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();
}
};