Using curl to test if a port is open: exit immediately upon connection if STDIN is /dev/null

From: Robin A. Meade via curl-users <curl-users_at_lists.haxx.se>
Date: Thu, 21 Apr 2022 16:52:37 -1000

Suggestion:

Make curl exit immediately with code 0 upon successful telnet connection
if STDIN is initially EOF.

That is, the following command would exit immediately after the
connection is established with exit code 0:

curl -s --connect-timeout 2 telnet://example.com:80 </dev/null


Discussion:

curl's telnet protocol support is often used interactively to test if a
port on a remote server is open [1][2][3]

[1]: https://www.cyberithub.com/check-test-port-connectivity-in-linux/
[2]: https://www.unixtutorial.org/test-tcp-connectivity-with-curl/
[3]:
https://www.computertechblog.com/testing-a-tcp-port-with-curl-in-linux/

When used in a script, though, there is the need to make curl exit
immediately upon successful connection. [1][2]

[1]: https://stackoverflow.com/q/42347271
[2]: https://stackoverflow.com/q/64106136

My best attempt at achieving this is to purposely pass an unknown telnet
option and test if the exit code is 48 (CURLE_UNKNOWN_OPTION):

curl -t 'DUMMY=1' -s --connect-timeout 2 telnet://example.com:80 </dev/null

Alternatively, pass an empty string as the telnet option and test for
exit code 49 (CURLE_SETOPT_OPTION_SYNTAX). I formalized this latter
approach as a function:


# Args:
# 1 - host
# 2 - port
tcp_port_is_open() {
   local exit_status_code
   curl -t '' --connect-timeout 2 -s telnet://"$1:$2" </dev/null
   exit_status_code=$?
   case $exit_status_code in
     49) return 0 ;;
     *) return "$exit_status_code" ;;
   esac
}


Tests:

tcp_port_is_open example.com 80
echo "$?"
# 0 ✔️

tcp_port_is_open example.com 1234
echo "$?"
# 28 - Operation timeout ✔️

tcp_port_is_open non.existent.example 80
echo "$?"
# 6 - Couldn't resolve host. ✔️

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Received on 2022-04-22