View source for Run as a daemon or service

{{draft task|Programming environment operations}}
A [[wp:Daemon_(computing)|daemon]] is a service that runs in the background independent of a users login session.

Demonstrate how a program disconnects from the terminal to run as a daemon in the background.

Write a small program that writes a message roughly once a second to its stdout which should be redirected to a file.

Note that in some language implementations it may not be possible to disconnect from the terminal, and instead the process needs to be started with stdout (and stdin) redirected to files before program start. If that is the case then a helper program to set up this redirection should be written in the language itself. A shell wrapper, as would be the usual solution on Unix systems, is not appropriate.

=={{header|BASIC}}==
==={{header|FreeBASIC}}===
{{trans|QB64}}
<syntaxhighlight lang="freebasic">ScreenLock()
Open "c:\Users\Default\Documents\myDaemon.log" For Append As #1
'Open "myDaemon.log" For Append As #1 'Ruta de archivo relativa para la salida // Relative file path for the output

For i As Byte = 1 To 5               'Entonces esto no se ejecuta ad infinitum // So this is not executed ad infinitum
    Print #1, Time + " - Running"    'Imprime en el archivo previamente abierto la hora actual del sistema (hh:mm:ss) y una nota
                                     'Prints in the previously opened file the current system time (hh: mm: ss) and a note
    Sleep 1000                       'Espera 1 segundo // Wait 1 second
Next i
Close #1                             'Cierra el archivo previamente abierto para escritura // Close the file previously open for writing
ScreenUnlock()
End</syntaxhighlight>

==={{header|QB64}}===
<syntaxhighlight lang="freebasic">'Metacommands and Statements for closing console or window
'' The following line is double commented ('') as some metacommands still execute even when singularly commented (')
''$Console     'Closes the console but does not close the main window, used for whole program run
'_Console Off  'Closes console but does not close main window, used for portion of a program run with _Console On
'_ScreenHide   'Closes main windowm used for a portion of a program run with _ScreenShow
$ScreenHide    'Closes main window, used for whole program run. (NB: this metacommand should not be commented in order to execute)

Open "/home/user/Documents/myDaemon.log" For Append As #1 'Fully qualified file path for output
'Open "./myDaemon.log" For Append As #1 'Relative file path for output

For i% = 1 To 5                        'So this doesn't run ad infinitum
    Print #1, Time$ + " - Running"     'Prints to the file previously opened the current system time (HH:MM:SS) and a note
    Sleep 1                            'Waits 1 second
Next i%
Close #1                               'Closes the file previously opened for writing
System</syntaxhighlight>
{{out}}
<pre>09:02:23 - Running
09:02:24 - Running
09:02:25 - Running
09:02:26 - Running
09:02:27 - Running</pre>

=={{header|C}}==
==={{libheader|BSD libc}}===
[[BSD]] provides a convenient [http://netbsd.gw.com/cgi-bin/man-cgi?daemon+3+NetBSD-current daemon(3)] function. [[GNU]] libc also provides [http://www.kernel.org/doc/man-pages/online/pages/man3/daemon.3.html daemon(3)], but [[POSIX]] omits it, so it is not portable. Other BSDisms in this program are __progname and <err.h>.

The task also wants to redirect stdout. This program does so with [http://netbsd.gw.com/cgi-bin/man-cgi?dup2+2+NetBSD-current dup2(2)]. Had we wanted to directly write to a file, we could open the file with <code>file = fopen(argv[1], "a")</code>, and write to ''file'' instead of ''stdout''.

<syntaxhighlight lang="c">#include <err.h>
#include <errno.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <syslog.h>
#include <time.h>
#include <unistd.h>

int
main(int argc, char **argv)
{
	extern char *__progname;
	time_t clock;
	int fd;

	if (argc != 2) {
		fprintf(stderr, "usage: %s file\n", __progname);
		exit(1);
	}

	/* Open the file before becoming a daemon. */
	fd = open(argv[1], O_WRONLY | O_APPEND | O_CREAT, 0666);
	if (fd < 0)
		err(1, argv[1]);

	/*
	 * Become a daemon. Lose terminal, current working directory,
	 * stdin, stdout, stderr.
	 */
	if (daemon(0, 0) < 0)
		err(1, "daemon");

	/* Redirect stdout. */
	if (dup2(fd, STDOUT_FILENO) < 0) {
		syslog(LOG_ERR, "dup2: %s", strerror(errno));
		exit(1);
	}
	close(fd);

