It's reading from a file named foo, which in this case was 4GB large and created with: dd if=/dev/zero of=foo bs=1024 count=$((1024*1024*4)) You probably do

It's reading from a file named foo, which in this case was 4GB large and created with: dd if=/dev/zero of=foo bs=1024 count=$((1024*1024*4)) You probably don't have a file of that name in the current working directory?