vi

Open a file

$ vi [file]

$ vi +[line_number] [file]

$ vi -R [file]

$ vi [file1] [file2]

$ vi -r [file]

vi [-rR] [+command] [file...]

vi is the classic Unix visual text editor, present on virtually all Unix-like systems. It uses modal editing where keys have different functions depending on the current mode.
On most modern Linux systems, vi is actually a link to vim (Vi IMproved) running in compatibility mode, or a minimal vi clone like nvi. True original vi is rare outside of commercial Unix systems.
The modal design allows powerful text manipulation without modifier keys, using the home row for navigation. While the learning curve is steep, proficiency enables very efficient editing.
Configuration is stored in ~/.exrc for traditional vi or ~/.vimrc for vim.

-R

Read-only mode

Recovery mode; list recoverable files or recover specific file

Execute command after loading

Start at specified line number

Execute command after loading (same as +)

Normal: Navigation and commands (default)
Insert: Text input (i, a, o)
Command: Ex commands (:)
Visual: Selection (v) - vim only
Press Esc to return to Normal mode.

Navigation: h/j/k/l (left/down/up/right), 0/$ (line start/end), gg/G (file start/end)
Insert: i (before cursor), a (after cursor), o (new line below), O (new line above)
Delete: x (character), dd (line), dw (word)
Copy/Paste: yy (copy line), p (paste after), P (paste before)
Undo: u (undo), Ctrl+r (redo - vim only)
Save/Quit: :w (save), :q (quit), :wq (save & quit), :q! (quit without saving), ZZ (save & quit)
Search: /pattern (forward), ?pattern (backward), n/N (next/previous)

Vi has a steep learning curve. Use vimtutor (if vim is installed) to learn interactively.
Without a proper terminal, vi may not function correctly. Set the TERM environment variable appropriately.
On minimal systems, only basic vi may be available without vim's enhancements. Know the common subset of commands.

Vi was created by Bill Joy at UC Berkeley in 1976 as the visual mode for the ex line editor. It became the standard Unix editor and influenced countless text editors. The name stands for "visual" as it was the visual interface to ex.

vim(1), nvim(1), nano(1), emacs(1), ed(1)