TIL how to check if a port is open using only Bash

Bash can open TCP connections via /dev/tcp without needing curl, wget, or nc installed. This is handy for Docker healthchecks in minimal images:

services:
  collabora:
    image: collabora/code
    healthcheck:
      test: bash -c "exec 6<> /dev/tcp/localhost/9980"
      interval: 15s
      retries: 5
      start_period: 60s
      timeout: 10s

exec 6<> /dev/tcp/localhost/9980 tries to open file descriptor 6 as a read-write connection to localhost:9980. If the port is not listening, it fails and the healthcheck reports unhealthy.