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, which work with containers on Linux, follow the OCI standards.

SingularityCE was initially developed to address difficulties with using Docker in shared HPC compute environments. Because of the ways these issues were addressed, SingularityCE’s default native runtime is not fully OCI compatible. However, over time, SingularityCE has continuously improved compatibility with OCI standards so that the majority of OCI container images can be run using it.

In SingularityCE 4.0, a new OCI-mode is now fully supported. This mode, enabled via the --oci CLI option or oci mode directive in singularity.conf, uses an OCI low-level runtime to execute containers that achieve compatibility that is not possible with SingularityCE’s native runtime. The familiar singularity command line interface is maintained and unique features such as the SIF image format continue to offer benefits for HPC environments.

OCI compatibility is discussed in three areas of this guide:

  1. The OCI-mode section of this page introduces the new OCI-mode (--oci), which runs OCI / Docker containers using a true OCI low-level runtime.

  2. The Support for Docker page discusses limitations, compatibility options, and best practices for running OCI / Docker containers with SingularityCE’s default runtime.

  3. The OCI Command Group section of this page documents the singularity oci commands, which provide a low-level means to run SingularityCE SIF containers with a command line that matches other OCI runtimes.

OCI Mode (--oci)

OCI-mode, enabled with the --oci command line option, or the oci mode directive in singularity.conf, is now fully supported from SingularityCE 4.0. When OCI-mode is enabled:

  • OCI containers are executed using crun or runc as the low-level runtime, for true OCI runtime compatibility.

  • Default behaviour (of bind mounts etc.) is comparable to using the --compat mode with the native runtime.

  • Containers retrieved from OCI sources are encapsulated within a single OCI-SIF file, maintaining the benefits of SIF while avoiding a full conversion into SingularityCE’s native container format.

Users are encouraged to employ OCI-mode when their primary use-case for SingularityCE is to run existing containers from Docker Hub or other OCI registries. Behavior will more closely match that described for Docker than with SingularityCE’s native runtime.

System Requirements

To use OCI mode, the following requirements must be met by the host system:

  • Unprivileged user namespace creation is supported by the kernel, and enabled.

  • Subuid and subgid mappings are configured for users who plan to run --oci mode.

  • The TMPDIR / SINGULARITY_TMPDIR is located on a filesystem that supports subuid/subgid mapping.

  • crun or runc are available on the PATH.

The majority of these requirements are shared with the those of an unprivileged installation of SingularityCE, as OCI mode does not use setuid. See the admin guide for further information on configuring a system appropriately.

Pulling and Running OCI Containers

To activate OCI-mode when running a container from an OCI source (e.g. Docker Hub), add the --oci flag to a run / shell / exec or pull command:

# Pull container to an OCI-SIF and run it
$ singularity pull --oci docker://ubuntu
Getting image source signatures
Copying blob 445a6a12be2b done
Copying config c6b84b685f done
Writing manifest to image destination
INFO:    Converting OCI image to OCI-SIF format
INFO:    Squashing image to single layer
INFO:    Writing OCI-SIF image
INFO:    Cleaning up.
$ singularity run --oci ubuntu_latest.oci.sif
dtrudg-sylabs@mini:~$ echo "HELLO OCI WORLD"

# Run directly from a URI
$ singularity exec --oci docker://ubuntu date
INFO:    Using cached OCI-SIF image
Mon Sep  4 12:24:26 UTC 2023

Running containers in this manner greatly improves compatibility between SingularityCE’s features and the OCI specification. For example, when running in --oci mode, Singularity honors the Dockerfile USER directive:

# I am joeuser outside of the container
$ whoami

# The Dockerfile adds a `testuser`
$ cat Dockerfile
FROM alpine
RUN addgroup -g 2000 testgroup
RUN adduser -D -u 2000 -G testgroup testuser
USER testuser
CMD id

# Create and save a docker archive from this Dockerfile
$ docker build --tag docker-user-demo .
$ docker save docker-user-demo > docker-user-demo.tar

# Run the docker archive from singularity
$ singularity run --oci docker-archive:./docker-user-demo.tar
Getting image source signatures
Copying blob 3f8df8c11beb done
Copying blob 78a822fe2a2d done
Copying blob f7cb6364f42b done
Copying config 59af11197a done
Writing manifest to image destination
INFO:    Converting OCI image to OCI-SIF format
INFO:    Squashing image to single layer
INFO:    Writing OCI-SIF image
INFO:    Cleaning up.
uid=2000(testuser) gid=2000(testgroup)

As the last line of output shows, the user inside the container run by singularity run --oci is testuser (the user added as part of the Dockerfile) rather than joeuser (the user on the host).

