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Using Apptainer (with Docker Images)


Singularity is now Apptainer! While Apptainer provides an singularity alias for backwards compatibility, it is recommanded to adapt all workflows to use the new binary apptainer.

Apptainer ( is a popular alternative to docker, because it does not require to run as a privileged user. Apptainer can run Docker images out-of-the-box by converting them to the apptainer image format. The following guide gives a quick dive into using docker images with apptainer.

Build on your workstation, run on the HPC

Building images using Apptainer requires root privileges. We cannot give you these permissions on the BIH HPC. Thus, you will have to build the images on your local workstation (or anywhere where you have root access). You can then run the built images on the BIH HPC.

This is also true for the --writeable flag. Apparently it needs root permissions which you don't have on the cluster.


Link ~/.apptainer to ~/work/.apptainer

Because you only have a quota of 1 GB in your home directory, you should symlink ~/.apptainer to ~/work/.apptainer.

host:~$ mkdir -p ~/work/.apptainer && ln -sr ~/work/.apptainer ~/.apptainer

In case you already have a apptainer directory:

host:~$ mv ~/.apptainer ~/work/.apptainer && ln -sr ~/work/.apptainer ~/.apptainer

Run a bash in a docker image:

host:~$ apptainer shell docker://godlovedc/lolcow

Run a command in a docker image:

host:~$ apptainer exec docker://godlovedc/lolcow echo "hello, hello!"

Run a bash in a docker image, enable access to the cuda driver (--nv) and mount a path (--bind or -B):

host:~$ apptainer shell --nv --bind /path_on_host/:/path_inside_container/ docker://godlovedc/lolcow

Some Caveats and Notes


  • The default apptainer images format (.sif) is read-only.
  • By default apptainer mounts /home/$USER, /tmp, and $PWD in the container.


  • Environment variables can be provided by setting them in the bash and adding the prefix APPTAINERENV_:
    host:~$ APPTAINERENV_HELLO=123 apptainer shell docker://godlovedc/lolcow echo $HELLO
  • Calling apptainer shell or apptainer exec uses as cwd the host callers cwd not the one set in the Dockerfile. One can change this by setting --pwd.

Referencing/Providing Docker Images

Option 1: Use Docker Images via Docker Hub

The easiest variant to run a docker image available via a docker hub is by specifying its url. This causes apptainer to download the image and convert it to a apptainer image:

host:~$ apptainer run docker://godlovedc/lolcow

or to open a shell inside the image

host:~$ apptainer shell docker://godlovedc/lolcow

Furthermore, similar to docker, one can pull (and convert) remote image with the following call:

host:~$ apptainer pull docker://godlovedc/lolcow

In case your registry requires authentication you can provide it via a prompt by adding the option --docker-login:

host:~$ apptainer pull --docker-login docker://ilumb/mylolcow

or by setting the following environment variables:

host:~$ export APPTAINER_DOCKER_USERNAME=ilumb
host:~$ export APPTAINER_DOCKER_PASSWORD=<redacted>
host:~$ apptainer pull docker://ilumb/mylolcow

More details can be found in the Apptainer documentation.

Option 2: Converting Docker Images

Another option is to convert your docker image into the Apptainer/Singularity image format. This can be easily done using the docker images provided by docker2singularity.

To convert the docker image docker_image_name to the apptainer image apptainer_image_name one can use the following command line. The output image will be located in output_directory_for_images.

host:~$ docker run -v /var/run/docker.sock:/var/run/docker.sock -v /output_directory_for_images/:/output --privileged -t --rm --name apptainer_image_name docker_image_name

The resulting image can then directly be used as image:

host:~$ apptainer exec apptainer_image_name.sif bash

Conversion Compatibility

Here are some tips for making Docker images compatible with Apptainer taken from docker2singulrity:

  • Define all environmental variables using the ENV instruction set. Do not rely on ~/.bashrc, ~/.profile, etc.
  • Define an ENTRYPOINT instruction set pointing to the command line interface to your pipeline.
  • Do not define CMD - rely only on ENTRYPOINT.
  • You can interactively test the software inside the container by overriding the ENTRYPOINT docker run -i -t --entrypoint /bin/bash bids/example.
  • Do not rely on being able to write anywhere other than the home folder and /scratch. Make sure your container runs with the --read-only --tmpfs /run --tmpfs /tmp parameters (this emulates the read-only behavior of Apptainer).
  • Don't rely on having elevated user permissions.
  • Don't use the USER instruction set.