linux kernels and system software reach production environments, many of the limitations that required SingularityCE to operate quite differently from OCI runtimes are becoming less-applicable. Over future releases, SingularityCE development will bring greater OCI compliance for typical usage, while maintaining the same ease-of-use and application focus.

You can read more about these plans in the following article and open community roadmap:

OCI Command Group

To run Singularity containers in an OCI Runtime Spec compliant manner, you can use the oci command group.


All commands in the oci command group currently require root privileges.

OCI containers follow a different lifecycle to containers that are run with singularity run/shell/exec. Rather than being a simple process that starts, and exits, they are created, run, killed, and deleted. This is similar to instances. Additionally, containers must be run from an OCI bundle, which is a specific directory structure that holds the container’s root filesystem and configuration file. To run a SingularityCE SIF image, you must mount it into a bundle.

Mounting an OCI Filesystem Bundle

Let’s work with a busybox container image, pulling it down with the default busybox_latest.sif filename:

$ singularity pull library://busybox
INFO:    Downloading library image
773.7KiB / 773.7KiB [===============================================================] 100 % 931.4 KiB/s 0s

Now use singularity oci mount to create an OCI bundle onto which the SIF is mounted:

$ sudo singularity oci mount ./busybox_latest.sif /var/tmp/busybox

By issuing the mount command, the root filesystem encapsulated in the SIF file busybox_latest.sif is mounted on /var/tmp/busybox with an overlay setup to hold any changes, as the SIF file is read-only.

Content of an OCI Compliant Filesystem Bundle

The OCI bundle, created by the mount command consists of the following files and directories:

  • config.json - a generated OCI container configuration file, which instructs the OCI runtime how to run the container, which filesystems to bind mount, what environment to set, etc.

  • overlay/ - a directory that holds the contents of the bundle overlay - any new files, or changed files, that differ from the content of the read-only SIF container image.

  • rootfs/ - a directory containing the mounted root filesystem from the SIF container image, with its overlay.

  • volumes/ - a directory used by the runtime to stage any data mounted into the container as a volume.

OCI config.json

The container configuration file, config.json in the OCI bundle, is generated by singularity mount with generic default options. It may not reflect the config.json used by an OCI runtime working directly from a native OCI image, rather than a mounted SIF image.

You can inspect and modify config.json according to the OCI runtime specification to influence the behavior of the container.

Running a Container

For simple interactive use, the oci run command will create and start a container instance, attaching to it in the foreground. This is similar to the way singularity run works, with SingularityCE’s native runtime engine:

$ sudo singularity oci run -b /var/tmp/busybox busybox1
/ # echo "Hello"
/ # exit

When the process running in the container (in this case a shell) exits, the container is automatically cleaned up, but note that the OCI bundle remains mounted.

Full Container Lifecycle

If you want to run a detached background service, or interact with SIF containers from 3rd party tools that are compatibile with OCI runtimes, you will step through the container lifecycle using a number of oci subcommands. These move the container between different states in the lifecycle.

Once an OCI bundle is available, you can create a instance of the container with the oci create subcommand:

$ sudo singularity oci create -b /var/tmp/busybox busybox1
INFO:    Container busybox1 created with PID 20105

At this point the runtime has prepared container processes, but the payload (CMD / ENTRYPOINT or runscript) has not been started.

Check the state of the container using the oci state subcommand:

$ sudo singularity oci state busybox1
  "ociVersion": "1.0.2-dev",
  "id": "busybox1",
  "pid": 20105,
  "status": "created",
  "bundle": "/var/tmp/busybox",
  "rootfs": "/var/tmp/busybox/rootfs",
  "created": "2022-04-27T15:39:08.751705502Z",
  "owner": ""

Start the container’s CMD/ENTRYPOINT or runscript with the oci start command:

$ singularity start busybox1

There is no output, but if you check the container state it will now be running. The container is detached. To view output or provide input we will need to attach to its input and output streams. with the oci attach command:

$ sudo singularity oci attach busybox1
/ # date
Wed Apr 27 15:45:27 UTC 2022
/ #

When finished with the container, first oci kill running processes, than oci delete the container instance:

$ sudo singularity oci kill busybox1
$ sudo singularity oci delete busybox1

Unmounting OCI Filesystem Bundles

When you are finished with an OCI bundle, you will need to explicitly unmount it using the oci umount subcommand:

$ sudo singularity oci umount /var/tmp/busybox

Technical Implementation

SingularityCE 3.10 uses runc as the low-level runtime engine to execute containers in an OCI Runtime Spec compliant manner. runc is expected to be provided by your Linux distribution.

To manage container i/o streams and attachment, conmon is used. SingularityCE ships with a suitable version of conmon to support the oci command group.

In SingularityCE 3.9 and prior, SingularityCE’s own low-level runtime was employed for oci operations. This was retired to simplify maintenance, improve OCI compliance, and make possible future development in the roadmap to 4.0.