Emacs is aweseome but sometimes it feels it's too smart for its own good. You may feel you learned everything there's to know about a function and then someone tells you to try a prefix argument and suddenly it does something you never knew about.

For example - you can jump to Magit without opening a project, by calling magit-status with 2 prefix arguments (it will prompt you with a "choose directory" dialog) - I just learned that

So my question is: Is it possible to build a list of functions that take prefix arguments and for each row grab the description (maybe from describe-function) that explains what calling the function with a prefix would do?

  • One idea would be to grep a particular library for (interactive and/or the lines containing relevant interactive codes -- e.g., P: gnu.org/software/emacs/manual/html_node/elisp/…
    – lawlist
    Oct 22, 2016 at 20:13
  • Figuring out the effect of a prefix argument from the docstring would require natural language parsing, so probably impossible to get it 100% correct, but you might be able to implement a 95% solution.
    – npostavs
    Oct 22, 2016 at 20:27

2 Answers 2


Here is one way to get the functions that mention prefix in the doc string. This saves the output to a file

(with-temp-file "prefix-cmds.org"
  (loop for cmd-name in smex-ido-cache
    (let* ((cmd (intern cmd-name))
           (doc (documentation cmd)))
      (when (and doc (string-match "prefix" doc))
         (format "** %s\n\n%s\n\n"
             (org-escape-code-in-string doc)))))))

This misses any function that doesn't specifically call it a prefix, e.g. a universal argument, or prfix, etc... You might catch those with a better string match. It is also limited to functions in smex-ido-cache, but that is what counsel-M-x uses, so I assume it is not missing any. Maybe there is a better way to get a full list of interactive commands.

Still, this listed 1457 commands for me!

  1. A list of all commands "that take prefix arguments" means a list of all commands.

  2. If you instead mean a list of only those commands whose doc mentions prefix-argument behavior then you need to search command doc strings.

    • An easy way to do this is to use command apropos-documentation, which is bound to C-h d. Give it the regexp prefix[- ]+arg, for example. And yes, use C-u if you have not customized apropos-do-all to non-nil (which I recommend).

       apropos-documentation is an interactive autoloaded compiled Lisp
       function in `apropos.el'.
       It is bound to C-h d, f1 d, help d, C-h M-a, f1 M-a, help M-a,
       menu-bar help-menu apropos apropos-doc, menu-bar help-menu
       search-documentation search-documentation-strings.
       (apropos-documentation PATTERN &optional DO-ALL)
       Show symbols whose documentation contains matches for PATTERN.
       PATTERN can be a word, a list of words (separated by spaces),
       or a regexp (using some regexp special characters).  If it is a word,
       search for matches for that word as a substring.  If it is a list of words,
       search for matches for any two (or more) of those words.
       Note that by default this command only searches in the file specified by
       `internal-doc-file-name'; i.e., the etc/DOC file.  With C-u prefix,
       or if `apropos-do-all' is non-nil, it searches all currently defined
       documentation strings.
       Returns list of symbols and documentation found.

      See the Emacs manual, node Apropos, for more information.

    • Another easy way to do this is to use Icicles command icicle-fundoc.

  • 1
    When I run it this, it also finds some variables (not many though), e.g. prefix-arg. Somewhat ironically, you have to call it with a prefix arg to get it to search all known doc-strings as noted in the docstring above;) Oct 23, 2016 at 0:02

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.