大体流程

我们执行一个react-native init或start指令的时候入口在
react-native-cli/package.json

"bin": {
    "react-native": "index.js"
  },

从这里可以看到入口指向了react-native-cli/index.js，进入index.js
分析代码前先看一下文件注释

react native cli已全局安装在人们的计算机上。这意味着
很难让他们升级版本因为只安装了一个全局版本，所以很容易打破改变。react native cli的唯一工作是初始化存储库，然后将所有命令转发到react native的本地版本。如果需要添加新命令，请将其添加到本地cli/。修改此文件的唯一原因是添加了更多警告和“react native init”命令的疑难解答信息。不要做破坏性的改变！我们绝对不想告诉人们更新他们的react native cl的全局版本

所以说全局库中只有init的指令方法，其他指令方法都在本地local-cli中，接下来分析一下代码

//获取指令参数
var options = require('minimist')(process.argv.slice(2));

//获取react-native包下的cli路径
var CLI_MODULE_PATH = function() {
  return path.resolve(
    process.cwd(),
    'node_modules',
    'react-native',
    'cli.js'
  );
};

//获取react-native包下的package.json路径
var REACT_NATIVE_PACKAGE_JSON_PATH = function() {
  return path.resolve(
    process.cwd(),
    'node_modules',
    'react-native',
    'package.json'
  );
};
...
//获取cli文件
var cli;
var cliPath = CLI_MODULE_PATH();
if (fs.existsSync(cliPath)) {
  cli = require(cliPath);
}

//执行指令
var commands = options._;
if (cli) {
  cli.run();
} else {
  if (options._.length === 0 && (options.h || options.help)) {
    console.log([
      '',
      '  Usage: react-native [command] [options]',
      '',
      '',
      '  Commands:',
      '',
      '    init <ProjectName> [options]  generates a new project and installs its dependencies',
      '',
      '  Options:',
      '',
      '    -h, --help    output usage information',
      '    -v, --version output the version number',
      '',
    ].join('\n'));
    process.exit(0);
  }

  if (commands.length === 0) {
    console.error(
      'You did not pass any commands, run `react-native --help` to see a list of all available commands.'
    );
    process.exit(1);
  }

  switch (commands[0]) {
  case 'init':
    if (!commands[1]) {
      console.error(
        'Usage: react-native init <ProjectName> [--verbose]'
      );
      process.exit(1);
    } else {
      init(commands[1], options);
    }
    break;
  default:
    console.error(
      'Command `%s` unrecognized. ' +
      'Make sure that you have run `npm install` and that you are inside a react-native project.',
      commands[0]
    );
    process.exit(1);
    break;
  }
}

前面是获取指令参数和路径信息，然后是require cli文件，开始执行指令，判断如果有cli则执行cli.run，否则执行当前文件的逻辑。

init指令逻辑

生成项目会走到init方法，最后执行到run方法

/**
 * @param name Project name, e.g. 'AwesomeApp'.
 * @param options.verbose If true, will run 'npm install' in verbose mode (for debugging).
 * @param options.version Version of React Native to install, e.g. '0.38.0'.
 * @param options.npm If true, always use the npm command line client,
 *                       don't use yarn even if available.
 */
function init(name, options) {
  validateProjectName(name);

  if (fs.existsSync(name)) {
    createAfterConfirmation(name, options);
  } else {
    createProject(name, options);
  }
}

function run(root, projectName, options) {
  // E.g. '0.38' or '/path/to/archive.tgz'
  const rnPackage = options.version;
  const forceNpmClient = options.npm;
  const yarnVersion = (!forceNpmClient) && getYarnVersionIfAvailable();
  var installCommand;
  if (options.installCommand) {
    // In CI environments it can be useful to provide a custom command,
    // to set up and use an offline mirror for installing dependencies, for example.
    installCommand = options.installCommand;
  } else {
    if (yarnVersion) {
      console.log('Using yarn v' + yarnVersion);
      console.log('Installing ' + getInstallPackage(rnPackage) + '...');
      installCommand = 'yarn add ' + getInstallPackage(rnPackage) + ' --exact';
      if (options.verbose) {
        installCommand += ' --verbose';
      }
    } else {
      console.log('Installing ' + getInstallPackage(rnPackage) + '...');
      if (!forceNpmClient) {
        console.log('Consider installing yarn to make this faster: https://yarnpkg.com');
      }
      installCommand = 'npm install --save --save-exact ' + getInstallPackage(rnPackage);
      if (options.verbose) {
        installCommand += ' --verbose';
      }
    }
  }
  try {
    execSync(installCommand, {stdio: 'inherit'});
  } catch (err) {
    console.error(err);
    console.error('Command `' + installCommand + '` failed.');
    process.exit(1);
  }
  checkNodeVersion();
  cli = require(CLI_MODULE_PATH());
  cli.init(root, projectName);
}

后面分别会创建项目、安装yarn依赖、安装react-native依赖，然后获取到react-native的cli路径，执行cli的init方法。

/**
 * Creates the template for a React Native project given the provided
 * parameters:
 * @param projectDir Templates will be copied here.
 * @param argsOrName Project name or full list of custom arguments
 *                   for the generator.
 * @param options Command line options passed from the react-native-cli directly.
 *                E.g. `{ version: '0.43.0', template: 'navigation' }`
 */
function init(projectDir, argsOrName) {
  const args = Array.isArray(argsOrName)
    ? argsOrName // argsOrName was e.g. ['AwesomeApp', '--verbose']
    : [argsOrName].concat(process.argv.slice(4)); // argsOrName was e.g. 'AwesomeApp'

  // args array is e.g. ['AwesomeApp', '--verbose', '--template', 'navigation']
  if (!args || args.length === 0) {
    console.error('react-native init requires a project name.');
    return;
  }

  const newProjectName = args[0];
  const options = minimist(args);

  if (listTemplatesAndExit(newProjectName, options)) {
    // Just listing templates using 'react-native init --template'
    // Not creating a new app.
    return;
  } else {
    console.log('Setting up new React Native app in ' + projectDir);
    generateProject(projectDir, newProjectName, options);
  }
}

至此init指令流程基本分析完毕，有兴趣的童鞋可以跟一下代码