Question: How can I find and read the MongoDB system message log files?  I'm troubleshooting some issues and need to gather some extra information 

Answer: There's a few different options for identifying and reading the  messaging log file . As a first step check the "mongod.conf" file , which has reference to the systemlog file 

The default installation for the log file is /var/log/mongodb/mongodb.log but if you don't find it at this location , them check the mongod.conf file

You'll see something like :

# where to write logging data.
systemLog:
  destination: file
  logAppend: true
  path: /var/log/mongodb/mongod.log

An alternative method to read 1024 mongod events which are logged is to use the getLog command. Just to clarify - the MongoDB command getLog reads only from RAM cache - the getLog doesn't read from the mongod Log.    Note: The getLog requires administrative privileges otherwise you'll get an error message

 "errmsg" : "not authorized on admin to execute command { getLog: \"*\", lsid: { id: UUID(\"xxxxxxxxxxxxxxxxxxxx\") }, $clusterTime: { clusterTime: Timestamp(1566284667, 1), signature: { hash: BinData(0, xxxxxxxxxxxxxxxxxxxxx), keyId: xxxxxxxxxxxxxx } }, $db: \"admin\" }",
        "code" : 13,
        "codeName" : "Unauthorized",

Read MongoDB cheatsheet for more MongoDB commands