Errr 'mongo.js:L112 Error: couldn't connect to server 127.0.0.1:27017 at src/mongo/shell/mongo.js:L112'

If you are running Ubuntu, then there is an issue with folder ownership.

Run these commands:

1. Stop the MongoDB service

   sudo service mongodb stop
   

2. Remove the MongoDB lock file

   sudo rm /var/lib/mongodb/mongod.lock
   

3. Change ownership from root to the MongoDB path

   sudo chown -R mongodb:mongodb /var/lib/mongodb/
   

4. Start the MongoDB service

   sudo service mongodb start
   

5. Test the mongo application

   mongo
   

Then you will be able to execute successfully (I hope).

Reference: an answer on Stack Exchange site Database Administrators to Error: couldn't connect to server 127.0.0.1:27017 src/mongo/shell/mongo.js:91 when changing mongodb data directory!

Try to remove /var/lib/mongodb/mongod.lock and restart mongdo service

sudo rm /var/lib/mongodb/mongod.lock
sudo service mongodb restart

This method only works if you want to repair your data files without preserving the original files. To find where you dbpath resides, use

vim /etc/mongodb.conf

Check for option dbpath=.

(I have dbpath=/var/lib/mongodb.)

Default: /data/db/

Typical locations include /srv/mongodb, /var/lib/mongodb or /opt/mongodb.

Replace the /var/lib/mongodb with your dbpath

sudo rm /var/lib/mongodb/mongod.lock
sudo mongod --dbpath /var/lib/mongodb/ --repair
sudo mongod --dbpath /var/lib/mongodb/ --journal

(Make sure that you leave you terminal running in which you have run the above lines. Don't press Ctrl + C or quit it.) Type the command to start mongo now in another window.

I hope this works for you! For those who want to repair your data files while preserving the original files, see mongo recover.