	/* Dump clock. */
	for (;;) {
		time(&clock);
		fputs(ctime(&clock), stdout);
		if (fflush(stdout) == EOF) {
			syslog(LOG_ERR, "%s: %s", argv[1], strerror(errno));
			exit(1);
		}
		sleep(1);	/* Can wake early or drift late. */
	}
}</syntaxhighlight>

<pre>$ make dumper
cc -O2 -pipe    -o dumper dumper.c 
$ ./dumper dump
$ tail -f dump
Fri Nov 18 13:50:41 2011
Fri Nov 18 13:50:42 2011
Fri Nov 18 13:50:43 2011
Fri Nov 18 13:50:44 2011
Fri Nov 18 13:50:45 2011
^C
$ pkill -x dumper
$ rm dump</pre>

=={{header|Go}}==
{{libheader|go-daemon}}
{{works with|Ubuntu 16.04}}
<syntaxhighlight lang="go">package main

import (
    "fmt"
    "github.com/sevlyar/go-daemon"
    "log"
    "os"
    "time"
)

func work() {
    f, err := os.Create("daemon_output.txt")
    if err != nil {
        log.Fatal(err)
    }
    defer f.Close()
    ticker := time.NewTicker(time.Second)
    go func() {
        for t := range ticker.C {
            fmt.Fprintln(f, t) // writes time to file every second
        }
    }()
    time.Sleep(60 * time.Second) // stops after 60 seconds at most
    ticker.Stop()
    log.Print("ticker stopped")
}

func main() {
    cntxt := &daemon.Context{
        PidFileName: "pid",
        PidFilePerm: 0644,
        LogFileName: "log",
        LogFilePerm: 0640,
        WorkDir:     "./",
        Umask:       027,
        Args:        []string{"[Rosetta Code daemon example]"},
    }

    d, err := cntxt.Reborn()
    if err != nil {
        log.Fatal("Unable to run: ", err)
    }
    if d != nil {
        return
    }
    defer cntxt.Release()

    log.Print("- - - - - - - - - - - - - - -")
    log.Print("daemon started")

    work()
}</syntaxhighlight>

{{out}}
Although the daemon will only run for a maximum of 60 seconds, we kill it after a few seconds so the output is not too long.
<pre>
$ go build daemon.go
$ ./daemon
$ kill `cat pid`
$ cat log
2019/01/31 22:14:50 - - - - - - - - - - - - - - -
2019/01/31 22:14:50 daemon started
$ cat daemon_output.txt
2019-01-31 22:14:51.641498031 +0000 GMT m=+1.089305363
2019-01-31 22:14:52.641672923 +0000 GMT m=+2.089480618
2019-01-31 22:14:53.641724473 +0000 GMT m=+3.089531868
2019-01-31 22:14:54.641557151 +0000 GMT m=+4.089364696
2019-01-31 22:14:55.641543175 +0000 GMT m=+5.089350596
</pre>

=={{header|Nim}}==
{{trans|Python}}
This is a direct translation of the Python Posix version.
<syntaxhighlight lang="nim">import os, posix, strutils

let pid = fork()
if pid != 0:
  echo "Child process detached with pid $#".format(pid)
  quit QuitSuccess

let
  oldStdin = stdin
  oldStdout = stdout
  oldStderr = stderr

stdin = open("/dev/null")
stdout = open("/tmp/dmn.log", fmWrite)
stderr = stdout

oldStdin.close()
oldStdout.close()
oldStderr.close()

discard setsid()

import times
var t = now()
while now() < t + initDuration(seconds = 10):
  echo "timer running, $# seconds".format((now() - t).inSeconds)
  sleep(1000)</syntaxhighlight>

{{out}}
The file "/tmp/dmn.log" contains the following data:
<pre>timer running, 0 seconds
timer running, 1 seconds
timer running, 2 seconds
timer running, 3 seconds
timer running, 4 seconds
timer running, 5 seconds
timer running, 6 seconds
timer running, 7 seconds
timer running, 8 seconds
timer running, 9 seconds</pre>

=={{header|PARI/GP}}==
GP scripts cannot run in this fashion directly, but can be compiled into PARI code with <code>gp2c</code>. PARI code, whether from <code>gp2c</code> or not, can be run as a daemon just as [[#C|C]] would be.

