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- Download and save a basic CentOS image from Docker : "singularity pull docker://centos:centos7.7.1908"with the following command. This command will save a file /tmp/centos_centos7.7.1908.sif to /tmp.
singularity pull docker://centos:centos7.7.1908
- Extract the squashfs image from the saved .sif file. A .sif has several objects inside, which you can exam by using "
singularity sif list /tmp/centos_centos7.7.1908.sif"command. The squashffs squashfs image we need is usually the 3rd object. Extract and save it with command "command
singularity sif dump 3 /tmp/centos_centos7.7.1908.sif > /tmp/centos_centos7.7.1908.sqsh
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- Unpack the files from the squashfs image: ". This will create a sub-directory squashfs-root, which contains the OS files. All files under this directory are own by you, not root.
/bin/unsquashfs \-no-xattrs /tmp/centos_centos7.7.1908.sqsh
- Singularity can use the files in /tmp/squashfs-root as an "image" (Singularity sandbox image). But one needs to change a few things in order to install additional rpms via yum in the image. So do this:
- Fix a few directory permission issue:
cd /tmp/squashfs-root; find usr bin lib* etc root var -type d -exec chmod u+w {} \;
- By default, yum checks if the user is root, and will refuse to install anything if it is not. So edit /tmp/squashfs-root/usr/share/yum-cli/yumcommands.py and change the following code and comment out the relevant code (in red)
def checkRootUID(base): """Verify that the program is being run by the root user.
:param base: a :class:`yum.Yumbase` object. :raises: :class:`cli.CliError` """ # if base.conf.uid != 0: # base.logger.critical(_('You need to be root to perform this command.')) # raise cli.CliError
- After the above change, you can start using the image and installing rpms, such as
singularity shell /tmp/squashfs-root
singularity exec \-w --no-home /tmp/squashfs-root yum install git
Step-by-step guide
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