Add bash/port_checking.bash

bash/port_checking.bash

@@ -0,0 +1,33 @@
+## Stop installing tools just to check if a port is open. Bash has it built in.
+
+# Instead of:
+telnet host 443
+# or
+nmap host -p 443
+# Just use:
+echo > /dev/tcp/host/443 && echo "open" || echo "closed"
+
+# No tools required. No sudo. No package manager. Works on any machine with bash.
+
+# /dev/tcp is a bash built-in pseudo-device. Bash handles the TCP connection itself — the kernel never sees a file open on /dev/tcp.
+
+# Real world examples:
+
+# Check if SSH is up
+echo > /dev/tcp/192.168.1.100/22 && echo "SSH up" || echo "SSH down"
+
+# Check if your web server is listening
+echo > /dev/tcp/localhost/80 && echo "nginx up" || echo "nginx down"
+
+# Check SSL port before running a cert check
+echo > /dev/tcp/example.com/443 && echo "open" || echo "closed"
+
+# Loop until a service comes up (great for scripts)
+until echo > /dev/tcp/localhost/5432; do
+    echo "waiting for postgres..."
+    sleep 2
+done
+
+# That last one is the killer use case — waiting for a service to become available in a deploy script without installing netcat or curl or anything else.
+# One caveat: this is bash-specific. Won't work in sh, zsh, or fish. If portability matters, use nc -z host port instead.
+# Works on Linux and macOS.