The Common Lisp Cookbook – Interfacing with your OS

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The Common Lisp Cookbook – Interfacing with your OS

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The ANSI Common Lisp standard doesn’t mention this topic. (Keep in mind that it was written at a time where Lisp Machines were at their peak. On these boxes Lisp was your operating system!) So almost everything that can be said here depends on your OS and your implementation. There are, however, some widely used libraries, which either come with your Common Lisp implementation, or are easily available through Quicklisp. These include:

Accessing Environment variables

UIOP comes with a function that’ll allow you to look at Unix/Linux environment variables on a lot of different CL implementations:

* (uiop:getenv "HOME")

Below is an example implementation, where we can see /feature flags/ used to run code on specific implementations:

* (defun my-getenv (name &optional default)
    "Obtains the current value of the POSIX environment variable NAME."
    (declare (type (or string symbol) name))
    (let ((name (string name)))
      (or #+abcl (ext:getenv name)
         #+ccl (ccl:getenv name)
         #+clisp (ext:getenv name)
         #+cmu (unix:unix-getenv name) ; since CMUCL 20b
         #+ecl (si:getenv name)
         #+gcl (si:getenv name)
         #+mkcl (mkcl:getenv name)
         #+sbcl (sb-ext:posix-getenv name)
* (my-getenv "HOME")
* (my-getenv "HOM")
* (my-getenv "HOM" "huh?")

You should also note that some of these implementations also provide the ability to set these variables. These include ECL (si:setenv) and AllegroCL, LispWorks, and CLISP where you can use the functions from above together with setf. This feature might be important if you want to start subprocesses from your Lisp environment.

To set an envionmental variable, you can setf with (uiop:getenv "lisp") in a implementation-independent way.

Also note that the Osicat library has the method (environment-variable "name"), on POSIX-like systems including Windows. It is also fset-able.

Accessing the command line arguments


Accessing command line arguments is implementation-specific but it appears most implementations have a way of getting at them. UIOP with uiop:command-line-arguments or Roswell as well as external libraries (see next section) make it portable.

SBCL stores the arguments list in the special variable sb-ext:*posix-argv*

$ sbcl my-command-line-arg


* sb-ext:*posix-argv*

("sbcl" "my-command-line-arg")

More on using this to write standalone Lisp scripts can be found in the SBCL Manual

LispWorks has system:*line-arguments-list*

* system:*line-arguments-list*
("/Users/cbrown/Projects/lisptty/tty-lispworks" "-init" "/Users/cbrown/Desktop/lisp/lispworks-init.lisp")

Here’s a quick function to return the argument strings list across multiple implementations:

(defun my-command-line ()
   #+SBCL *posix-argv*
   #+LISPWORKS system:*line-arguments-list*))

Now it would be handy to access them in a portable way and to parse them according to a schema definition.

Parsing command line arguments

We have a look at the Awesome CL list#scripting section and we’ll show how to use clingon.

Please see our scripting recipe.

Running external programs

uiop has us covered, and is probably included in your Common Lisp implementation.


uiop:run-program either takes a string as argument, denoting the name of the executable to run, or a list of strings, for the program and its arguments:

(uiop:run-program "firefox")


(uiop:run-program (list "firefox" "http:url"))

This will process the program output as specified and return the processing results when the program and its output processing are complete.

Use :output t to print to standard output.

This function has the following optional arguments:

run-program (command &rest keys &key
               (force-shell nil force-shell-suppliedp)
               (if-input-does-not-exist :error)
               (if-output-exists :supersede)
               (if-error-output-exists :supersede)
               (element-type *default-stream-element-type*)
               (external-format *utf-8-external-format*)

It will always call a shell (rather than directly executing the command when possible) if force-shell is specified. Similarly, it will never call a shell if force-shell is specified to be nil.

Signal a continuable subprocess-error if the process wasn’t successful (exit-code 0), unless ignore-error-status is specified.

If output is a pathname, a string designating a pathname, or nil (the default) designating the null device, the file at that path is used as output. If it’s :interactive, output is inherited from the current process; beware that this may be different from your *standard-output*, and under slime will be on your *inferior-lisp* buffer. If it’s t, output goes to your current *standard-output* stream. Otherwise, output should be a value that is a suitable first argument to slurp-input-stream (qv.), or a list of such a value and keyword arguments. In this case, run-program will create a temporary stream for the program output; the program output, in that stream, will be processed by a call to slurp-input-stream, using output as the first argument (or the first element of output, and the rest as keywords). The primary value resulting from that call (or nil if no call was needed) will be the first value returned by run-program. E.g., using :output :string will have it return the entire output stream as a string. And using :output '(:string :stripped t) will have it return the same string stripped of any ending newline.

if-output-exists, which is only meaningful if output is a string or a pathname, can take the values :error, :append, and :supersede (the default). The meaning of these values and their effect on the case where output does not exist, is analogous to the if-exists parameter to open with :direction :output.

error-output is similar to output, except that the resulting value is returned as the second value of run-program. t designates the *error-output*. Also :output means redirecting the error output to the output stream, in which case nil is returned.

if-error-output-exists is similar to if-output-exist, except that it affects error-output rather than output.

input is similar to output, except that vomit-output-stream is used, no value is returned, and T designates the *standard-input*.

if-input-does-not-exist, which is only meaningful if input is a string or a pathname, can take the values :create and :error (the default). The meaning of these values is analogous to the if-does-not-exist parameter to open with :direction :input.

element-type and external-format are passed on to your Lisp implementation, when applicable, for creation of the output stream.

One and only one of the stream slurping or vomiting may or may not happen in parallel in parallel with the subprocess, depending on options and implementation, and with priority being given to output processing. Other streams are completely produced or consumed before or after the subprocess is spawned, using temporary files.

run-program returns 3 values:


With uiop:launch-program.

