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.
build command accepts a target as input and produces a container as
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
- compressed read-only Singularity Image File (SIF) format suitable for production (default)
- writable (ch)root directory called a sandbox for interactive development
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 (
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
Downloading an existing container from Docker Hub¶
You can use
build to download layers from Docker Hub and assemble them into
$ sudo singularity build lolcow.sif docker://godlovedc/lolcow
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
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
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.
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
xz and then trying to run that image on a CentOS 6 node, where the only compressor available is
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.
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
When used in combination with the
--remote option, the
will detach the build from your terminal and allow it to build in the background
without echoing any output to your terminal.
Specifies that Singularity should use a secret saved in either the
environment variable to build an encrypted container. See encrypted
containers for more details.
Gives users a way to build containers completely unprivileged. See the fakeroot feature for details.
--force option will delete and overwrite an existing Singularity image
without presenting the normal interactive prompt.
--json option will force Singularity to interpret a given definition
file as a json.
This command allows you to set a different library. (The default library is “https://library.sylabs.io”)
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
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.
This flag allows you to pass a plaintext passphrase to encrypt the container file system at build time. See encrypted containers for more details.
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.
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”.)
Build a sandbox (chroot directory) instead of the default SIF format.
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
none or any combination of
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.
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
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.
--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.