The effect of the timeout option might surprise users

[djeikyb]

The timeout is implemented by doing N attempts and waiting one second between each attempt. If the attempt isn't instantaneous, the effective timeout balloons. Some versions of netcat allow you to control the tcp connection timeout. The version of netcat that shipped with my copy of macos sierra takes 75 seconds to timeout by default:

18:00:40]~% time nc google.com 807
nc google.com 807  0.00s user 0.00s system 0% cpu 1:15.25 total
18:02:01]~% time nc google.com 807
nc google.com 807  0.00s user 0.00s system 0% cpu 1:15.08 total

[bgehman]

I concur -- using alpine:3.6 docker image:

/app # time nc -z www.google.com 81
Command exited with non-zero status 1
real	2m 7.38s
user	0m 0.00s
sys	0m 0.00s

So, 2m7s multiple by 15 iterations and you are looking at ~30min "hang" with zero output being logged.

[ianfixes]

What do you get with time nc -w 1 -z www.google.com 81 ?

[bgehman]

@ianfixes Easy to test:

$ time docker run --rm -it alpine:3.6 nc -w 1 -z www.google.com 81

0.06s user 0.13s system 4% cpu 4.175 total

[ianfixes]

Aah, that's a nice one-liner! I put the time command in a different spot and the measurement looks more accurate:

$ docker run --rm -it alpine:3.6 time nc -w 1 -z www.google.com 81
Command exited with non-zero status 1
real	0m 1.00s
user	0m 0.00s
sys	0m 0.00s

[bgehman]

FYI, feel free to use my solution:

wait_for() {
  # $1 hostname    $2 port
  n=0
  until [ $n -ge 15 ]; do
    nc -z -w 10 "$1" "$2" && break
    n=$(($n+1))
    echo "`date`: Waiting for $1:$2 to be available... n=$n"
    sleep 10
  done

  if [ $n -ge 15 ]; then
    echo "`date`: ERROR: Timeout out waiting for $1:$2."
    exit 1
  else
    echo "`date`: Dependency $1:$2 is online."
  fi
}

[ianfixes]

The problem is that the nc version that ships with OSX splits the timeout setting into 2 different parameters, the -G one being almost more important (via stackoverflow). -G is unfortunately not very standard.

$ echo $OSTYPE
darwin17
$ time nc -w 1 www.google.com 81

real	1m6.857s
user	0m0.003s
sys	0m0.004s
$ time nc -w 1 -G 1 www.google.com 81

real	0m1.011s
user	0m0.003s
sys	0m0.004s

There's also the timeout command which works great but is not a built-in.