1. /bin/sh

prog > /dev/null 2>&1             <-- ok
prog > /dev/null 2>/dev/null      <-- ok
prog >& /dev/null                 <-- ok


2. /bin/csh

prog > /dev/null 2>&1             <-- fail
Ambiguous output redirect.

prog > /dev/null 2>/dev/null      <-- fail
Ambiguous output redirect.

prog >& /dev/null                 <-- ok, recommend


3. FreeBSD /etc/crontab
SHELL=/bin/sh

prog >& /dev/null                 <-- fail, bad FD
prog &> /dev/null                 <-- not good, is bg
prog > /dev/null 2>&1             <-- ok


*참고) /bin/bash
prog &> /dev/null                 <-- not good
prog >& /dev/null                 <-- ok