scripting: enable shebang line 🚀

docs/scripting.md

@@ -49,7 +49,7 @@ Output:
 
 
 ```
-$ ciel myscript.lisp you
+$ ciel script.lisp you
 =>
 
 Hello you!
@@ -73,6 +73,38 @@ ciel-user>
 Access them with `(uiop:command-line-arguments)`.
 
 
+## Executable file and shebang line
+
+We can also make a CIEL file executable and run it directly:
+
+```
+$ chmod +x script.lisp
+$ ./script.lisp
+```
+
+Add the following shebang at the beginning:
+
+```sh
+#!/bin/sh
+#|-*- mode:lisp -*-|#
+#|
+exec /path/to/ciel `basename $0` "$@"
+|#
+
+(in-package :ciel-user)
+;; lisp code follows.
+```
+
+How it works:
+
+- it starts as a /bin/sh script
+  - all lines starting by `#` are shell comments
+- the exec calls the `ciel` binary with this file name as first argument,
+  the rest of the file (lisp code) is not read by the shell.
+  - before LOAD-ing this Lisp file, we remove the #!/bin/sh shebang line.
+  - Lisp ignores comments between `#|` and `|#`.
+
+
 ---
 
 Now, let us iron out the details ;)

scripting.lisp

@@ -1,20 +1,77 @@
 
 (in-package :ciel)
 
+(defun maybe-ignore-shebang (in)
+  "If this file starts with #!, delete the shebang line,
+  so we can LOAD the file.
+  Return: a stream (it is LOADable)."
+  ;; thanks Roswell for the trick.
+  (let ((first-line (read-line in)))
+    (make-concatenated-stream
+     ;; remove shebang:
+     (make-string-input-stream
+      (format nil "~a"
+              (if (str:starts-with-p "#!" first-line)
+                  ""
+                  first-line)))
+     ;; rest of the file:
+     in)))
+
+(defun load-without-shebang (file)
+  "LOAD this file, but exclude the first line if it is a shebang line."
+  (with-open-file (file-stream file)
+    (load
+     (maybe-ignore-shebang file-stream))))
+
+(defun has-shebang (file)
+  "Return T if the first line of this file is a shell shebang line (starts with #!)."
+  (with-open-file (s file)
+    (str:starts-with-p "#!" (read-line s))))
+
 (defun main ()
   "Run a lisp file as a script.
-  If no file is given or if it doesn't exist, run the top-level CIEL REPL.
+  If no argument is given or if the file doesn't exist, run the top-level CIEL
 
-  It should begin with:
+  The script should begin with:
 
     (in-package :ciel-user)
 
+  We have two ways to run a CIEL script:
+
+  1) by calling the ciel binary with a file as argument:
+
+    $ ciel myscript.lisp
+
+  2) by using a shebang. It's a little bit convoluted:
+
+  #!/bin/sh
+  #|-*- mode:lisp -*-|#
+  #|
+  exec /path/to/ciel `basename $0` \"$@\"
+  (print \"hello CIEL!\")
+
+  How it works:
+
+  - it starts as a /bin/sh script
+    - all lines starting by # are shell comments
+  - the exec calls the ciel binary with this file name as first argument,
+    the rest of the file (lisp code) is not read by the shell.
+    - before LOAD-ing this Lisp file, we remove the #!/bin/sh shebang line.
+    - Lisp ignore comments between #| and |#
+
   Exciting things to come!"
   ;; (format t "Hello main! ~S~&" uiop:*command-line-arguments*)
   (let ((file (first (uiop:command-line-arguments))))
     (cond
       ((and file
             (uiop:file-exists-p file))
-       (load file))
+       (if (has-shebang file)
+           ;; I'm a bit cautious about this function.
+           ;; (mostly, small issues when testing at the REPL, should be fine though…)
+           (load-without-shebang file)
+           ;; So the one with no risk:
+           (load file)))
       (t
+       (when (and file (not (uiop:file-exists-p file)))
+         (format t "warn: file ~S does not exist.~&" file))
        (sbcli::repl)))))