adding dev-tcp

shell/dev-tcp.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+#
+# bash /dev/tcp redirection to check Internet connection
+#
+# SPDX-License-Identifier: MIT
+
+TCP_HOST=example.com       # A given website
+TCP_PORT=80                # Port 80 is http
+
+# Try to connect. (Somewhat similar to a 'ping' . . .)
+echo "HEAD / HTTP/1.0" >/dev/tcp/${TCP_HOST}/${TCP_PORT}
+MYEXIT=$?
+
+: <<EXPLANATION
+If bash was compiled with --enable-net-redirections, it has the capability of
+using a special character device for both TCP and UDP redirections. These
+redirections are used identically as STDIN/STDOUT/STDERR. The device entries
+are 30,36 for /dev/tcp:
+
+  mknod /dev/tcp c 30 36
+
+>From the bash reference:
+/dev/tcp/host/port
+    If host is a valid hostname or Internet address, and port is an integer
+port number or service name, Bash attempts to open a TCP connection to the
+corresponding socket.
+EXPLANATION
+
+
+if [ "X$MYEXIT" = "X0" ]; then
+  echo "Connection successful. Exit code: $MYEXIT"
+else
+  echo "Connection unsuccessful. Exit code: $MYEXIT"
+fi
+
+exit $MYEXIT