sudo, write, talk, mesg & others

sudo let user to execute command in root privillege with out root passwd, when user input correct passwd of him/her/it self. There is only prerequisite: user must be configured in /etc/sudoers.
Common options of sudo is -l, -v, -K.
user should use visudo to edit etc/sudoers. visudo will choose editor according to EDITOR variable.

write, talk are used to send msg to other user. mesg is a switch, when invoked as mesg n, it will disable any msg sent except root.

wall = write all

etc/motd
etc/issue

su stands for substitute but not super

How to be a super user in UNIX? I know 2 ways - login as super or su
How to return to normal? 'exit', yes. '^D', yes. And 'suspend'. I prefer this one. Since I can play around by typing 'fg' - 'suspend' - 'fg' ...

Login shell: you can guess the meaning from its name. It will use /etc/profile ,~/.bash_profile, /etc/bashrc, ~/.bashrc
On the other hand non - login shell use only the former 2 files. That's the distinction I just know about profile and bashrc. When runing 'su -', it indeed means 'su -l'/'su --login'. I now know the magic under the hook.

What needs mentioning is 'passwd' & 'xscreensaver'. On RH linux, the former is used to change user password and the later is used to invoke x server screen saver.