bug#24276: head without pipline is not reading complete STDIN, resulting in unwilling and unexpected command execution

[Paul Eggert]

This is not a bug. It's quite OK for 'head' to read just as much input as is 
required, and to not bother to read the rest of input. In fact, POSIX requires 
this behavior if standard input is a seekable file. See:

http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html
and look for "seekable input".