node.js中的fs.realpath方法使用说明

方法说明：

获取真实路径。

可以使用process.cwd解决相对路径。

语法：

fs.realpath(path, [cache], [callback(err , resolvedPath)])

由于该方法属于fs模块，使用前需要引入fs模块（var fs= require(“fs”) ）

接收参数：

path                             路径

cache                           可选，一个文字的映射路径可用于强制一个特定的路径解决或避免额外的fs.stat需要知道真正的路径对象。

callback                       回调

err                                异常

resolvedPath               真实地址

例子：

var cache = {'/etc':'/private/etc'};

fs.realpath('/etc/passwd', cache, function (err, resolvedPath) {

  if (err) throw err;

  console.log(resolvedPath);

});

源码：

fs.realpath = function realpath(p, cache, cb) {

  if (!util.isFunction(cb)) {

    cb = maybeCallback(cache);

    cache = null;

  }

  // make p is absolute

  p = pathModule.resolve(p);

  if (cache && Object.prototype.hasOwnProperty.call(cache, p)) {

    return process.nextTick(cb.bind(null, null, cache[p]));

  }

  var original = p,

      seenLinks = {},

      knownHard = {};

  // current character position in p

  var pos;

  // the partial path so far, including a trailing slash if any

  var current;

  // the partial path without a trailing slash (except when pointing at a root)

  var base;

  // the partial path scanned in the previous round, with slash

  var previous;

  start();

  function start() {

    // Skip over roots

    var m = splitRootRe.exec(p);

    pos = m[0].length;

    current = m[0];

    base = m[0];

    previous = '';

    // On windows, check that the root exists. On unix there is no need.

    if (isWindows && !knownHard[base]) {

      fs.lstat(base, function(err) {

        if (err) return cb(err);

        knownHard[base] = true;

        LOOP();

      });

    } else {

      process.nextTick(LOOP);

    }

  }

  // walk down the path, swapping out linked pathparts for their real

  // values

  function LOOP() {

    // stop if scanned past end of path

    if (pos >= p.length) {

      if (cache) cache[original] = p;

      return cb(null, p);

    }

    // find the next part

    nextPartRe.lastIndex = pos;

    var result = nextPartRe.exec(p);

    previous = current;

    current += result[0];

    base = previous + result[1];

    pos = nextPartRe.lastIndex;

    // continue if not a symlink

    if (knownHard[base] || (cache && cache[base] === base)) {

      return process.nextTick(LOOP);

    }

    if (cache && Object.prototype.hasOwnProperty.call(cache, base)) {

      // known symbolic link. no need to stat again.

      return gotResolvedLink(cache[base]);

    }

    return fs.lstat(base, gotStat);

  }

  function gotStat(err, stat) {

    if (err) return cb(err);

    // if not a symlink, skip to the next path part

    if (!stat.isSymbolicLink()) {

      knownHard[base] = true;

      if (cache) cache[base] = base;

      return process.nextTick(LOOP);

    }

    // stat & read the link if not read before

    // call gotTarget as soon as the link target is known

    // dev/ino always return 0 on windows, so skip the check.

    if (!isWindows) {

      var id = stat.dev.toString(32) + ':' + stat.ino.toString(32);

      if (seenLinks.hasOwnProperty(id)) {

        return gotTarget(null, seenLinks[id], base);

      }

    }

    fs.stat(base, function(err) {

      if (err) return cb(err);

      fs.readlink(base, function(err, target) {

        if (!isWindows) seenLinks[id] = target;

        gotTarget(err, target);

      });

    });

  }

  function gotTarget(err, target, base) {

    if (err) return cb(err);

    var resolvedLink = pathModule.resolve(previous, target);

    if (cache) cache[base] = resolvedLink;

    gotResolvedLink(resolvedLink);

  }

  function gotResolvedLink(resolvedLink) {

    // resolve the link, then start over

    p = pathModule.resolve(resolvedLink, p.slice(pos));

    start();

  }

};