Authentication with OCI registries

By default, the run / shell / exec and pull commands will attempt to use the login credentials found in the user’s $HOME/.singularity/docker-config.json file to authenticate with the OCI registry in use (e.g. DockerHub). This file is created and populated by the registry login command.

If this file does not exist, or exists but does not contain credentials for the registry in question, anonymous authentication will be used instead.

However, the run / shell / exec and pull commands can also use credentials stored in a different file of the user’s choosing, by specifying the --authfile <path> flag. See the documentation of the –authfile flag for details on how to create and use custom credential files.

Default Behaviour & --no-compat

By and large, the user experience when running in OCI-mode is similar to using the --compat flag with the native runtime, or running containers with other tools such as Docker. Particularly:

  • A writable in-memory overlay is provided by default. The container can be written to, but changes to the filesystem are lost when the container exits.

  • The home directory and current working directory are not mounted into the container.

While these defaults make it simpler to translate docker run commands to singularity run, OCI-mode can also be used with the --no-compat option to emulate SingularityCE’s traditional native runtime behaviour:

$ singularity shell --oci --no-compat docker://ubuntu

# The container is read-only
Singularity> touch /foo
touch: cannot touch '/foo': Read-only file system

# The current working directory was bind mounted, and is the default entry point
Singularity> pwd

# The user's home directory is bind mounted
Singularity> echo $HOME
Singularity> ls $HOME
file1   file2   file3

Feature set

With the release of SingularityCE 4.0, the functionality available in OCI mode - that is, when running SingularityCE shell / exec / run commands with the --oci flag - is approaching feature-parity with the native SingularityCE runtime, with some important exceptions noted below.


SingularityCE’s OCI mode also supports the Container Device Interface (CDI) standard for making GPUs and other devices from the host available inside the container. See the CDI section, below, for details.

The following features are supported in --oci mode:

  • docker://, docker-archive:, docker-daemon:, oci:, oci-archive:, library://, oras://, http://, and https:// image sources.

  • --fakeroot for effective root in the container.

  • Bind mounts via --bind or --mount.

  • --overlay to mount a SquashFS image (read-only), an EXT3 (read-only or writable), or a directory (read-only or writable), as overlays within the container.

    • Allows changes to the filesystem to persist across runs of the OCI container

    • Multiple simultaneous overlays are supported (though all but one must be mounted as read-only).

  • --cwd (synonym: --pwd) to set a custom starting working-directory for the container.

  • --home to set the in-container user’s home directory. Supplying a single location (e.g. --home /myhomedir) will result in a new tmpfs directory being created at the specified location inside the container, and that dir being set as the in-container user’s home dir. Supplying two locations separated by a colon (e.g. --home /home/user:/myhomedir) will result in the first location on the host being bind-mounted as the second location in-container, and set as the in-container user’s home dir.

  • --scratch (shorthand: -S) to mount a tmpfs scratch directory in the container.

  • –workdir <workdir>: if specified, will map /tmp and /var/tmp in the container to <workdir>/tmp and <workdir>/var_tmp, respectively, on the host (rather than to tmpfs storage, which is the default). If –scratch <scratchdir> is used in conjunction with –workdir, scratch directories will be mapped to subdirectories nested under <workdir>/scratch on the host, rather than to tmpfs storage.

  • --no-home to prevent the container home directory from being mounted.

  • --no-mount to disable the mounting of proc, sys, devpts, tmp, and home mounts in the container. Note: dev cannot be disabled in OCI-mode, and bind-path mounts are not supported.

  • Support for the SINGULARITY_CONTAINLIBS environment variable, to specify libraries to bind into /.singularity.d/libs/ in the container.

  • --hostname to set a custom hostname inside the container. (This requires a UTS namespace, therefore this flag will infer --uts.)

