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 *Âü°í) /bin/bash prog &> /dev/null <-- not good prog >& /dev/null <-- ok