Building a Basic Container

The information can be taken from https://github.com/NIH-HPC/Singularity-Tutorial. I’m just documenting what I understand step-by-step

In Order for you to build a Container, you have to use a build command. The build command does the following

• Installs an OS
• Setup the Container’s Environment
• Install the apps required

Singularity Flow – Standard Development Cycle

• Create a writable container (called a sandbox)
• Shell into the container with the –writable option and tinker with it interactively
• Record changes that we like in our definition file
• Rebuild the container from the definition file if we break it
• Rinse and Repeat
• Rebuild the container from the final definition file as a read-only singularity image format (SIF) image for use in production

Let’s Build – Using the examples.

You can git clone Singularity Download Page and the examples is provided there.

% git clone https://github.com/sylabs/singularity.git

% mkdir ~lolcow
% cd singularity/examples/debian/Singularity ~/lolcow/lolcow.def
% cd ~/lolcow
% vim lolcow.def

BootStrap: debootstrap
OSVersion: stable
MirrorURL: http://ftp.us.debian.org/debian/

%runscript
    echo "This is what happens when you run the container..."

%post
    echo "Hello from inside the container"
    apt-get -y --allow-unauthenticated install vim

You can find more information on the Singularity Definition Files

Developing a new container

To build a container, you will need sudo privilege or root access

% sudo singularity build --sandbox lolcow lolcow.def

• Container: lolcow
• Definition File: lolcow.def
• Build a Container for Development Purposes: –sandbox

After firing the command, you will have a basic Debian contained saved in a local directory called lolcow

Explore and modify the container

In order to obtain root privileges within the root container, you will need root at the Host System

% sudo singularity shell --writable lolcow

The –writable option allows us to modify the container. The corresponding changes will be saved into the container and persist across uses.

Installing Packages in Singularity

Once inside the shell, you can modify the container and it will be saved persistently.

Singularity > apt-get install -y fortune cowsay lolcat

you have to PATH it so that container can find the binary

Singularity > export PATH=$PATH:/usr/games

Singularity> fortune | cowsay | lolcat
 ________________________________________
/ Q: Why haven't you graduated yet? A:   \
| Well, Dad, I could have finished years |
| ago, but I wanted                      |
|                                        |
\ my dissertation to rhyme.              /
 ----------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

Building Production Grade

 If you ultimately want all of these changes to be reflected in your definition file. Let’s update our definition file

% nano lolcow.def

BootStrap: debootstrap
OSVersion: stable
MirrorURL: http://ftp.us.debian.org/debian/

%runscript
    echo "This is what happens when you run the container..."

%post
    echo "Hello from inside the container"
    apt-get update
    apt-get -y install fortune cowsay lolcat

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

To rebuild the image from the definition, make sure you have the necessary yum package in your host.

# yum install debootstrap
# yum install squashfs-tools

If you are having further issues, make sure your hosts has the prerequistics packages installed. It can be found at https://github.com/NIH-HPC/Singularity-Tutorial/tree/master/01-installation

Building the Container

# singularity build lolcow.sif lolcow.def

.....
.....
INFO:    Adding environment to container
INFO:    Adding runscript
INFO:    Creating SIF file...
INFO:    Build complete: lolcow.sif

References:

• https://github.com/NIH-HPC/Singularity-Tutorial
• https://github.com/sylabs/singularity

The information can be taken from https://github.com/NIH-HPC/Singularity-Tutorial. I’m just documenting what I understand step-by-step

EXEC Command

Using the exec command, we can run commands within the container from the host system.

[user1@node ~]$ singularity exec lolcow_latest.sif cowsay 'How is today?'
 _______________
< How is today? >
 ---------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

RUN Command

Sometimes you can run a container

 [user1@node ~]$ singularity run lolcow_latest.sif
 _____________________________________
/ Fine day to work off excess energy. \
\ Steal something heavy.              /
 -------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

A special script “runscript” is activated when the “run” command is exercised. To have a closer look

[user1@node ~]$ singularity inspect --runscript lolcow_latest.sif
#!/bin/sh

    fortune | cowsay | lolcat

runscript consists of three simple commands with the output of each command piped to the subsequent command. Altenatively, you can do a

[user1@node ~]$ ./lolcow_latest.sif

Pipes and Redirection

Singularity allows piping services so that the host system can interact with the container. Here we are executing a command in the container and piping the output out into a file called output.txt

[user1@node ~]$ singularity exec lolcow_latest.sif cowsay moo > output.txt

You will notice that the output has been pushed to output.txt. Make sure you create output.txt prior

[user1@node ~]$ vim output.txt

Inside output.txt

 How are you >
 -------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

Let’s say we create a file called HelloThere and put in a text called “Shame On You”. The command is then fed into cowsay.

% cat HelloThere | singularity exec lolcow_latest.sif cowsay -n
______________
< Shame on You >
 --------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

References:

1. https://github.com/NIH-HPC/Singularity-Tutorial