running shell commands with gnu clisp

Question

I'm trying to create a "system" command for clisp that works like this

(setq result (system "pwd"))

;;now result is equal to /my/path/here

I have something like this:

(defun system (cmd)
 (ext:run-program :output :stream))

But, I am not sure how to transform a stream into a string. I've reviewed the hyperspec and google more than a few times.

edit: working with Ranier's command and using with-output-to-stream,

(defun system (cmd)
  (with-output-to-string (stream)
    (ext:run-program cmd :output stream)))

And then trying to run grep, which is in my path...

[11]> (system "grep")

*** - STRING: argument #<OUTPUT STRING-OUTPUT-STREAM> should be a string, a
      symbol or a character
The following restarts are available:
USE-VALUE      :R1      Input a value to be used instead.
ABORT          :R2      Abort main loop
Break 1 [12]> :r2

Answer 1

Per the CLISP documentation on run-program, the :output argument should be one of

• :terminal - writes to the terminal
• :stream - creates and returns an input stream from which you can read
• a pathname designator - writes to the designated file
• nil - ignores the output

If you're looking to collect the output into a string, you'll have to use a read-write copying loop to transfer the data from the returned stream to a string. You already have with-output-to-string in play, per Rainer's suggestion, but instead of providing that output stream to run-program, you'll need to write to it yourself, copying the data from the input stream returned by run-program.

Answer 2

Something like this?

Version 2:

(defun copy-stream (in out)
   (loop for line = (read-line in nil nil)
         while line
         do (write-line line out)))

(defun system (cmd)
  (with-open-stream (s1 (ext:run-program cmd :output :stream))
    (with-output-to-string (out)
      (copy-stream s1 out))))


[6]> (system "ls")
"#.emacs#
Applications
..."

Answer 3

You are asking specifically about clisp. I'll add here that if you are using Clozure CL then you can also easily run os subprocesses.

Some examples:

;;; Capture the output of the "uname" program in a lisp string-stream
;;; and return the generated string (which will contain a trailing
;;; newline.)
? (with-output-to-string (stream)
    (run-program "uname" '("-r") :output stream))
;;; Write a string to *STANDARD-OUTPUT*, the hard way.
? (run-program "cat" () :input (make-string-input-stream "hello") :output t)
;;; Find out that "ls" doesn't expand wildcards.
? (run-program "ls" '("*.lisp") :output t)
;;; Let the shell expand wildcards.
? (run-program "sh" '("-c" "ls *.lisp") :output t)

Do a search for run-program in the CCL docs located here: http://ccl.clozure.com/ccl-documentation.html

There are a couple nice Lisp ways of doing this in this stackoverflow answer: Making a system call that returns the stdout output as a string Once again, Rainer to the rescue. Thanks Ranier.

Answer 4

This is a shorter one

(defun system(cmd)
  (ext:shell (string cmd)))

> (system '"cd ..; ls -lrt; pwd")