What's the difference between :w and :sav in spacemacs?

Question

I'm using spacemacs and while when I create a new buffer with a given name and try to save it into a real file I find that :w won't work but :sav will. What is the difference between those two commands ?

Answer 1

:w and :sav are Evil ex-commands, provided by Evil. The "bindings" of ex-commands are stored in the variable evil-ex-commands, which is an alist. To examine the variable, run the command C-h v evil-ex-commands (in Spacemacs you can use SPC h d v evil-ex-commands instead). The output would be something like this:

evil-ex-commands is a variable defined in ‘evil-vars.el’.
Its value is shown below.

This variable may be risky if used as a file-local variable.

Documentation:
Association list of command bindings and functions.

Value: (("edit" . evil-edit)
        ("e" . "edit")
        ("write" . evil-write)
        ("w" . "write")
        ("wall" . evil-write-all)
        ("wa" . "wall")
        ("saveas" . evil-save)
        ("sav" . "saveas")
        ("read" . evil-read)
        ("r" . "read")
--- I trimmed the rest (the list continues) ---

When invoking :<key>, Evil looks for <key> in evil-ex-commands and invokes the corresponding command. As you can see, ":w" is a shortcut for ":write", and ":write" invokes the function evil-write. Similarly, ":sav" is a shortcut for ":saveas", which invokes evil-save.

To find the differences between evil-write and evil-save, you can read their documentation by calling C-h f or SPC h d f (as db48x wrote). It seems that ":w" saves to current file, while ":sav" lets you save to a new file.

Answer 2

I don't use spacemacs, but emacs in general is self-documenting; you can always query the help system for information about keybindings, functions, variables, etc. The normal emacs keyboard shortcut for getting help with a key binding is C-h k. From a quick glance at the spacemacs manual (http://spacemacs.org/doc/DOCUMENTATION.html#orgheadline91), it looks like you should use SPC h d k, or possibly SPC h d f (I'm not sure if the ex-commands like :w are classified as keyboard shortcuts or functions; probably the former but it might be the latter.)

You should be able to figure out the differnce by comparing the documentation for the two.