Files in Singularity containers

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How do I make data available in a Singularity container?

• What data is made available by default in a Singularity container?

Objectives

• Understand that some data from the host system is usually made available by default within a container

• Learn more about how Singularity handles users and binds directories from the host filesystem.

The way in which user accounts and access permissions are handeld in Singularity containers is very different from that in Docker (where you effectively always have superuser/root access). When running a Singularity container, you only have the same permissions to access files as the user you are running as on the host system.

In this episode we’ll look at working with files in the context of Singularity containers and how this links with Singularity’s approach to users and permissions within containers.

Users within a Singularity container

The first thing to note is that when you ran whoami within the container shell you started at the end of the previous episode, you should have seen the username that you were signed in as on the host system when you ran the container.

For example, if my username were jc1000, I’d expect to see the following:

$ singularity shell hello-world.sif
Singularity> whoami
jc1000

But hang on! I downloaded the standard, public version of the hello-world.sif image from Singularity Hub. I haven’t customised it in any way. How is it configured with my own user details?!

If you have any familiarity with Linux system administration, you may be aware that in Linux, users and their Unix groups are configured in the /etc/passwd and /etc/group files respectively. In order for the shell within the container to know of my user, the relevant user information needs to be available within these files within the container.

Assuming this feature is enabled within the installation of Singularity on your system, when the container is started, Singularity appends the relevant user and group lines from the host system to the /etc/passwd and /etc/group files within the container [1].

This means that the host system can effectively ensure that you cannot access/modify/delete any data you should not be able to on the host system and you cannot run anything that you would not have permission to run on the host system since you are restricted to the same user permissions within the container as you are on the host system.

Files and directories within a Singularity container

Singularity also binds some directories from the host system where you are running the singularity command into the container that you’re starting. Note that this bind process is not copying files into the running container, it is making an existing directory on the host system visible and accessible within the Singularity container environment. If you write files to this directory within the running container, when the container shuts down, those changes will persist in the relevant location on the host system.

There is a default configuration of which files and directories are bound into the container but ultimate control of how things are set up on the system where you are running Singularity is determined by the system administrator. As a result, this section provides an overview but you may find that things are a little different on the system that you’re running on.

One directory that is likely to be accessible within a container that you start is your home directory. You may also find that the directory from which you issued the singularity command (the current working directory) is also mapped.

The mapping of file content and directories from a host system into a Singularity container is illustrated in the example below showing a subset of the directories on the host Linux system and in a Singularity container:

Host system:                                                      Singularity container:
-------------                                                     ----------------------
/                                                                 /
├── bin                                                           ├── bin
├── etc                                                           ├── etc
│   ├── ...                                                       │   ├── ...
│   ├── group  ─> user's group added to group file in container ─>│   ├── group
│   └── passwd ──> user info added to passwd file in container ──>│   └── passwd
├── home                                                          ├── usr
│   └── jc1000 ───> user home directory made available ──> ─┐     ├── sbin
├── usr                 in container via bind mount         │     ├── home
├── sbin                                                    └────────>└── jc1000
└── ...                                                           └── ...

Questions and exercises: Files in Singularity containers

Q1: What do you notice about the ownership of files in a container started from the hello-world image? (e.g. take a look at the ownership of files in the root directory (/))

Exercise 1: In this container, try editing (for example using the editor vi which should be avaiable in the container) the /rawr.sh file. What do you notice?

If you’re not familiar with vi there are many quick reference pages online showing the main commands for using the editor, for example this one.

Exercise 2: In your home directory within the container shell, try and create a simple text file. Is it possible to do this? If so, why? If not, why not?! If you can successfully create a file, what happens to it when you exit the shell and the container shuts down?

Answers

A1: Use the ls -l command to see a detailed file listing including file ownership and permission details. You should see that most of the files in the / directory are owned by root, as you’d probably expect on any Linux system. If you look at the files in your home directory, they should be owned by you.