Its signature is the following:

launch-program (command &rest keys &key
                    (if-input-does-not-exist :error)
                    (if-output-exists :supersede)
                    (if-error-output-exists :supersede)
                    (element-type *default-stream-element-type*)
                    (external-format *utf-8-external-format*)
                    #+allegro separate-streams

Output (stdout) from the launched program is set using the output keyword:

if-output-exists, which is only meaningful if output is a string or a pathname, can take the values :error, :append, and :supersede (the default). The meaning of these values and their effect on the case where output does not exist, is analogous to the if-exists parameter to open with :DIRECTION :output.

error-output is similar to output. T designates the *error-output*, :output means redirecting the error output to the output stream, and :stream causes a stream to be made available via process-info-error-output.

launch-program returns a process-info object, which look like the following (source):

(defclass process-info ()
     ;; The advantage of dealing with streams instead of PID is the
     ;; availability of functions like `sys:pipe-kill-process`.
     (process :initform nil)
     (input-stream :initform nil)
     (output-stream :initform nil)
     (bidir-stream :initform nil)
     (error-output-stream :initform nil)
     ;; For backward-compatibility, to maintain the property (zerop
     ;; exit-code) <-> success, an exit in response to a signal is
     ;; encoded as 128+signum.
     (exit-code :initform nil)
     ;; If the platform allows it, distinguish exiting with a code
     ;; >128 from exiting in response to a signal by setting this code
     (signal-code :initform nil)))

See the docstrings.

Test if a subprocess is alive

uiop:process-alive-p tests if a process is still alive, given a process-info object returned by launch-program:

* (defparameter *shell* (uiop:launch-program "bash"
                            :input :stream :output :stream))

;; inferior shell process now running
* (uiop:process-alive-p *shell*)

;; Close input and output streams
* (uiop:close-streams *shell*)
* (uiop:process-alive-p *shell*)

Get the exit code

We can use uiop:wait-process. If the process is finished, it returns immediately, and returns the exit code. If not, it waits for the process to terminate.

(uiop:process-alive-p *process*)
(uiop:wait-process *process*)

An exit code to 0 means success (use zerop).

The exit code is also stored in the exit-code slot of our process-info object. We see from the class definition above that it has no accessor, so we’ll use slot-value. It has an initform to nil, so we don’t have to check if the slot is bound. We can do:

(slot-value *my-process* 'uiop/launch-program::exit-code)

The trick is that we must run wait-process beforehand, otherwise the result will be nil.

Since wait-process is blocking, we can do it on a new thread:

  (lambda ()
    (let ((exit-code (uiop:wait-process
                       (uiop:launch-program (list "of" "commands"))))
      (if (zerop exit-code)
          (print :success)
          (print :failure)))))
  :name "Waiting for <program>")

Note that run-program returns the exit code as the third value.

Input and output from subprocess

If the input keyword is set to :stream, then a stream is created and can be written to in the same way as a file. The stream can be accessed using uiop:process-info-input:

;; Start the inferior shell, with input and output streams
* (defparameter *shell* (uiop:launch-program "bash"
                           :input :stream :output :stream))
;; Write a line to the shell
* (write-line "find . -name '*.md'"
     (uiop:process-info-input *shell*))
;; Flush stream
* (force-output (uiop:process-info-input *shell*))

where write-line writes the string to the given stream, adding a newline at the end. The force-output call attempts to flush the stream, but does not wait for completion.

Reading from the output stream is similar, with uiop:process-info-output returning the output stream:

* (read-line (uiop:process-info-output *shell*))

In some cases the amount of data to be read is known, or there are delimiters to determine when to stop reading. If this is not the case, then calls to read-line can hang while waiting for data. To avoid this, listen can be used to test if a character is available:

* (let ((stream (uiop:process-info-output *shell*)))
     (loop while (listen stream) do
         ;; Characters are immediately available
         (princ (read-line stream))

There is also read-char-no-hang which reads a single character, or returns nil if no character is available. Note that due to issues like buffering, and the timing of when the other process is executed, there is no guarantee that all data sent will be received before listen or read-char-no-hang return nil.

Capturing standard and error output

Capturing standard output, as seen above, is easily done by telling :output to be :string, or using :output '(:string :stripped t) to strip any ending newline.

You can ask the same to :error-output and, in addition, you can ask uiop:run-program to not signal an error, thus to not enter the interactive debugger, with :ignore-error-status t.

In that case, you can check the success or the failure of the program with the returned exit-code. 0 is success.

Here’s everything together:

(uiop:run-program (list "git"
                  :output :string
                  :error-output :string
                  :ignore-error-status t)
;; =>
"error: pathspec 'me/does-not-exist did not match any file(s) known to git

uiop:run-program returns 3 values:

We can bind them with multiple-value-bind:

(multiple-value-bind (output error-output exit-code)
    (uiop:run-program (list …))
  (unless (zerop exit-code)
    (format t "error output is: ~a" error-output)))

Running interactive and visual commands (htop)

Use uiop:run-program and set both :input and :output to :interactive:

(uiop:run-program "htop"
                  :output :interactive
                  :input :interactive)

This will spawn htop in full screen, as it should.

It works for more commands (sudo, vim, less…).


Here’s an example to do the equivalent of ls | sort. Note that “ls” uses launch-program (async) and outputs to a stream, where “sort”, the last command of the pipe, uses run-program and outputs to a string.

(uiop:run-program "sort"
                    (uiop:launch-program "ls"
                                         :output :stream))
                   :output :string)

Get Lisp’s current Process ID (PID)

Implementations provide their own functions for this.



It is possible portably with the osicat library:


Here again, we could find it by using the apropos function:

CL-USER> (apropos "pid")

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