=={{header|Perl}}==
First off, start with a standalone script and let's call it "count.pl"
<syntaxhighlight lang="perl"># 20211217 Perl programming solution

use strict;
use warnings;

use IO::Handle;
use File::Temp qw/ tempfile tempdir /;

my ($fh, $tempfile) = tempfile();
my $count = 0;

open $fh, '>', $tempfile or die;

print "\n\nOutput goes to $tempfile\n";

while (1) {
   sleep 1;
   print $fh "Sheep number ",$count++," just leaped over the fence ..\n";
   $fh->flush();
}</syntaxhighlight>
Make sure it works and then we can control/run it in daemon mode with another script
<syntaxhighlight lang="perl">use warnings;
use strict;

use Daemon::Control;

exit Daemon::Control->new(
   name       => 'Counter daemon',
   program    => '/home/hkdtam/count.pl',
   pid_file   => '/tmp/counter.pid',
)->run;</syntaxhighlight>
{{out}}
<pre>
./daemon.pl status
Counter daemon                                            [Not Running]
./daemon.pl start
Counter daemon                                                [Started]

Output goes to /tmp/5wHmHKu4fN

./daemon.pl status
Counter daemon                                                [Running]
tail -3 /tmp/5wHmHKu4fN
Sheep number 76 just leaped over the fence ..
Sheep number 77 just leaped over the fence ..
Sheep number 78 just leaped over the fence ..
cat /tmp/counter.pid
32178
./daemon.pl restart
Counter daemon                                                [Stopped]
Counter daemon                                                [Started]

Output goes to /tmp/ttTTS4oGY_

tail -3 /tmp/ttTTS4oGY_
Sheep number 32 just leaped over the fence ..
Sheep number 33 just leaped over the fence ..
Sheep number 34 just leaped over the fence ..
cat /tmp/counter.pid
32188
./daemon.pl stop
Counter daemon                                                [Stopped]
./daemon.pl status
Counter daemon                                            [Not Running]
cat /tmp/counter.pid
cat: /tmp/counter.pid: No such file or directory
</pre>

=={{header|Phix}}==
Assuming this task is simply "a program that can write to stdout(/stderr) or a file".<br>
In some cases free_console() may help, should an attached console have been incorrectly created in the first place.<br>
Be warned however that some methods of invocation and (obviously) console capture may die horribly when that is invoked inappropriately.<br>(Specifically, you can spend an awful long time waiting for a message or signal, from an agent that has just been surrepticiously murdered.)<br>
Phix does not have an implicit stdout, instead (eg) printf(1,fmt,args) writes
to stdout, whereas printf(fn,fmt,args) writes to some previously opened fn.<br>
Of course there is nothing at all to stop fn from being set to 1 for stdout, or 2 for stderr.<br>
It is also strongly against the core philosophy of Phix to have or permit anything that would somehow make printf(1,x) do something it does not look like it should be doing, especially given printf(fn,x) has no such ambiguity. Not having to change your code may sound dandy, not being able to tell what your code is doing by looking at it, isn't.
<!--<syntaxhighlight lang="phix">(notonline)-->
 <span style="color: #008080;">without</span> <span style="color: #008080;">javascript_semantics</span>
 <span style="color: #008080;">include</span> <span style="color: #004080;">timedate</span><span style="color: #0000FF;">.</span><span style="color: #000000;">e</span>
 <span style="color: #004080;">integer</span> <span style="color: #000000;">fn</span> <span style="color: #0000FF;">=</span> <span style="color: #008080;">iff</span><span style="color: #0000FF;">(</span><span style="color: #7060A8;">rand</span><span style="color: #0000FF;">(</span><span style="color: #000000;">2</span><span style="color: #0000FF;">)==</span><span style="color: #000000;">2</span><span style="color: #0000FF;">?</span><span style="color: #7060A8;">open</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"log.txt"</span><span style="color: #0000FF;">,</span><span style="color: #008000;">"w"</span><span style="color: #0000FF;">):</span><span style="color: #000000;">1</span><span style="color: #0000FF;">)</span>
 <span style="color: #008080;">for</span> <span style="color: #000000;">i</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span> <span style="color: #008080;">to</span> <span style="color: #000000;">5</span> <span style="color: #008080;">do</span>
     <span style="color: #7060A8;">printf</span><span style="color: #0000FF;">(</span><span style="color: #000000;">fn</span><span style="color: #0000FF;">,</span><span style="color: #7060A8;">format_timedate</span><span style="color: #0000FF;">(</span><span style="color: #7060A8;">date</span><span style="color: #0000FF;">(),</span><span style="color: #008000;">"'The time is' h:mm:sspm\n"</span><span style="color: #0000FF;">))</span>
     <span style="color: #7060A8;">sleep</span><span style="color: #0000FF;">(</span><span style="color: #000000;">1</span><span style="color: #0000FF;">)</span>
 <span style="color: #008080;">end</span> <span style="color: #008080;">for</span>
 <span style="color: #7060A8;">close</span><span style="color: #0000FF;">(</span><span style="color: #000000;">fn</span><span style="color: #0000FF;">)</span> <span style="color: #000080;font-style:italic;">-- [close(1) quietly does nothing]</span>
<!--</syntaxhighlight>-->
{{out}}
<small>(When fn is 1, or the content of log.txt when it is not.)</small>
<pre>
The time is 3:18:01pm
The time is 3:18:02pm
The time is 3:18:03pm
The time is 3:18:04pm
The time is 3:18:05pm
</pre>

