UNIX is case sensitive
vi is case-sensitive:
Remember that UNIX is case sensitive. You can cause yourself all sorts of problems by using variable names with mixed case and then forgetting
how you have defined those variables.
Shell scripts do not perform any checking on variables like regular programming languages do, so it is up to you to be consistent in how you use variables so that your scripts are easy to debug.
The keystroke commands in vi are case-sensitive. The actions of the uppercase letter (such as A) and the lowercase letter (such as a) are slightly different.
To change from Insert Mode to Command Mode, press the Esc key.
To change from Command Mode to Last Line Mode, type : (or SHIFT-: in some UNIX operating systems).
When that is done, characters are echoed or shown on the last line on the screen.
To terminate Last Line Mode, type Enter after commands.