Build a Container

build is the “Swiss army knife” of container creation. You can use it to download and assemble existing containers from external resources like the Container Library and Docker Hub. You can use it to convert containers between the formats supported by Singularity. And you can use it in conjunction with a Singularity definition file to create a container from scratch and customized it to fit your needs.

Overview

The build command accepts a target as input and produces a container as output.

The target defines the method that build uses to create the container. It can be one of the following:

  • URI beginning with library:// to build from the Container Library
  • URI beginning with docker:// to build from Docker Hub
  • URI beginning with shub:// to build from Singularity Hub
  • path to a existing container on your local machine
  • path to a directory to build from a sandbox
  • path to a Singularity definition file

build can produce containers in two different formats that can be specified as follows.

  • compressed read-only Singularity Image File (SIF) format suitable for production (default)
  • writable (ch)root directory called a sandbox for interactive development ( --sandbox option)

Because build can accept an existing container as a target and create a container in either supported format you can convert existing containers from one format to another.

Downloading an existing container from the Container Library

You can use the build command to download a container from the Container Library.

$ sudo singularity build lolcow.simg library://sylabs-jms/testing/lolcow

The first argument (lolcow.simg) specifies a path and name for your container. The second argument (library://sylabs-jms/testing/lolcow) gives the Container Library URI from which to download. By default the container will be converted to a compressed, read-only SIF. If you want your container in a writable format use the --sandbox option.

Downloading an existing container from Docker Hub

You can use build to download layers from Docker Hub and assemble them into Singularity containers.

$ sudo singularity build lolcow.sif docker://godlovedc/lolcow

Creating writable --sandbox directories

If you wanted to create a container within a writable directory (called a sandbox) you can do so with the --sandbox option. It’s possible to create a sandbox without root privileges, but to ensure proper file permissions it is recommended to do so as root.

$ sudo singularity build --sandbox lolcow/ library://sylabs-jms/testing/lolcow

The resulting directory operates just like a container in a SIF file. To make changes within the container, use the --writable flag when you invoke your container. It’s a good idea to do this as root to ensure you have permission to access the files and directories that you want to change.

$ sudo singularity shell --writable lolcow/

Converting containers from one format to another

If you already have a container saved locally, you can use it as a target to build a new container. This allows you convert containers from one format to another. For example if you had a sandbox container called development/ and you wanted to convert it to SIF container called production.sif you could:

$ sudo singularity build production.sif development/

Use care when converting a sandbox directory to the default SIF format. If changes were made to the writable container before conversion, there is no record of those changes in the Singularity definition file rendering your container non-reproducible. It is a best practice to build your immutable production containers directly from a Singularity definition file instead.

Building containers from Singularity definition files

Of course, Singularity definition files can be used as the target when building a container. For detailed information on writing Singularity definition files, please see the Container Definition docs. Let’s say you already have the following container definition file called lolcow.def, and you want to use it to build a SIF container.

Bootstrap: docker
From: ubuntu:16.04

%post
    apt-get -y update
    apt-get -y install fortune cowsay lolcat

%environment
    export LC_ALL=C
    export PATH=/usr/games:$PATH

%runscript
    fortune | cowsay | lolcat

You can do so with the following command.

$ sudo singularity build lolcow.sif lolcow.def

The command requires sudo just as installing software on your local machine requires root privileges.

Note

Beware that it is possible to build an image on a host and have the image not work on a different host. This could be because of the default compressor supported by the host. For example, when building an image on a host in which the default compressor is xz and then trying to run that image on a CentOS 6 node, where the only compressor available is gzip.

Building encrypted containers

Beginning in Singularity 3.4.0 it is possible to build and run encrypted containers. The containers are decrypted at runtime entirely in kernel space, meaning that no intermediate decrypted data is ever present on disk or in memory. See encrypted containers for more details.

Build options

--builder

Singularity 3.0 introduces the option to perform a remote build. The --builder option allows you to specify a URL to a different build service. For instance, you may need to specify a URL to build to an on premises installation of the remote builder. This option must be used in conjunction with --remote.

--detached

When used in combination with the --remote option, the --detached option will detach the build from your terminal and allow it to build in the background without echoing any output to your terminal.

--encrypt

Specifies that Singularity should use a secret saved in either the SINGULARITY_ENCRYPTION_PASSPHRASE or SINGULARITY_ENCRYPTION_PEM_PATH environment variable to build an encrypted container. See encrypted containers for more details.

--fakeroot

Gives users a way to build containers completely unprivileged. See the fakeroot feature for details.

--force

The --force option will delete and overwrite an existing Singularity image without presenting the normal interactive prompt.

--json

The --json option will force Singularity to interpret a given definition file as a json.

--library

This command allows you to set a different library. (The default library is “https://library.sylabs.io”)

--notest

If you don’t want to run the %test section during the container build, you can skip it with the --notest option. For instance, maybe you are building a container intended to run in a production environment with GPUs. But perhaps your local build resource does not have GPUs. You want to include a %test section that runs a short validation but you don’t want your build to exit with an error because it cannot find a GPU on your system.

--passphrase

This flag allows you to pass a plaintext passphrase to encrypt the container file system at build time. See encrypted containers for more details.

--pem-path

This flag allows you to pass the location of a public key to encrypt the container file system at build time. See encrypted containers for more details.

--remote

Singularity 3.0 introduces the ability to build a container on an external resource running a remote builder. (The default remote builder is located at “https://cloud.sylabs.io/builder”.)

--sandbox

Build a sandbox (chroot directory) instead of the default SIF format.

--section

Instead of running the entire definition file, only run a specific section or sections. This option accepts a comma delimited string of definition file sections. Acceptable arguments include all, none or any combination of the following: setup, post, files, environment, test, labels.

Under normal build conditions, the Singularity definition file is saved into a container’s meta-data so that there is a record showing how the container was built. Using the --section option may render this meta-data useless, so use care if you value reproducibility.

--update

You can build into the same sandbox container multiple times (though the results may be unpredictable and it is generally better to delete your container and start from scratch).

By default if you build into an existing sandbox container, the build command will prompt you to decide whether or not to overwrite the container. Instead of this behavior you can use the --update option to build _into_ an existing container. This will cause Singularity to skip the header and build any sections that are in the definition file into the existing container.

The --update option is only valid when used with sandbox containers.

More Build topics

  • If you want to customize the cache location (where Docker layers are downloaded on your system), specify Docker credentials, or any custom tweaks to your build environment, see build environment.
  • If you want to make internally modular containers, check out the getting started guide here
  • If you want to build your containers on the Remote Builder, (because you don’t have root access on a Linux machine or want to host your container on the cloud) check out this site
  • If you want to build a container with an encrypted file system look here.