nc -v -z すると 2 つ結果を返すのはなぜか？ ref: http://qiita.com/kitsuyui/items/cbac88800dfe5cfb8dcb

file0.txt

$ nc -v -z localhost 8000
nc: connectx to localhost port 8000 (tcp) failed: Connection refused
nc: connectx to localhost port 8000 (tcp) failed: Connection refused

file10.txt

$ nc -v -z  localhost 8000 ; echo $?
nc: connectx to localhost port 8000 (tcp) failed: Connection refused
found 0 associations
found 1 connections:
     1: flags=XX<CONNECTED,PREFERRED>
    outif XXX
    src 127.0.0.1 port XXXXX
    dst 127.0.0.1 port 8000
    rank info not available
    TCP aux info available

Connection to localhost port 8000 [tcp/irdmi] succeeded!
0

file2.txt

$ nc -v -z 127.0.0.1 8000
nc: connectx to 127.0.0.1 port 8000 (tcp) failed: Connection refused

file3.txt

$ nc -v -z ::1 8000
nc: connectx to ::1 port 8000 (tcp) failed: Connection refused

file4.txt

$ man nc

file5.txt

$ nc -v -z -4 localhost 8000
nc: connectx to localhost port 8000 (tcp) failed: Connection refused

file6.txt

$ nc -v -z -6 localhost 8000
nc: connectx to localhost port 8000 (tcp) failed: Connection refused

file7.txt

$ /usr/local/bin/nc -v -z localhost 8000
localhost [127.0.0.1] 8000 (irdmi): Connection refused

file9.txt

$ /usr/bin/nc -v -z -G 1 localhost 8000
nc: connectx to localhost port 8000 (tcp) failed: Connection refused
nc: connectx to localhost port 8000 (tcp) failed: Connection refused
nc: connectx to localhost port 8000 (tcp) failed: Operation timed out
nc: connectx to localhost port 8000 (tcp) failed: Operation timed out
nc: connectx to localhost port 8000 (tcp) failed: Operation timed out

hosts

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
127.0.0.2   localhost
127.0.0.3   localhost
127.0.0.4   localhost
255.255.255.255 broadcasthost
::1             localhost