elisp parse output of asynchronous shell command

To answer the question directly, you can definitely parse the output of an asyncronous shell command, using start-process and set-process-filter:

(let ((proc (start-process "find" "find" "find"                   (expand-file-name "~") "-name" "*el")))     (set-process-filter proc (lambda (proc line)                    (message "process output: %s" line))))

(Docs for filter function)

However, note that line above is not necessarily a line, and may include multiple lines or broken lines. Your filter is called whenever the process or emacs decides to flush some ouput:

... /home/user/gopath/src/github.com/gongo/json-reformat/test/json-reformat-test.el /home/user/gopath/src/github.com/gongo/json-reformat/test/test- process output: helper.el

In your case, this could mean that your port number might be broken into two separate process-filter calls.

To fix this, we can introduce a line-buffering and line-splitting wrapper, which calls your filter for each process output line:

(defun process-filter-line-buffer (real-filter) (let ((cum-string-sym (gensym "proc-filter-buff"))       (newline (string-to-char "\n"))       (string-indexof (lambda (string char start)             (loop for i from start below (length string)                   thereis (and (eq char (aref string i))                        i)))))   (set cum-string-sym "")   `(lambda (proc string)      (setf string (concat ,cum-string-sym string))      (let ((start 0) new-start)    (while (setf new-start            (funcall ,string-indexof string ,newline start))      ;;does not include newline      (funcall ,real-filter proc (substring string start new-start))      (setf start (1+ new-start)));;past newline    (setf ,cum-string-sym (substring string start))))))

Then, you can safely expect your lines to be whole:

(let* ((test-output "\nREPL server started on port 59795 on host 127.0.0.1 - \nrepl://127.0.0.1:59795 Implicit target dir is deprecated, please use the target task instead. Set BOOT_EMIT_TARGET=no to disable implicit target dir.")     (proc (start-process "echo-test" "echo-test" "echo" test-output))) (set-process-filter proc (process-filter-line-buffer               (lambda (proc line)                 (when (string-match                    "REPL server started on port \\([0-9]+\\)"                    line)                   (let ((port (match-string 1 line)))                 ;;take whatever action here with PORT                 (message "port found: %s" port)))))))

Finally, I'm not familiar with cider but this kind of low-level work probably belongs in an inferior-type mode and has probably already been solved.

shell-command

allows to name optional output- and error-buffers. Than the error should appear inside the latter and not clutter the output any more.

A better answer to the other one I provided is to simply use cider as you suggested:

(progn     (package-refresh-contents)     (package-install 'cider)     (cider-jack-in))