  • Handling --dns and resolv.conf on a par with native mode: the --dns flag can be used to pass a comma-separated list of DNS servers that will be used in the container; if this flag is not used, the container will use the same resolv.conf settings as the host.

  • Additional namespace requests with --net, --uts, --user.

  • --no-privs to drop all capabilities from the container process and enable the NoNewPrivileges flag.

  • --keep-privs to keep effective capabilities for the container process (bounding set only for non-root container users).

  • --add-caps and --drop-caps, to modify capabilities of the container process.

  • --rocm to bind ROCm GPU libraries and devices into the container.

  • --nv to bind NVIDIA driver / basic CUDA libraries and devices into the container.

  • --apply-cgroups, and the --cpu*, --blkio*, --memory*, --pids-limit flags to apply resource limits.

Features that are not yet supported include but are not limited to:

  • Custom --security options.

  • Support for instances (starting containers in the background).

Running existing non-OCI Singularity containers

OCI-mode can also be used to run containers in SingularityCE’s native format, which were created with singularity build or pulled without --oci. Note that the --no-compat option must still be specified to achieve behavior matching the native runtime defaults, otherwise the container will behave as if --compat was specified.

When running a native SIF container in OCI-mode, a compatibility warning is shown, as it is not possible to perfectly emulate the behaviour of the native runtime.

$ singularity run --oci --no-compat ubuntu_latest.sif
INFO:    Running a non-OCI SIF in OCI mode. See user guide for compatibility information.

Environment and action scripts are run using the container’s shell, rather than an embedded shell interpreter. Complex environment scripts may exhibit different behavior. Bare images that do not contain /bin/sh cannot be run.

OCI-SIF Images

When an OCI container is pull-ed or run directly from a URI in OCI-mode, it is encapsulated within a single OCI-SIF file.


OCI-SIF files are only supported by SingularityCE 4.0 and above. They cannot be run using older versions of SingularityCE.

An OCI-SIF file provides the same benefits as a native SIF images:

  • The single file is easy to share and transport between systems, including air-gapped hosts.

  • The container root filesystem is mounted at the point of execution, directly from the SIF. This avoids excessive metadata traffic when images are stored on shared network filesystems.

  • Digital signatures may be added to the OCI-SIF, and later verifed, using the singularity sign and singularity verify commands.

An OCI-SIF differs from a native-runtime SIF as it aims to minimize the ways in which the encapsulated container differs from its source:

  • Container configuration and files are stored as OCI Blobs. This means that the container can be pushed from the OCI-SIF to a registry as a native OCI image, consisting of these blobs, rather than an ORAS entry / artifact.

  • The container’s OCI image manifest and config are preserved, and directly inserted into the OCI-SIF.

  • The container’s root filesystem is squashed to a single layer, in squashfs format, but using an approach that better preserves metadata than for native SIF images.

  • The container root filesystem is not modified by the addition of Singularity-specific files and directories.

The differences in the layout of native SIF and OCI-SIF images can be seen by inspecting the same OCI container, pulled with and without the --oci flag.

$ singularity pull docker://ubuntu

$ singularity sif list ubuntu_latest.sif
ID   |GROUP   |LINK    |SIF POSITION (start-end)  |TYPE
1    |1       |NONE    |32176-32208               |Def.FILE
2    |1       |NONE    |32208-35190               |JSON.Generic
3    |1       |NONE    |35190-35386               |JSON.Generic
4    |1       |NONE    |36864-29818880            |FS (Squashfs/*System/amd64)

The native SIF, shown above, includes SingularityCE specific entries, such as a definition file and metadata.

$ singularity pull --oci docker://ubuntu

$ singularity sif list ubuntu_latest.oci.sif
ID   |GROUP   |LINK    |SIF POSITION (start-end)  |TYPE
1    |1       |NONE    |32176-29806000            |OCI.Blob
2    |1       |NONE    |29806000-29806807         |OCI.Blob
3    |1       |NONE    |29806807-29807216         |OCI.Blob
4    |1       |NONE    |29807216-29807456         |OCI.RootIndex

The OCI-SIF contains three OCI.Blob entries. These are the container root filesystem (as a single squashfs format layer), the image config, and the image manifest, respectively. There is no definition file or Singularity specific JSON metadata.

