Linux Network Port Watching

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This issue is from a machine without net-tools.x86_64 : Basic networking tools pre-installed, so netstat command does not exist. When watching docker registry pod setup, ansible runs netstat -tunlp | grep 5000 to check 5000 port, failed.

Is there other way around to check if the 5000 port is up or not? Yes.

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- name: Wait for docker registry to come up
  any_errors_fatal: true
  shell: |
    declare -a array=($(cat /proc/net/tcp6 | cut -d":" -f"3"|cut -d" " -f"1"))
    for port in ${array[@]};
    do
      # $((0x$port)) is 16 based, will output 10 based
      val=$(echo $((0x$port)) | grep 5000)
      if ! [[ "X${val}" == "X" ]]; then
        break
      fi
    done
    echo ${val} | grep 5000
  register: docker_registry
  until: docker_registry.rc == 0
  retries: "{{ 10 }}"

Here I watch /proc/net/tcp6 kernel file to see what ipv6 port is running (docker registry port is in ipv6 scope here). for ipv4, use /proc/net/tcp.

This file is not plain text, after use cut to extra the port field, then convert the number to decimal. see https://www.kernel.org/doc/Documentation/networking/proc_net_tcp.txt