/etc/update is an antiquated command which is no longer
necessary to be used.  It is not a shell script, but could easily be
one.  The executable basically takes one argument, the number of
seconds to sleep, or defaults to 30 (I believe).  It then sleeps that
interval and then issues a sync(2) system call.  This is a continuous
process -- it detaches itself from the current tty with the setpgrp(2)
call, and closes all file descriptors 0, 1, and 2 (stdin, stdout, and
stderr).  It was to be run by /etc/init, as a boot-time process, and
was to remain there for the duration of the machine's uptime.