How can I find the port of mongo server started by homebrew using?

I started the mongo server by

   brew services restart mongodb@3.4
Stopping `mongodb@3.4`... (might take a while)
==> Successfully stopped `mongodb@3.4` (label: homebrew.mxcl.mongodb@3.4)
==> Successfully started `mongodb@3.4` (label: homebrew.mxcl.mongodb@3.4)

And I can check it is really running by

   brew services list
Name        Status  User     Plist
mongodb@3.4 started root     /Library/LaunchDaemons/homebrew.mxcl.mongodb@3.4.plist

But I when I tried to connect it via the standard port, it fails:

   lsof -i:27017
COMMAND     PID     USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME

the GUI:

By cat the plist file listed in the result of brew services list, I get the mongodb config file path:

cat /Library/LaunchDaemons/homebrew.mxcl.mongodb@3.4.plist
<?xml version="1.0" encoding="UTF-8"?>
...
    <string>/usr/local/etc/mongod.conf</string>
...

So I cat the conf file:

cat /usr/local/etc/mongod.conf 
systemLog:
  destination: file
  path: /usr/local/var/log/mongodb/mongo.log
  logAppend: true
storage:
  dbPath: /usr/local/var/mongodb
net:
  bindIp: 127.0.0.1

So again I cat the system log again:

cat /usr/local/var/log/mongodb/mongo.log
2018-10-10T11:02:19.071+0800 I NETWORK  [initandlisten] shutdown: going to close listening sockets...
2018-10-10T11:02:19.071+0800 I NETWORK  [initandlisten] removing socket file: /tmp/mongodb-27017.sock
2018-10-10T11:02:19.071+0800 I NETWORK  [initandlisten] shutdown: going to flush diaglog...
2018-10-10T11:02:19.071+0800 I CONTROL  [initandlisten] now exiting
2018-10-10T11:02:19.071+0800 I CONTROL  [initandlisten] shutting down with code:100

So the reason why lsof -i:27017 gives no result is the mongo server met some error and existed. Sad...

So the question becomes: How can I find the root cause the error that mongo server exits? and how to fix it? Thanks!