MongoDB: log mongod to stdout - mongodb

Question

I have been running MongoDB on my home laptop for awhile now. running mongod outputs to stdout as it should. super...
I just installed it on my work laptop but it is logging to a file.
$ sudo mongod
all output going to: /usr/local/var/log/mongodb/mongo.log
MongoDB should by default be logging to stdout according to the docs. This installation of mongoDB was done via home brew. Why is mongod logging to a file, and how do i get it to log to stdout instead?

Answer 1

If you run cat `which mongod` you can see that the Ruby wrapper around the mongod binary appends the default config if the --config arguments isn't passed.
ARGV << '--config' << '/usr/local/etc/mongod.conf' unless ARGV.find { |arg| arg =~ /--config/ }
exec "/usr/local/Cellar/mongodb/2.2.3-x86_64/mongod", *ARGV
The default config contains the following
# Store data in /usr/local/var/mongodb instead of the default /data/db
dbpath = /usr/local/var/mongodb
# Append logs to /usr/local/var/log/mongodb/mongo.log
logpath = /usr/local/var/log/mongodb/mongo.log
logappend = true
# Only accept local connections
bind_ip = 127.0.0.1
as you can see both logappend and logpath is set. The logpath setting is normally None meaning /dev/stdout.
To the point. mongod --config false will output to STDOUT. That will run the mongod binary without a config. If that isn't ideal, change the config settings in /usr/local/etc/mongod.conf.

Answer 2

Besides turning off file logging as others have suggested, you can also do the following:
sudo mongod; tail -f /usr/local/var/log/mongodb/mongo.log

Answer 3

from the docs:
systemLog.destination Type: string
The destination to which MongoDB sends all log output. Specify either
file or syslog. If you specify file, you must also specify
systemLog.path.
If you do not specify systemLog.destination, MongoDB sends all log
output to standard output.

Answer 4

As other answers have said, it is likely that you have the "logpath" setting made in a configuration file. If you don't know where that is, you can find that out by running mongod like:
sudo su -
strace -e open mongod
This spits out some information - every file mongod tries to open - in which you should be able to find the config file. For me, it looked like this (partially):
…
open("/proc/cpuinfo", O_RDONLY) = 3
open("/dev/urandom", O_RDONLY) = 3
open("/etc/mongodb.conf", O_RDONLY) = 4
all output going to: /var/log/mongodb/mongodb.log
open("/var/log/mongodb/mongodb.log", O_WRONLY|O_CREAT|O_APPEND, 0666) = 4
open("/var/log/mongodb/mongodb.log", O_WRONLY|O_CREAT|O_APPEND, 0666) = 1
…
After you have found the file, comment out (with a #) the logpath= line and your log should be output to stdout.

Answer 5

On my machine brew created a mongod.conf file in /usr/local/etc/.
Moreover, you should see something like this in line 7 (or so) of your log file /usr/local/var/log/mongodb/mongo.log:
Tue Jan 8 10:14:25 [initandlisten] options: { bind_ip: "127.0.0.1", config: "/usr/local/etc/mongod.conf", dbpath: "/usr/local/var/mongodb" }

Answer 6

It's an old question and above answer might not be valid anymore.
Documentation says:
systemLog.destination Type: string
The destination to which MongoDB sends all log output. Specify either
file or syslog. If you specify file, you must also specify
systemLog.path.
If you do not specify systemLog.destination, MongoDB sends all log
output to standard output.
Changing the configuration file /etc/mongod.conf does not work for me.
$ cat /etc/mongod.conf
# mongod.conf for MongoDB local Server
# where to write logging data.
#systemLog:
# destination: file
# logAppend: true
# logRotate: reopen
# path: /var/log/mongodb/mongod.log
# Where and how to store data.
storage:
dbPath: /var/lib/mediation/mongodb
directoryPerDB: true
$ mongod --config /etc/mongod.conf
BadValue: --fork has to be used with --logpath or --syslog
Also this one is failing:
$ mongod --config /etc/mongod.conf --logpath ""
{"t":{"$date":"2022-10-24T12:43:20.863Z"},"s":"F", "c":"CONTROL", "id":20574, "ctx":"-","msg":"Error during global initialization","attr":{"error":{"code":2,"codeName":"BadValue","errmsg":"logpath cannot be empty if supplied"}}}
The solution for me is this one:
mongod --config /etc/mongod.conf --logpath /proc/$$/fd/0