sudo ulimit -s 16384 does not start a shell. Here sudo runs ulimit with arguments directly. This will throw command not found unless ulimit exists as a separate executable (and it may exist, like cd). If ulimit exists as a separate executable in the right place then sudo ulimit -s 16384 will "succeed", still without affecting the current shell. The conclusion and the gist of this answer is right; "sudo started a new shell, where it ran ulimit" is wrong.