=={{header|PicoLisp}}==
<syntaxhighlight lang="picolisp">(unless (fork)
   (out "file.log"
      (println *Pid)    # First write the daemon's PID to the file
      (for N 3600       # Write count for about one hour (if not killed)
         (wait 1000)
         (println N)
         (flush) ) )
   (bye) )              # Child terminates after one hour

(bye)                   # Parent terminates immediately</syntaxhighlight>

=={{header|Pike}}==
<code>__FILE__</code> is a preprocessor definition that contains the current filename.
if the first argument is "daemon" the program will be restarted with stdout redirected to "foo".

<syntaxhighlight lang="pike">int main(int argc, array argv)
{
    if (sizeof(argv)>1 && argv[1] == "daemon")
    {
        Stdio.File newout = Stdio.File("foo", "wc");
        Process.spawn_pike(({ __FILE__ }), ([ "stdout":newout ]));
        return 1;
    }

    int i = 100;
    while(i--)
    {
        write(i+"\n");
        sleep(0.1);
    }
}</syntaxhighlight>

=={{header|Python}}==
<syntaxhighlight lang="python">
#!/usr/bin/python3
import posix
import os
import sys

pid = posix.fork()
if pid != 0:
    print("Child process detached with pid %s" % pid)
    sys.exit(0)

old_stdin = sys.stdin
old_stdout = sys.stdout
old_stderr = sys.stderr

sys.stdin = open('/dev/null', 'rt')
sys.stdout = open('/tmp/dmn.log', 'wt')
sys.stderr = sys.stdout

old_stdin.close()
old_stdout.close()
old_stderr.close()

posix.setsid()

import time
t = time.time()
while time.time() < t + 10:
    print("timer running, %s seconds" % str(time.time() - t))
    time.sleep(1)
</syntaxhighlight>

=={{header|Racket}}==
<syntaxhighlight lang="racket">
#lang racket
(require ffi/unsafe)
((get-ffi-obj 'daemon #f (_fun _int _int -> _int)) 0 0)
(with-output-to-file "/tmp/foo"
  (λ() (for ([i 10]) (displayln (random 1000)) (flush-output) (sleep 1))))
</syntaxhighlight>

=={{header|Raku}}==
(formerly Perl 6)
<syntaxhighlight lang="raku" line># Reference:
# https://github.com/hipek8/p6-UNIX-Daemonize/

use v6;
use UNIX::Daemonize;
use File::Temp;

my ($output, $filehandle) = tempfile(:tempdir("/tmp"),:!unlink) or die;

say "Output now goes to ",$output;

daemonize();

