exits:
EX_OK 0 # successful termination
EX_USAGE 64 # command line usage error
EX_DATAERR 65 # data format error
EX_NOINPUT 66 # cannot open input
EX_NOUSER 67 # addressee unknown
EX_NOHOST 68 # host name unknown
EX_UNAVAILABLE 69 # service unavailable
EX_SOFTWARE 70 # internal software error
EX_OSERR 71 # system error (e.g., can't fork)
EX_OSFILE 72 # critical OS file missing
EX_CANTCREAT 73 # can't create (user) output file
EX_IOERR 74 # input/output error
EX_TEMPFAIL 75 # temp failure; user is invited to retry
EX_PROTOCOL 76 # remote error in protocol
EX_NOPERM 77 # permission denied