How do I add custom tab completion to my own Ex commands?

Explanation

When defining custom commands with :command, the -complete option adds tab completion for arguments. Vim provides many built-in completion types, and you can also write custom completion functions for full control over suggestions.

How it works

• -complete=file — completes file names
• -complete=dir — completes directory names
• -complete=buffer — completes buffer names
• -complete=command — completes Ex commands
• -complete=custom,FuncName — uses a custom function
• -nargs=1 — command takes exactly one argument

Example

" Command with file completion
:command -complete=file -nargs=1 E edit <args>

" Custom completion function
function! ColorComplete(A, L, P)
  return "red\ngreen\nblue\nyellow"
endfunction
:command -complete=custom,ColorComplete -nargs=1 SetColor echo <args>

:SetColor <Tab>
Completes: red, green, blue, yellow

Tips

• Use -complete=customlist,Func for returning a List instead of newline-separated string
• The function receives: ArgLead (current arg), CmdLine (full line), CursorPos
• Built-in types include: color, event, highlight, mapping, option, shellcmd, tag
• Combine with -nargs=* for commands accepting multiple arguments