Mongo copies logs in /var/log/mongodb/mongod.log. Consequently, you can take a look at the file by typing cat /var/log/mongodb/mongod.log. For your information, if you run
Here is a log when Mongodb does not start:
[email protected]:/lib/systemd/system$ mongod -v
2018-09-21T20:56:41.063+0200 I CONTROL [main] Automatically disabling TLS 1.0, to force-enable TLS 1.0 specify --sslDisabledProtocols 'none'
2018-09-21T20:56:41.066+0200 D NETWORK [main] fd limit hard:1048576 soft:1024 max conn: 819
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] MongoDB starting : pid=5202 port=27017 dbpath=/data/db 64-bit host=jojo-Z83-F
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] db version v4.0.2
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] git version: fc1573ba18aee42f97a3bb13b67af7d837826b47
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] OpenSSL version: OpenSSL 1.1.0g 2 Nov 2017
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] allocator: tcmalloc
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] modules: none
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] build environment:
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] distmod: ubuntu1804
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] distarch: x86_64
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] target_arch: x86_64
2018-09-21T20:56:41.079+0200 I CONTROL [initandlisten] options: { systemLog: { verbosity: 1 } }
2018-09-21T20:56:41.079+0200 D - [initandlisten] User Assertion: NonExistentPath: Data directory /data/db not found. src/mongo/db/storage/storage_engine_init.cpp 212
2018-09-21T20:56:41.080+0200 I STORAGE [initandlisten] exception in initAndListen: NonExistentPath: Data directory /data/db not found., terminating
2018-09-21T20:56:41.080+0200 I CONTROL [initandlisten] now exiting
2018-09-21T20:56:41.080+0200 I CONTROL [initandlisten] shutting down with code:100