A Ex1: We’ve already seen from the previous answer that the files in / are owned by root so we wouldn’t expect to be able to edit them if we’re not the root user. However, if you tried to edit /rawr.sh you probably saw that the file was read only and, if you tried for example to delete the file you would have seen an error similar to the following: cannot remove '/rawr.sh': Read-only file system.

A Ex2: Within your home directory, you should be able to successfully create a file. Since you’re seeing your home directory on the host system which has been bound into the container, when you exit and the container shuts down, the file that you created within the container should still be present when you look at your home directory on the host system.

Binding additional host system directories to the container

You will sometimes need to bind additional host system directories into a container you are using over and above those bound by default. For example:

• There may be a shared dataset in a shared location that you need access to in the container
• You may require access, within the running container, to additional executables and software libraries stored on the host system

The -B option to the singularity command is used to specify additonal binds. For example, to bind the /work/z19/shared directory into a container you could use (note this directory is unlikely to exist on the host system you are using so you’ll need to test this using a different directory):

$ singularity shell -B /work/z19/shared hello-world.sif
Singularity> ls /work/z19/shared

CP2K-regtest	    cube	     eleanor		   image256x192.pgm		kevin		    pblas			    q-e-qe-6.7 
ebe		    evince.simg	     image512x384.pgm	   low_priority.slurm           pblas.tar.gz	                                    q-qe
Q1529568	    edge192x128.pgm  extrae		   image768x1152.pgm		mkdir		    petsc			    regtest-ls-rtp_forCray
adrianj		    edge256x192.pgm  gnuplot-5.4.1.tar.gz  image768x768.pgm		moose.job	    petsc-hypre			    udunits-2.2.28.tar.gz
antlr-2.7.7.tar.gz  edge512x384.pgm  hj			   job-defmpi-cpe-21.03-robust	mrb4cab		    petsc-hypre-cpe21.03	    xios-2.5
cdo-archer2.sif     edge768x768.pgm  image192x128.pgm	   jsindt			paraver		    petsc-hypre-cpe21.03-gcc10.2.0

Note that, by default, a bind is mounted at the same path in the container as on the host system. You can also specify where a host directory is mounted in the container by separating the host path from the container path by a colon (:) in the option:

$ singularity shell -B /work/z19/shared:/shared-data hello-world.sif
Singularity> ls /shared-data

CP2K-regtest	    cube	     eleanor		   image256x192.pgm		kevin		    pblas			    q-e-qe-6.7 
ebe		    evince.simg	     image512x384.pgm	   low_priority.slurm           pblas.tar.gz	                                    q-qe
Q1529568	    edge192x128.pgm  extrae		   image768x1152.pgm		mkdir		    petsc			    regtest-ls-rtp_forCray
adrianj		    edge256x192.pgm  gnuplot-5.4.1.tar.gz  image768x768.pgm		moose.job	    petsc-hypre			    udunits-2.2.28.tar.gz
antlr-2.7.7.tar.gz  edge512x384.pgm  hj			   job-defmpi-cpe-21.03-robust	mrb4cab		    petsc-hypre-cpe21.03	    xios-2.5
cdo-archer2.sif     edge768x768.pgm  image192x128.pgm	   jsindt			paraver		    petsc-hypre-cpe21.03-gcc10.2.0

You can also specify multiple binds to -B by separating them by commas (,).

An alternative option to binding a directory from the host system into a container is to copy required data into a container image at build time. This is useful when there is some static data required in the image. We cover this later in the section on building Singularity images.

References

[1] Gregory M. Kurzer, Containers for Science, Reproducibility and Mobility: Singularity P2. Intel HPC Developer Conference, 2017. Available at: https://www.intel.com/content/dam/www/public/us/en/documents/presentation/hpc-containers-singularity-advanced.pdf

Key Points

• Your current directory and home directory are usually available by default in a container.

• You have the same username and permissions in a container as on the host system.

• You can specify additional host system directories to be available in the container.