[Singularity][a] enables users to have full control of their environment. A non-privileged user can "swap out" the operating system on the host for one they control. So if the host system is running RHEL6 but your application runs in Ubuntu/RHEL7, you can create an Ubuntu/RHEL7 image, install your applications into that image, copy the image to another host, and run your application on that host in its native Ubuntu/RHEL7 environment.
Singularity also allows you to leverage the resources of whatever host you are on. This includes HPC interconnects, resource managers, file systems, GPUs and/or accelerators, etc. Singularity does this by enabling several key facets:
* Encapsulation of the environment
* Containers are image based
* No user contextual changes or root escalation allowed
* No root owned daemon processes
This documentation is for Singularity version 2.4 and newer.
[Apptainer][a] is a container platform. It allows you to create and run containers that package up pieces of software in a way that is portable and reproducible. You can build a container using Apptainer on your laptop, and then run it on many of the largest HPC clusters in the world, local university or company clusters, a single server, in the cloud, or on a workstation down the hall. Your container is a single file, and you don’t have to worry about how to install all the software you need on each different operating system.
## Using Docker Images
Singularity can import, bootstrap, and even run Docker images directly from [Docker Hub][b]. You can easily run an RHEL7 container like this:
Apptainer can import, bootstrap, and even run Docker images directly from [Docker Hub][b]. You can easily run an CentOS container like this:
In this case, the image is downloaded from Docker Hub, extracted to a temporary directory, and Singularity interactive shell is invoked. This procedure can take a lot of time, especially with large images.
In this case, the image is downloaded from Docker Hub, extracted to a temporary directory, and Apptainer interactive shell is invoked. This procedure can take a lot of time, especially with large images.
## Importing Docker Image
Singularity containers can be in three different formats:
Apptainer containers can be in three different formats:
* read-only **squashfs** (default) - best for production
* writable **ext3** (--writable option)
* writable **(ch)root directory** (--sandbox option) - best for development
Squashfs and (ch)root directory images can be built from Docker source directly on the cluster, no root privileges are needed. It is strongly recommended to create a native Singularity image to speed up the launch of the container.
Squashfs and (ch)root directory images can be built from Docker source directly on the cluster, no root privileges are needed. It is strongly recommended to create a native Apptainer image to speed up the launch of the container.
The interactive shell can be invoked by the `singularity shell` command. This is useful for development purposes. Use the `-w | --writable` option to make changes inside the container permanent.
The interactive shell can be invoked by the `apptainer shell` command. This is useful for development purposes. Use the `-w | --writable` option to make changes inside the container permanent.
```console
hra0031@login4:~$singularity shell -w ubuntu.img
Singularity: Invoking an interactive shell within container...
$apptainer shell ubuntu.sif
Singularity ubuntu.img:~>cat /etc/lsb-release
Apptainer>cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=16.04
DISTRIB_CODENAME=xenial
DISTRIB_DESCRIPTION="Ubuntu 16.04.3 LTS"
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.1 LTS"
```
A command can be run inside the container (without an interactive shell) by invoking the `singularity exec` command.
A command can be run inside the container (without an interactive shell) by invoking the `apptainer exec` command.
A singularity image can contain a runscript. This script is executed inside the container after the `singularity run` command is used. The runscript is mostly used to run an application for which the container is built. In the following example, it is the `fortune | cowsay` command:
A Apptainer image can contain a runscript. This script is executed inside the container after the `apptainer run` command is used. The runscript is mostly used to run an application for which the container is built. In the following example, it is the `fortune | cowsay` command:
