Tests performed and outcome:

1. clean install singularity: PASSED

   yum install singularity --enablerepo=epel-testing
   

2. build, exec, run *.simg from definition file: PASSED

   sudo singularity build ubuntu.simg ubuntu.def
   singularity exec ubuntu.simg lsb_release -a
   singularity run ubuntu.simg
   

3. build and exec *.simg from Docker Hub: PASSED

   sudo singularity build ubuntu-docker.simg docker://ubuntu
   singularity exec ubuntu-docker.simg cat /etc/os-release
   

4. build, exec, run *.simg from Singulartiy Hub: PASSED

   sudo singularity build ubuntu-shub.simg shub://singularityhub/ubuntu
   singularity exec ubuntu-shub.simg cat /etc/os-release
   singularity run ubuntu-shub.simg
   

5. convert from .simg to .img and modify *.img with shell --writable: PASSED

   singularity image.create --size 2048 ubuntu.img
   sudo singularity build --writable ubuntu.img ubuntu.simg
   sudo singularity shell --writable ubuntu.img
   > apt-get install -y python
   > exit
   

6. exec and run modified *.img: PASSED

   singularity exec ubuntu.img python --version
   singularity run ubuntu.img