loop {
   sleep(1);
   spurt $output, DateTime.now.Str~"\n", :append;
}</syntaxhighlight>
{{out}}<pre>root@ubuntu:~# su - david
david@ubuntu:~$ ./dumper.p6
Output now goes to /tmp/x2ovx9JG8b
david@ubuntu:~$ tail -f /tmp/x2ovx9JG8b
2018-12-11T20:20:01.510484+08:00
2018-12-11T20:20:02.513732+08:00
2018-12-11T20:20:03.517063+08:00
2018-12-11T20:20:04.520394+08:00
2018-12-11T20:20:05.524871+08:00
2018-12-11T20:20:06.528244+08:00
2018-12-11T20:20:07.531985+08:00
2018-12-11T20:20:08.537776+08:00
2018-12-11T20:20:09.541606+08:00
2018-12-11T20:20:10.545796+08:00
2018-12-11T20:20:11.549047+08:00
2018-12-11T20:20:12.552704+08:00
^C
david@ubuntu:~$ exit
logout
root@ubuntu:~# tail -f /tmp/x2ovx9JG8b
2018-12-11T20:20:28.623690+08:00
2018-12-11T20:20:29.626978+08:00
2018-12-11T20:20:30.634309+08:00
2018-12-11T20:20:31.637481+08:00
2018-12-11T20:20:32.640794+08:00
2018-12-11T20:20:33.643947+08:00
2018-12-11T20:20:34.647146+08:00
2018-12-11T20:20:35.651008+08:00
^C
root@ubuntu:~# su - david
david@ubuntu:~$ tail -f /tmp/x2ovx9JG8b
2018-12-11T20:20:51.711357+08:00
2018-12-11T20:20:52.715044+08:00
2018-12-11T20:20:53.718921+08:00
2018-12-11T20:20:54.722134+08:00
2018-12-11T20:20:55.725970+08:00
2018-12-11T20:20:56.729160+08:00
2018-12-11T20:20:57.732376+08:00
2018-12-11T20:20:58.735409+08:00
2018-12-11T20:20:59.738886+08:00
2018-12-11T20:21:00.743045+08:00
2018-12-11T20:21:01.748113+08:00
2018-12-11T20:21:02.753204+08:00
2018-12-11T20:21:03.756665+08:00
2018-12-11T20:21:04.759902+08:00
^C
david@ubuntu:~$ pkill -c moar
1
</pre>

=={{header|Sidef}}==
When the "daemon" argument is specified, a fork of the program is created with its STDOUT redirected into the file "foo.txt", and the main process is exited.
<syntaxhighlight lang="ruby">var block = {
    for n in (1..100) {
        STDOUT.say(n)
        Sys.sleep(0.5)
    }
}

if (ARGV[0] == 'daemon') {
    STDERR.say("Daemon mode")
    STDOUT{:fh} = %f'foo.txt'.open_w(){:fh}
    STDOUT.autoflush(true)
    block.fork
    STDERR.say("Exiting")
    Sys.exit(0)
}

STDERR.say("Normal mode")
block.run</syntaxhighlight>

=={{header|Smalltalk}}==
{{works with |Smalltalk/X}}
<syntaxhighlight lang="smalltalk">#! /usr/bin/env stx --script
(pid := OperatingSystem fork) == 0 ifTrue:[
    Stdin close.
    Stdout := '/tmp/daemon.log' asFilename writeStream.
    Stdout buffered:false. "so we can watch it growing"
    Transcript := Stderr := Stdout.

    Stdout showCR: e'here is the child'.
    1 to:5 do:[:n |
        Delay waitForSeconds:5.
        Transcript showCR: e'another hello on Transcript {n}'.
    ].
] ifFalse:[
    Stdout showCR: e'forked new process pid={pid}; now exiting'.
    OperatingSystem exit:0
]</syntaxhighlight>
{{out}}
<pre>bash-3.2$ ./daemon.st
forked new process pid=77897; now exiting
bash-3.2$
... after a few seconds:
bash-3.2$ cat /tmp/daemon.log
here is the child
another hello on Transcript 1
another hello on Transcript 2
another hello on Transcript 3
another hello on Transcript 4
another hello on Transcript 5</pre>

=={{header|Tcl}}==
Tcl doesn't come with tools for converting the process into a daemon, but can build them easily enough. Here's the BSD daemon function mapped into a Tcl command in a package.
{{libheader|Critcl}}
<syntaxhighlight lang="tcl">package provide daemon 1
package require critcl

critcl::ccode {
    #include <stdlib.h>
}
critcl::cproc daemon {Tcl_Interp* interp} ok {
    if (daemon(0, 0) < 0) {
	Tcl_AppendResult(interp, "cannot switch to daemon operation: ",
		Tcl_PosixError(interp), NULL);
	return TCL_ERROR;
    }
    return TCL_OK;
}</syntaxhighlight>
These tools can then be used to solve this task:
<syntaxhighlight lang="tcl">### Command line argument parsing
if {$argc < 1} {
    puts "usage: $argv0 file ?message...?"
    exit 1
} elseif {$argc == 1} {
    set filename [lindex $argv 0]
    set message "Hi there!"
} else {
    set message [join [lassign $argv filename]]
}

