Question

What’s the lisp way of replacing a string in a file.

There is a file identified by *file-path*, a search string *search-term* and a replacement string *replace-term*.

How to make file with all instances of *search-term*s replaced with *replace-term*s, preferably in place of the old file?

Answer 1

One more take at the problem, but few warnings first:

1. To make this really robust and usable in the real-life situation you would need to wrap this into handler-case and handle various errors, like insufficient disc space, device not ready, insufficient permission for reading / writing, insufficient memory to allocate for the buffer and so on.

2. This does not do regular expression-like replacement, it’s simple string replacement. Making a regular expression based replacement on large files may appear far less trivial than it looks like from the start, it would be worth writing a separate program, something like sed or awk or an entire language, like Perl or awk 😉

3. Unlike other solutions it will create a temporary file near the file being replaced and will save the data processed so far into this file. This may be worse in the sense that it will use more disc space, but this is safer because in case the program fails in the middle, the original file will remain intact, more than that, with some more effort you could later resume replacing from the temporary file if, for example, you were saving the offset into the original file in the temporary file too.

---

(defun file-replace-string (search-for replace-with file
                            &key (element-type 'base-char)
                              (temp-suffix ".tmp"))
  (with-open-file (open-stream
                   file
                   :direction :input
                   :if-exists :supersede
                   :element-type element-type)
    (with-open-file (temp-stream
                     (concatenate 'string file temp-suffix)
                     :direction :output
                     :element-type element-type)
      (do ((buffer (make-string (length search-for)))
           (buffer-fill-pointer 0)
           (next-matching-char (aref search-for 0))
           (in-char (read-char open-stream nil :eof)
                    (read-char open-stream nil :eof)))
          ((eql in-char :eof)
           (when (/= 0 buffer-fill-pointer)
             (dotimes (i buffer-fill-pointer)
               (write-char (aref buffer i) temp-stream))))
        (if (char= in-char next-matching-char)
            (progn
              (setf (aref buffer buffer-fill-pointer) in-char
                    buffer-fill-pointer (1+ buffer-fill-pointer))
              (when (= buffer-fill-pointer (length search-for))
                (dotimes (i (length replace-with))
                  (write-char (aref replace-with i) temp-stream))
                (setf buffer-fill-pointer 0)))
            (progn
              (dotimes (i buffer-fill-pointer)
                (write-char (aref buffer i) temp-stream))
              (write-char in-char temp-stream)
              (setf buffer-fill-pointer 0)))
        (setf next-matching-char (aref search-for buffer-fill-pointer)))))
  (delete-file file)
  (rename-file (concatenate 'string file temp-suffix) file))