The final OCI.RootIndex is for internal use, and indexes the content of the OCI-SIF.

Future Development

Subsequent 4.x releases of SingularityCE will aim to improve OCI-mode to address missing features vs native mode, and improve the utility of the OCI-SIF format. Development items include:

  • Support for overlays embedded in OCI-SIF files.

  • Multi-layer OCI-SIF images, allowing full preservation of the structure of an OCI image embedded in an OCI-SIF. This will permit faithful 2-way translation to/from OCI-SIF.

Container Device Interface (CDI)

Beginning in SingularityCE 4.0, --oci mode supports the Container Device Interface (CDI) standard for making GPUs and other devices from the host available inside the container. It offers an alternative to previous approaches that were vendor specific, and unevenly supported across different container runtimes. Users of NVIDIA GPUs, and other devices with CDI configurations, will benefit from a consistent way of using them in containers that spans the cloud native and HPC fields.

SingularityCE’s “action” commands (run / exec / shell), when run in OCI mode, now support a --device flag:

--device strings                fully-qualified CDI device name(s).
                                A fully-qualified CDI device name
                                consists of a VENDOR, CLASS, and
                                NAME, which are combined as follows:
                                <VENDOR>/<CLASS>=<NAME> (e.g.
                                Multiple fully-qualified CDI device
                                names can be given as a comma
                                separated list.

This allows device from the host to be mapped into the container with the added benefits of the CDI standard, including:

  • Exposing multiple nodes on /dev as part of what is, notionally, a single “device”.

  • Mounting files from the runtime namespace required to support the device.

  • Hiding procfs entries.

  • Performing compatibility checks between the container and the device to determine whether to make it available in-container.

  • Performing runtime-specific operations (e.g. VM vs Linux container-based runtimes).

  • Performing device-specific operations (e.g. scrubbing the memory of a GPU or reconfiguring an FPGA).

In addition, SingularityCE’s OCI mode provides a --cdi-dirs flag, which enables the user to override the default search directory for CDI definition files:

--cdi-dirs strings              comma-separated list of directories
                                in which CDI should look for device
                                definition JSON files. If omitted,
                                default will be: /etc/cdi,/var/run/cdi

OCI Command Group

To support execution of containers via a CLI conforming to the OCI runtime specification, Singularity provides the oci command group.

The oci command group is not intended for end users, but as a low-level interface that can be leveraged by other software. In most circumstances, the OCI-mode (--oci) should be used instead of the oci command group.


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

OCI containers follow a different lifecycle from 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 and newer use 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 facilitate the development of OCI-mode.

OCI Specification Coverage

OCI Image Spec

In the default native mode, SingularityCE can convert container images that satisfy the OCI Image Format Specification into its own native 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 various limitations and workarounds may be required for some containers.

In OCI-mode, SingularityCE encapsulates OCI images in an OCI-SIF. The image config is preserved, and the container runs via a low-level OCI runtime for compatibiility with features of the image specification.

OCI Distribution Spec

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

Native SIF images can be pushed, as a single file, to registries that permit artifacts with arbitrary content types using oras:// URIs.

OCI-SIF images can be pushed to registries as a single file with oras:// URIs, or as an OCI image with docker:// URIs.


Although OCI-SIF images can be pushed to a registry as an OCI image, the squashfs layer format is not currently supported by other runtimes. The images can only be retrieved and run by SingularityCE 4.

OCI Runtime Spec

In its default mode, using the native runtime, 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 --compat option will apply various other flags to achieve behaviour that is closer (but not identical) to OCI runtimes such as Docker.

In OCI-mode, SingularityCE executes containers with a true OCI compatible low-level runtime. This allows compatibility with features of the OCI runtime specification that are not possible with the native runtime.

OCI Runtime CLI

The singularity oci commands were added to provide a mode of operation in which SingularityCE does implement the OCI runtime specification and container lifecycle, via a command line compatible with other low-level OCI runtimes. These commands are primarily of interest to tooling that might use SingularityCE as a container runtime, rather than end users. End users will general use the OCI-mode (--oci) with run / shell / exec.