Singularity Compatibility

Since the community decided to move the project into the Linux Foundation with the constraint of a name change to the project, it has been a goal of the project to minimize the impact to the user base. If you experience issues making the move, please reach out to the community so we can help you!

SIF Image Compatibility

The Apptainer project has decided to make no changes related to the project renaming at the image format level. This means that default metadata within SIF images and their filesystems will retain the singularity name without change. This will ensure that containers built with Apptainer will continue to work with installations of Singularity.

This decision was made to ensure that users can continue to package their applications and data with Apptainer without concerns of image format incompatibility when running in a different computing environment or collaborating with colleagues that still use a Singularity installation.

Singularity Prefixed Environment Variable Support

Apptainer respects environment variables with the SINGULARITY_ and SINGULARITYENV_ prefixes when their respective APPTAINER_ and APPTAINERENV_ counterparts are not set.

This first command does not use any environment variable compatibility behavior as it is using the APPTAINERENV_ prefix to command Apptainer to create an environment variable in the container with the name FOO and the value BAR:

$ APPTAINERENV_FOO=BAR apptainer exec docker://alpine env

We can see from the env output that this environment variable was properly set inside the container.

Next we can use the SINGULARITYENV_ prefix to do the same thing, but this time we wil have the intended value be BAZ:

$ SINGULARITYENV_FOO=BAZ apptainer exec  docker://alpine env
WARNING: DEPRECATED USAGE: Forwarding SINGULARITYENV_FOO as environment variable will not be supported in the future, use APPTAINERENV_FOO instead

Notice that we have a warning for DEPRECATED USAGE when doing so. This is because a future version of Apptainer may stop supporting environment variable compatibility once it is past this current period of transition.

Finally, if both are set, the value set by the APPTAINERENV_ variable will take priority over the SINGULARITYENV_ variable.

$ APPTAINERENV_FOO=BAR SINGULARITYENV_FOO=BAZ apptainer exec iqube_latest.sif env
WARNING: Skipping environment variable [SINGULARITYENV_FOO=BAZ], FOO is already overridden with different value [BAR]

In this case a warning is emitted to let the user know that two variables were set to create the same environment variable in the container in case they were unaware of one of them existing in their environment.

Automatic User Configuration Migration

Apptainer stores user configuration in files and directories under ~/.apptainer. Invocation of the apptainer command will automatically trigger Apptainer to create this directory, if it doesn’t exist. In order to ease the transition of users from Singularity to Apptainer, Apptainer will look for a ~/.singularity directory when the ~/.apptainer directory is being created and will migrate user configuration files and keyrings automatically. The following data will be migrated if it is found:

  • Remote endpoint configurations

  • OCI registry configurations stored in the Docker config format

  • Singularity Public PGP keyring

  • Singularity Private PGP keyring

Below we can see example output from when user configuration is being migrated:

$ apptainer exec docker://alpine echo
INFO:    Detected Singularity user configuration directory
INFO:    Detected Singularity remote configuration, migrating...
INFO:    Detected Singularity docker configuration, migrating...
INFO:    Detected public Singularity pgp keyring, migrating...
INFO:    Detected private Singularity pgp keyring, migrating...
INFO:    Converting OCI blobs to SIF format
INFO:    Starting build...

We can also see that subsequent use of Apptainer will not perform this migration again:

$ apptainer exec docker://alpine echo
WARNING: /usr/local/etc/singularity/ exists, migration to apptainer by system administrator is not complete
INFO:    Using cached SIF image


Apptainer will not migrate cached container data such as OCI blobs and SIF images. User caches will need to be manually migrated or reconstructed through normal use of Apptainer.

The default remote configuration has changed in Apptainer, so if you did not previously set any remotes and want to continue to use the previous default for library:// and/or pgp keys, see Restoring pre-Apptainer library behavior.