CLI: Creating Sparse Files (Can be used for virtual loopback block devices)

create_sparse_file.sh

#!/usr/bin/env bash

# seems to work on any filesystem
# final size is (bs * seek) in bytes
# this creates a 4MB sparse file

dd if=/dev/zero of=/tmp/file bs=1 count=0 seek=4194304

# or 

dd if=/dev/zero of=/tmp/file bs=1 count=0 seek=4M

# linux specific
# this doesn't work on some filesystems like FAT32 and ZFS, but easier to remember
# its unique feature is that it does actually reserve the space
# preventing the space from being used by other things

fallocate -l 4M /tmp/file

# so you could create a sparse file larger than the amount of space you really have
# seems equivalent to the dd command, doesn't actually reserve the space

truncate -s 4M /tmp/file