### Daemonize
package require daemon
daemon
close stdout; open $filename    ;# Redirects stdout!

### Print the message to the file every second until killed
proc every {ms body} {eval $body; after $ms [info level 0]}
every 1000 {puts "[clock format [clock seconds]]: $message"}
vwait forever</syntaxhighlight>
On Windows, there is a commercial extension to Tcl which allows a script to be installed as a service. Such a script would be much like the one above, but without the daemonization section as that has become a property of the runtime.

=={{header|Wren}}==
{{libheader|Wren-date}}
An embedded program but the actual daemon is of course the C host even though the process is orchestrated by the Wren code which handles any errors and prints the time every second to the file.

The following script is based on the C example though modified a little to run on Ubuntu 22.04.
<syntaxhighlight lang="wren">/* Run_as_a_daemon_or_service.wren */

import "./date" for Date

var O_WRONLY = 1
var O_APPEND = 1024
var O_CREAT  = 64

var STDOUT_FILENO = 1

class C {
    foreign static fileName

    foreign static open(pathName, flags, mode)

    foreign static daemon(nochdir, noclose)

    foreign static redirectStdout(oldfd, newfd)

    foreign static close(fd)

    foreign static time

    foreign static sleep(seconds)
}

// gets a Date object from a Unix time in seconds
var UT2Date = Fn.new { |ut| Date.unixEpoch.addSeconds(ut) }

Date.default = "ddd| |mmm| |dd| |hh|:|MM|:|ss| |yyyy" // default format for printing

// open file before becoming a daemon
var fd = C.open(C.fileName, O_WRONLY | O_APPEND | O_CREAT, 438)
if (fd < 0) {
    System.print("Error opening %(C.fileName)")
    return
}

// become a daemon
if (C.daemon(0, 0) < 0) {
    System.print("Error creating daemon.")
    return
}

// redirect stdout
if (C.redirectStdout(fd, STDOUT_FILENO) < 0) {
    System.print("Error redirecting stdout.")
    return
}

// close file
if (C.close(fd) < 0) {
    System.print("Error closing %(C.fileName)")
    return
}

// dump time every second
while (true) {
    System.print(UT2Date.call(C.time))
    C.sleep(1)
}</syntaxhighlight>
<br>
We now embed this in the following C program, compile and run it.
<syntaxhighlight lang="c">/* gcc Run_as_a_daemon_or_service.c -o dumper -lwren -lm */

#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include "wren.h"

char *fileName;

void C_fileName(WrenVM* vm) {
    wrenSetSlotString(vm, 0, fileName);
}

void C_open(WrenVM* vm) {
    const char *pathName = wrenGetSlotString(vm, 1);
    int flags = (int)wrenGetSlotDouble(vm, 2);
    mode_t mode = (mode_t)wrenGetSlotDouble(vm, 3);
    int fd = open(pathName, flags, mode);
    wrenSetSlotDouble(vm, 0, (double)fd);
}

void C_daemon(WrenVM* vm) {
    int nochdir = (int)wrenGetSlotDouble(vm, 1);
    int noclose = (int)wrenGetSlotDouble(vm, 2);
    int d = daemon(nochdir, noclose);
    wrenSetSlotDouble(vm, 0, (double)d);
}

void C_redirectStdout(WrenVM* vm) {
    int oldfd = (int)wrenGetSlotDouble(vm, 1);
    int newfd = (int)wrenGetSlotDouble(vm, 2);
    newfd = dup2(oldfd, newfd);
    wrenSetSlotDouble(vm, 0, (double)newfd);
}

void C_close(WrenVM* vm) {
    int fd = (int)wrenGetSlotDouble(vm, 1);
    int ret = close(fd);
    wrenSetSlotDouble(vm, 0, (double)ret);
}

void C_time(WrenVM* vm) {
    time_t t = time(NULL);
    wrenSetSlotDouble(vm, 0, (double)t);
}

void C_sleep(WrenVM* vm) {
    unsigned int seconds = (unsigned int) wrenGetSlotDouble(vm, 1);
    unsigned int ret = sleep(seconds);
    wrenSetSlotDouble(vm, 0, (double)ret);
}

