Singularity

On Linux, Singularity can be used as an alternative to Docker. Using Singularity instead of Docker may be easier or not, depending on your use case. Furthermore, on computing centers Singularity might be the only allowed solution, e.g. due to security issues.

One advantage of Singularity is that it preserves user's identity, contrary to Docker. In practice, the user's $HOME volume is automatically mounted when starting a container, and graphics are forwarded.

However, Singularity does not have an ecosystem as rich as Docker, requiring to convert existing Docker image(s) to Singularity one(s). Fortunately, this is usually straightforward.

Installation

On Debian and derivatives, the like Ubuntu, Singularity can be installed from the system package manager as singularity-container. For example,

sudo apt update
sudo apt install singularity-container

Please refer to the online documentation for alternative installation instructions.

Converting from Docker

Converting a Docker image to a Singularity one can be done with a single command line. For example, a Singularity image of the GRANDLIB development environment is created as

sudo singularity build --sandbox grandenv docker://grandlib/dev:latest

With the --sandbox option, the image is built as a regular folder, named grandenv in this case. Omitting this option, builds a SIF file (Singularity Image Format) instead.

In addition, note that root privileges are explicitly invoked (with the sudo command) when building the Singularity image. However, root privileges are not needed (nor recommended) at runtime. Only, when building the image.

Starting a shell

An interactive shell can be started in an image with the singularity shell command. For example,

singularity shell grandenv

starts an interactive shell within the previously created GRANDLIB environment. Note that Singularity preserves your identity. That is, you have the exact same rights within the image and outside. In addition, files and directories created from within the image have proper ownership.

If your $HOME volume is properly located, then it is automatically mounted when starting the Singularity image. Note that at CC-IN2P3 this fails due to non standard /home location. Then, the -H option can be used in order to explicitly mount your $HOME volume, e.g. as

singularity shell -H $HOME grandenv

Please refer to the command line --help for additional options.