Originally Posted by CT_Wiebe SeijiSensei
-- I seriously doubt that it can be done without invoking "sudo" (or requiring a password entry at some point in the process) -- that would fly in the face of the security processes. I'm talking about changing my primary "login" password (only one "account" is involved).
If you've already logged in with your username and password, you can change your own password just by typing "passwd" at the terminal prompt. You've already validated yourself by logging in so you don't need to do so again.
You use the "sudo" command to run as the "root" user (uid 0) which has full rights. You'd need to use sudo to change someone else's password.
Remember that Unix was developed in the late 1960's before GUIs ever existed. The graphical tools that you see often do nothing more than construct a terminal command from the options you select and submit it to the operating system in the background. The tools to manage tasks like creating users and groups, adding passwords, changing file and directory permissions, and the like can all be run from the command prompt.
If you're curious to see what you can accomplish at the terminal, I suggest taking a look at the Bash Guide for Beginners
from the Linux Documentation Project
You might also be surprised to discover that just about everything important in terms of configuration is stored in plain-text files, usually in the /etc directory. For instance, all the users are listed in /etc/passwd, the groups in /etc/group, and the encrypted password hashes in /etc/shadow. This file is readable only by root; the others can be read by all users. (Originally the passwords were also kept in /etc/passwd, but that created security problems since /etc/passwd must be world-readable. The "shadow password" system was invented to store the passwords in /etc/shadow which only the root user can read. You'll need to use "sudo cat /etc/shadow" to list this file.)