WrenForeignMethodFn bindForeignMethod(
    WrenVM* vm,
    const char* module,
    const char* className,
    bool isStatic,
    const char* signature) {
    if (strcmp(module, "main") == 0) {
        if (strcmp(className, "C") == 0) {
            if (isStatic && strcmp(signature, "fileName") == 0)            return C_fileName;
            if (isStatic && strcmp(signature, "open(_,_,_)") == 0)         return C_open;
            if (isStatic && strcmp(signature, "daemon(_,_)") == 0)         return C_daemon;
            if (isStatic && strcmp(signature, "redirectStdout(_,_)") == 0) return C_redirectStdout;
            if (isStatic && strcmp(signature, "close(_)") == 0)            return C_close;
            if (isStatic && strcmp(signature, "time") == 0)                return C_time;
            if (isStatic && strcmp(signature, "sleep(_)") == 0)            return C_sleep;
        } 
    }
    return NULL;
}

static void writeFn(WrenVM* vm, const char* text) {
    printf("%s", text);
    fflush(stdout); // as we're redirecting stdout to a file
}

void errorFn(WrenVM* vm, WrenErrorType errorType, const char* module, const int line, const char* msg) {
    switch (errorType) {
        case WREN_ERROR_COMPILE:
            printf("[%s line %d] [Error] %s\n", module, line, msg);
            break;
        case WREN_ERROR_STACK_TRACE:
            printf("[%s line %d] in %s\n", module, line, msg);
            break;
        case WREN_ERROR_RUNTIME:
            printf("[Runtime Error] %s\n", msg);
            break;
    }
}

char *readFile(const char *fileName) {
    FILE *f = fopen(fileName, "r");
    fseek(f, 0, SEEK_END);
    long fsize = ftell(f);
    rewind(f);
    char *script = malloc(fsize + 1);
    size_t ret = fread(script, 1, fsize, f);
    if (ret != fsize) printf("Error reading %s\n", fileName);
    fclose(f);
    script[fsize] = 0;
    return script;
}

static void loadModuleComplete(WrenVM* vm, const char* module, WrenLoadModuleResult result) {
    if( result.source) free((void*)result.source);
}

WrenLoadModuleResult loadModule(WrenVM* vm, const char* name) {
    WrenLoadModuleResult result = {0};
    if (strcmp(name, "random") != 0 && strcmp(name, "meta") != 0) {
        result.onComplete = loadModuleComplete;
        char fullName[strlen(name) + 6];
        strcpy(fullName, name);
        strcat(fullName, ".wren");
        result.source = readFile(fullName);
    }
    return result;
}

int main(int argc, char **argv) {
    fileName = argv[1];
    WrenConfiguration config;
    wrenInitConfiguration(&config);
    config.writeFn = &writeFn;
    config.errorFn = &errorFn;
    config.bindForeignMethodFn = &bindForeignMethod;
    config.loadModuleFn = &loadModule;
    WrenVM* vm = wrenNewVM(&config);
    const char* module = "main";
    const char* fileName = "Run_as_a_daemon_or_service.wren";
    char *script = readFile(fileName);
    WrenInterpretResult result = wrenInterpret(vm, module, script);
    switch (result) {
        case WREN_RESULT_COMPILE_ERROR:
            printf("Compile Error!\n");
            break;
        case WREN_RESULT_RUNTIME_ERROR:
            printf("Runtime Error!\n");
            break;
        case WREN_RESULT_SUCCESS:
            break;
    }
    wrenFreeVM(vm);
    free(script);
    return 0;
}</syntaxhighlight>

{{out}}
Sample output:
<pre>
$ ./dumper dump
$ tail -f dump
Sat Feb 03 20:02:20 2024
Sat Feb 03 20:02:21 2024
Sat Feb 03 20:02:22 2024
Sat Feb 03 20:02:23 2024
Sat Feb 03 20:02:24 2024
Sat Feb 03 20:02:25 2024
Sat Feb 03 20:02:26 2024
Sat Feb 03 20:02:27 2024
Sat Feb 03 20:02:28 2024
Sat Feb 03 20:02:29 2024
^C
$ pkill -x dumper
$ rm dump
</pre>

{{omit from|Brlcad}}
{{omit from|GUISS}}
{{omit from|Lilypond}}
{{omit from|Openscad}}
{{omit from|TPP}}
{{omit from|ZX Spectrum Basic}}