The docker save command streams a tar archive of the specified image(s) to standard output by default. To capture this STDOUT stream into a file, you use the shell redirection operator >, as in docker save centos > centos-all.tar. The < operator instead redirects a file's content into a command's standard input, which is the opposite of what is needed here, and would not correctly capture the image data into the tar file. The file name must exactly match centos-all.tar as required by the task, ruling out variants with a space instead of a hyphen.