set the correct major mode for script files based on interpreter

Question

I want to make a function that will set the correct major mode based on the interpreter used for a script.

I don't want to include extensions in my scripts (because that can be annoying if making cli tools), but I still want the appropriate mode to be set when I edit those scripts.

This is to be done by reading the first line of the script where we find the sh-bang.

for example, if the script starts with

#!/usr/bin/bash

I want shell-script-mode to be called on the buffer.

If the interpreter is

#!/usr/bin/python3

I want python mode to be called.

This is what I currently have:

(defun set-major-mode (interpreter-name)
  "Set major mode acording to INTERPRETER-NAME."
  (message (concat "INTERPRETER: " interpreter-name))
  (when (string-match interpreter-name "bash")
    (shell-script-mode 1)
    (message "Setting Shell script mode."))
  (when (string-match interpreter-name "python")
    (python-mode 1)
    (message "Setting python mode."))
  (when (string-match interpreter-name "")
    (message "Not setting an interpreter"))
  )

(defun get-interpreter-name (file-name)
"Get the interpreter described in the first line of FILE-NAME."
  (let ((saved-point (point)))
    (goto-line 0)
    (set-major-mode
     (file-name-nondirectory (buffer-substring-no-properties (line-beginning-position) (line-end-position))))
    (goto-char saved-point))
  )

(defun choose-mode-for-file-without-extension ()
  "Choose what major mode to use for files without extensions."
  (let ((file-name (file-name-nondirectory (buffer-file-name))))
    (message (concat "FILE OPENED: " file-name))
    (if (eq (string-match "[[:alnum:]]\\.[[:alnum:]+$]" file-name) nil)
        (get-interpreter-name file-name))
    ))

(add-hook 'find-file-hook 'choose-mode-for-file-without-extension)

The messages get called as expected, however when I try to open a shell script (file without the .sh extension), shell-script-mode does not get called. Any tips?

EDIT solved

Turns out that my script is fine. The path on my sh-bang was wrong. The file I was trying to open had

#!\usr\bin\bash

which is of course wrong.

Answer 1

Such a functionality is already built into Emacs. Interpreters are mapped to modes with the variable interpreter-mode-alist. It contains by default

("python[0-9.]*" . python-mode)

and many other interpreters. It does work for bash and python out of the box.

As an aside, you should use absolute path for interpreters, e.g. #!/bin/bashinstead of #!bin/bash.