## 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.