Wait for Port Open Shell Script

Docker Compose, Kubernetes init containers, and classic deploy scripts all need the same primitive: wait until MySQL listens on 3306 before running migrations. This portable Bash loop uses /dev/tcp (or nc when available) with a hard timeout.

What this script does

Arguments: host, port, optional timeout seconds
Polls every 2 seconds until the port accepts TCP
Fails with exit 1 after timeout (CI/CD can abort deploy)
Works in minimal images when bash is present
Safe for use in systemd ExecStartPre or entrypoint scripts

Prerequisites

Bash 4+ with /dev/tcp enabled (default on most Linux)
Network route to target host
Firewall allows probe from runner host

Step 1: Save the script

sudo nano /usr/local/bin/wait-for-port.sh
sudo chmod +x /usr/local/bin/wait-for-port.sh

Step 2: Full script (scroll to read)

wait-for-port.sh

#!/usr/bin/env bash
set -euo pipefail

HOST="${1:?usage: wait-for-port.sh HOST PORT [TIMEOUT_SEC]}"
PORT="${2:?usage: wait-for-port.sh HOST PORT [TIMEOUT_SEC]}"
TIMEOUT="${3:-120}"
INTERVAL=2

log(){ echo "[$(date '+%F %T')] $*"; }

port_open(){
  if command -v nc >/dev/null 2>&1; then
    nc -z -w 2 "$HOST" "$PORT" >/dev/null 2>&1
  else
    timeout 2 bash -c "echo >/dev/tcp/$HOST/$PORT" >/dev/null 2>&1
  fi
}

deadline=$(( $(date +%s) + TIMEOUT ))
attempt=0

log "Waiting for $HOST:$PORT (timeout ${TIMEOUT}s)"
while ! port_open; do
  attempt=$((attempt + 1))
  if [[ $(date +%s) -ge $deadline ]]; then
    log "TIMEOUT after ${attempt} attempts"
    exit 1
  fi
  sleep "$INTERVAL"
done

log "Port is open (attempt $attempt)"

Scroll inside the box to read the full script.

Step 3: Configure settings

Does not validate TLS or application health — only TCP open
For HTTP health use API health check script
Pair with git deploy script

Shell script waiting for TCP port to open before deploy

Step 4: Test manually

./wait-for-port.sh 127.0.0.1 22 30

echo exit=$?

Wait for Port Open Shell Script (Bash Loop for MySQL, Redis, Deploy)