Problem  : Failed to unlink socket file /tmp/mongodb-27017 error

Check the /tmp/mongodb-27017.sock permission, whether owner is root or mongodb?

$ ls -ls /tmp/mongodb-27017.sock
0 srwx------ 1 root root 0 Jan 2 01:56 /tmp/mongodb-27017.sock

To fix it, delete the /tmp/mongodb-27017.sock file manually and start the mongod process. Check the /tmp/mongodb-27017.sock permission again, the ownership is changed to the ‘mongodb’ user.

$ sudo rm -rf /tmp/mongodb-27017.sock
$ sudo service mongod start
$ sudo systemctl start mongod ( for centos 7 and above)

$ ls -ls /tmp/mongodb-27017.sock
0 srwx------ 1 mongodb mongodb 0 Jan 2 02:01 /tmp/mongodb-27017.sock