Ubuntu - How do I change my username?

Unix-like operating systems decouple the user name from the user identity, so you may safely change the name without affecting the ID. All permissions, files, etc are tied to your identity (uid), not your username.

To manage every aspect of the user database, you use the usermod tool.

1- To change username (it is probably best to do this without being logged in):

sudo usermod -l newUsername oldUsername

This however, doesn't rename the home folder.

2- To change home-folder, use

sudo usermod -d /home/newHomeDir -m newUsername

after you changed the username.

For instance, you could logout, drop to a console (Ctrl+Alt+F1), and sudo su - to become true root (as opposed to sudo -s, where $HOME is still /home/yourname.) Maybe you also have to kill some still running processes from this user first. To do so, enter ps -u username, look for the matching PID and kill them by kill PID-number.

Update: as arrange mentioned, some files may reference your old home directory. You can either keep a symlink for backward compatibility, e g ln -s /home/newname /home/oldname or you can change the file contents with sed -i.bak 's/*oldname*/*newname*/g' *list of files* It creates a backup for each file with a .bak extension.

Some additional information for not so experienced users like me:
As I only have ONE user account (administrator), it would not let me change the username ("you are already logged in" was the response in TTY1 (Ctrl+Alt+F1). To get around this:

1. Login with your old credentials and add a new user, e.g. "temporary" in TTY1:
   

   sudo adduser temporary
   

   set the password.
2. Allow the temporary user to run sudo by adding the user to sudo group:
   

   sudo adduser temporary sudo
   

3. Log out with the command exit.
4. Return to tty1: Login with the 'temporary' user account and password. Change your username and folder as mentioned above. exit (until you get the login prompt)
5. Go back to TTY7 (Ctrl+Alt+F7) to login on the GUI/normal desktop screen and see if this works.
6. Delete temporary user and folder:
   

   sudo deluser temporary
   sudo rm -r /home/temporary
   

NB: Copied from several resources. Thanks for the contribution. 

Mac OS X - Reset MySQL Root Password

Have you forgotten your Mac OS X 'ROOT' password  and need to reset it?  Follow these 4 simple steps:

1.  Stop the mysqld server.  Typically this can be done by from 'System Prefrences' > MySQL > 'Stop MySQL Server'

2.  Start the server in safe mode with privilege bypass

     From a terminal:

     sudo /usr/local/mysql/bin/mysqld_safe --skip-grant-tables

3.  In a new terminal window:

     sudo /usr/local/mysql/bin/mysql -u root

For MySQL older than MySQL 5.7 use:

UPDATE mysql.user SET Password=PASSWORD('your-password') WHERE User='root';

For MySQL 5.7+ use:

USE mysql;

UPDATE mysql.user SET authentication_string=PASSWORD("your-password") WHERE User='root';

Refresh and quit:

     FLUSH PRIVILEGES;

     \q

4.  Stop the mysqld server again and restart it in normal mode.