In VI, when an arrow key is pressed, a character (e.g. “A“) is sent.

That usually happens when vi is running in a mode that doesn’t understand your terminal’s arrow key escape sequences.

Here’s why:

• Arrow keys don’t send a single character; they send an escape sequence (for example, the Up arrow sends ESC [ A).
• If vi (or more commonly the very old ex/vi implementation, not vim) doesn’t recognize these escape sequences, it just displays the trailing characters like A, B, C, D (or sometimes M) instead of moving the cursor.

Common reasons

1. You’re actually running vi, not vim.
   Some minimal Linux/Unix systems only ship with the original BSD vi, which lacks proper arrow key support.

2. Terminal type mismatch.
   If your $TERM environment variable is wrong (e.g., set to dumb instead of xterm-256color), vi doesn’t know how to interpret escape sequences.

3. Insert mode vs command mode.

   • In command mode, arrows usually work.
   • In insert mode, some old vi versions don’t handle them and show A, B, etc.

Fixes

• Use vim instead of vi:

  
  sudo apt install vim      # Ubuntu/Debian
  sudo yum install vim      # RHEL/CentOS

• Check your terminal type:

  
  echo $TERM

  If it says dumb, set it properly:

  
  export TERM=xterm-256color