Where is the mongodb log file on Linux?

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 mongod -v at the prompt you see the last lines of the mongod.log file

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