Day 1 - The Three Modes of the vi Editor

📌 Key Points

• Editors are divided into line editors that work one line at a time (ed, ex) and screen editors that work on the whole screen (vi, emacs).
• vi was created by Bill Joy. He is the same person who created the C shell (csh).
• vi has three modes: command mode, insert mode, and ex mode. It starts in command mode.
• You switch modes with three keys: i (to insert), ESC (to command), and : (to ex).
• Save with :w, quit with :q, save and quit with :wq, and force quit with :q!.

Branches of Editors

You may be asked directly who created each one. vi is Bill Joy, emacs is Richard Stallman. It also helps to remember that Bill Joy created the C shell (csh) too.

How They Came About

   ed (line editor)  →  ex (line editor, Bill Joy)  →  vi (ex's screen mode, 1976)  →  vim (Bram Moolenaar, 1991)
 
   pico (editor of the mail program Pine)  →  nano (GNU)

• vi is ex with a "visual mode for editing while looking at the screen" attached. The name also comes from visual. The commands you call with : inside vi are precisely ex commands, which is why the third mode is called ex mode.
• sed is a non-interactive stream editor. It does not open a file and edit it; it processes edits specified by commands as the data flows through. It works line by line, so it is classified with the line editors.
• On modern Linux, running vi usually launches vim in reality. Depending on the distribution, a reduced-feature version may be installed.

• Questions that ask you to pick the one of a different kind come in one of two criteria: line/screen or console/graphical.

The Three Modes

              ┌──────────────────┐
              │   Command mode    │  ← you start here
              │  (command mode)  │     move · delete · copy
              └──────────────────┘
            i a o  │        ▲  │  :
                   ▼   ESC  │  ▼
      ┌──────────────┐   ┌──────────────┐
      │  Insert mode  │   │   ex mode     │
      │ (insert mode)│   │ (last line)  │
      │  type text    │   │ save · quit   │
      └──────────────┘   └──────────────┘

• When you open vi, you are in command mode. If you start typing right away, the characters are not entered but interpreted as commands. This is where beginners get most flustered.
• Whatever mode you are in, pressing ESC brings you back to command mode. When confused, press ESC first.
• You can enter ex mode only from command mode. If you type : in insert mode, a plain colon character is entered.

Same Mode, Different Names

Textbooks and questions call the modes by different names. You only need to know they refer to the same thing.

• There is no direct route between insert mode and ex mode. You must go through command mode with ESC. That is why there is no arrow between the two boxes in the figure.
• A command in ex mode runs when you press Enter and automatically returns you to command mode. To cancel, press ESC.

Reading the Bottom Line of the Screen

   ~
   ~
   "test.txt" [New File]              ← when you opened a nonexistent file
   -- INSERT --                       ← after pressing i (vim)
   :wq                                ← after ESC, pressing : makes input appear here
   "test.txt" 3L, 42C written         ← save result — 3 lines, 42 characters

• The bottom line tells you the current state. In vim, if you see -- INSERT -- it is insert mode, and if there is nothing it is command mode.
• The ~ at the start of a line is not file content. It marks the empty screen after the end of the file.
• ex mode is called "last line mode" because the command is typed on the last line of the screen.

Keys That Enter Insert Mode

• Lowercase and uppercase come in pairs. Lowercase works near the cursor, uppercase works relative to the whole line.
• The difference between i and a — before or after is the fork.

   Original line    Linux        (cursor is on the n)
 
   i then X  →  LiXnux       before the cursor
   a then X  →  LinXux       after the cursor
   I then X  →  XLinux       start of the line
   A then X  →  LinuxX       end of the line

There are also keys that enter insert mode while making changes.

• r does not go to insert mode. It changes one character and stays in command mode. r or x appear in the answer choices as "keys that do not switch to insert mode."

What You Do in Command Mode

Moving

• Questions ask for the order of h j k l. It follows the keyboard layout: left, down, up, right. The middle two are easy to mix up.

• Putting a number in front repeats it that many times. 5j is five lines down, and 3w is three words ahead.
• $ means different things depending on where it is used. $ in command mode is the end of the line, while :$ in ex mode is the last line of the file.
• There are several routes to the same place — the first line is gg, 1G, or :1, and the last line is G or :$.
• G is uppercase, gg is lowercase twice. You can also move with the arrow keys, but the exam asks about h j k l.

Delete, Copy, Paste

• dd is both delete and cut. The deleted content stays in the buffer and can be pasted with p.
• yy (copy), dd (cut), and p (paste) are a set.

• There is one rule — action (d delete, y copy, c change) + range (w word, $ end of line, G end of file). Typing the same letter twice (dd, yy, cc) targets the whole line.
• 3dd is three lines downward starting from the line where the cursor is. The lines above are untouched.
• Where p pastes depends on what is in the buffer. If it is line-based (dd, yy), it goes on the next line down, and if it is character-based (x, dw), it goes after the cursor.

ex Mode — Saving and Quitting

! means force. If you edit and then do :q, a warning "not saved" appears. To discard changes and leave, use :q!. For questions asking about this command, you need to distinguish it from the wrong answer choices.

vi test.txt        # open a file (creates it if it does not exist)
vi +10 test.txt    # open at line 10
vi -R test.txt     # open read-only

Warning Messages and the Exclamation Mark

   E37: No write since last change (add ! to override)     ← when you did :q after editing
   E45: 'readonly' option is set (add ! to override)       ← when you did :w in a file opened read-only

• Both messages tell you "add ! to force it." In the first case it is :q! or :wq, and in the second it is :w!.
• However, even :w! cannot get past permissions. If a regular user opened a file they have no write permission for, such as /etc/passwd, :w! will not save it either. In that case, leave with :q! and reopen with root privileges.

• :w is write, :r is read in. The directions are opposite.
• ZZ has no colon. You press uppercase Z twice in command mode. It is not an ex command.
• There are two ways to discard changes — :q! discards and exits, :e! discards and keeps editing.

Options Given at Launch

• -R (uppercase) is read-only, -r (lowercase) is recovery. One letter's case changes the meaning.
• While you edit, vim creates a hidden swap file such as .test.txt.swp. If the terminal was cut off before you could save, you use this file to bring the work back.

Tomorrow we cover vi's search and replace, settings, and other editors.

📖 Glossary

• Line editor : An editor that edits one line at a time. ed and ex belong here.
• Screen editor : An editor that edits while viewing the whole screen. vi, emacs, and nano belong here.
• Command mode : The mode you are in when you first launch vi. You move, delete, and copy.
• Insert mode : The mode where you actually type text. Entered with i, a, o, etc.
• ex mode : The mode entered with a colon. It performs saving, quitting, and substitution.
• dd : The command that cuts one line. It stays in the buffer and can be pasted with p.
• yy : The command that copies one line (yank).
• :q! : The command that force-quits without saving changes.