Specify a port or randomly generate a new port number

[microlazy]

Version
3.10.0

Reproduction link
https://github.com/microlazy/vuecli_demo

Environment info
System:
OS: Linux 3.10 CentOS Linux 7 (Core)
CPU: (4) x64 Intel(R) Xeon(R) CPU E5-2695 v3 @ 2.30GHz
Binaries:
Node: 10.16.3 - /usr/local/node/bin/node
Yarn: 1.17.3 - /usr/local/node/bin/yarn
npm: 6.9.0 - /usr/local/node/bin/npm
Browsers:
Chrome: Not Found
Firefox: Not Found
npmPackages:
@vue/babel-helper-vue-jsx-merge-props: 1.0.0
@vue/babel-plugin-transform-vue-jsx: 1.0.0
@vue/babel-preset-app: 3.10.0
@vue/babel-preset-jsx: 1.1.0
@vue/babel-sugar-functional-vue: 1.0.0
@vue/babel-sugar-inject-h: 1.0.0
@vue/babel-sugar-v-model: 1.0.0
@vue/babel-sugar-v-on: 1.1.0
@vue/cli-overlay: 3.10.0
@vue/cli-plugin-babel: ^3.10.0 => 3.10.0
@vue/cli-plugin-eslint: ^3.10.0 => 3.10.0
@vue/cli-service: ^3.10.0 => 3.10.0
@vue/cli-shared-utils: 3.10.0
@vue/component-compiler-utils: 2.6.0 (3.0.0)
@vue/eslint-config-prettier: ^5.0.0 => 5.0.0
@vue/preload-webpack-plugin: 1.1.1
@vue/web-component-wrapper: 1.2.0
babel-helper-vue-jsx-merge-props: 2.0.3
eslint-plugin-vue: ^5.0.0 => 5.2.3 (4.7.1)
vue: ^2.6.10 => 2.6.10
vue-eslint-parser: 5.0.0 (2.0.3)
vue-hot-reload-api: 2.3.3
vue-loader: 15.7.1
vue-router: ^3.0.3 => 3.1.2
vue-style-loader: 4.1.2
vue-template-compiler: ^2.6.10 => 2.6.10
vue-template-es2015-compiler: 1.9.1
vuex: ^3.0.1 => 3.1.1
npmGlobalPackages:
@vue/cli: 3.10.0
Steps to reproduce
Configure devServer.port in vue.config.js to be equal to 8899 and save it. Execute yarn run serve to display the port that is running is not specified 8899
Import the project into vueui, specify the port as 8899 in the variable of the serve task in the task and save it. After clicking the run button, the port displayed in the output panel is not specified 8899.
What is expected?
I hope you can specify a fixed port number.

What is actually happening?
I also want to know exactly what caused this problem!

Excluded issue:

Port 8899 is not used
No other port number is specified in other locations.

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