RFE: singularity shrink

[akkornel]

When I am bootstrapping a container, I'm never too sure how big that container image needs to be. However, once I'm done bootstrapping it, I should never need to make changes to it.

It would be nice to have a singularity shrink command, which will shrink a container image to its smallest size.

It's actually possible to do this in shell today, with something like this script:

image=PATH_TO_IMAGE
stripped_img=`tempfile --directory=.`
tail -n +2 $image > $stripped_img
e2fsck -f $stripped_img
resize2fs -M $stripped_img
shrunk_img=`tempfile --directory=.`
head -n 1 $image > $shrunk_img
cat $stripped_img >> $shrunk_img
rm $stripped_img
mv $shrunk_img $image
chmod a+x $image

The above shell is inefficient (space utilization is up to 2x the image size), but safe (the original image isn't deleted until the very end).

But, it would probably be better implemented in the same way as singularity expand.

[dtrudg]

This issue is not really present in Singularity 3.x as we now build to SIF images, which are based on a squashfs which takes the size of the content.