the famous daemon class from Joseph Ernest

daemon.py

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+# Shamelessly stolen from https://gist.github.com/josephernest/77fdb0012b72ebdf4c9d19d6256a1119
+#
+# From "A simple unix/linux daemon in Python" by Sander Marechal
+# See http://stackoverflow.com/a/473702/1422096 and
+# http://web.archive.org/web/20131017130434/http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/
+#
+# Modified to add quit() that allows to run some code before closing the daemon
+# See http://stackoverflow.com/a/40423758/1422096
+#
+# Modified for Python 3
+# (see also: http://web.archive.org/web/20131017130434/http://www.jejik.com/files/examples/daemon3x.py)
+#
+# Joseph Ernest, 20200507_1220
+
+import atexit
+import os
+import sys
+import time
+from signal import SIGTERM, signal
+
+
+class Daemon:
+    """
+    A generic daemon class.
+
+    Usage: subclass the Daemon class and override the run() method
+    """
+
+    def __init__(self, pidfile='_.pid', stdin='/dev/null', stdout='/dev/null', stderr='/dev/null'):
+        self.stdin = stdin
+        self.stdout = stdout
+        self.stderr = stderr
+        self.pidfile = pidfile
+
+    def daemonize(self):
+        """
+        do the UNIX double-fork magic, see Stevens' "Advanced
+        Programming in the UNIX Environment" for details (ISBN 0201563177)
+        http://www.erlenstar.demon.co.uk/unix/faq_2.html#SEC16
+        """
+        try:
+            pid = os.fork()
+            if pid > 0:
+                # exit first parent
+                sys.exit(0)
+        except OSError as e:
+            sys.stderr.write("fork #1 failed: %d (%s)\n" % (e.errno, e.strerror))
+            sys.exit(1)
+
+        # decouple from parent environment
+        os.setsid()
+        os.umask(0)
+
+        # do second fork
+        try:
+            pid = os.fork()
+            if pid > 0:
+                # exit from second parent
+                sys.exit(0)
+        except OSError as e:
+            sys.stderr.write("fork #2 failed: %d (%s)\n" % (e.errno, e.strerror))
+            sys.exit(1)
+
+        # redirect standard file descriptors
+        sys.stdout.flush()
+        sys.stderr.flush()
+        si = open(os.devnull, 'r')
+        so = open(os.devnull, 'a+')
+        se = open(os.devnull, 'a+')
+        os.dup2(si.fileno(), sys.stdin.fileno())
+        os.dup2(so.fileno(), sys.stdout.fileno())
+        os.dup2(se.fileno(), sys.stderr.fileno())
+
+        atexit.register(self.onstop)
+        signal(SIGTERM, lambda signum, stack_frame: exit())
+
+        # write pidfile
+        pid = str(os.getpid())
+        open(self.pidfile, 'w+').write("%s\n" % pid)
+
+    def onstop(self):
+        self.quit()
+        os.remove(self.pidfile)
+
+    def start(self):
+        """
+        Start the daemon
+        """
+        # Check for a pidfile to see if the daemon already runs
+        try:
+            pf = open(self.pidfile, 'r')
+            pid = int(pf.read().strip())
+            pf.close()
+        except IOError:
+            pid = None
+
+        if pid:
+            message = "pidfile %s already exist. Daemon already running?\n"
+            sys.stderr.write(message % self.pidfile)
+            sys.exit(1)
+
+        # Start the daemon
+        self.daemonize()
+        self.run()
+
+    def stop(self):
+        """
+        Stop the daemon
+        """
+        # Get the pid from the pidfile
+        try:
+            pf = open(self.pidfile, 'r')
+            pid = int(pf.read().strip())
+            pf.close()
+        except IOError:
+            pid = None
+
+        if not pid:
+            message = "pidfile %s does not exist. Daemon not running?\n"
+            sys.stderr.write(message % self.pidfile)
+            return  # not an error in a restart
+
+        # Try killing the daemon process
+        try:
+            while 1:
+                os.kill(pid, SIGTERM)
+                time.sleep(0.1)
+        except OSError as err:
+            err = str(err)
+            if err.find("No such process") > 0:
+                if os.path.exists(self.pidfile):
+                    os.remove(self.pidfile)
+            else:
+                print(str(err))
+                sys.exit(1)
+
+    def restart(self):
+        """
+        Restart the daemon
+        """
+        self.stop()
+        self.start()
+
+    def run(self):
+        """
+        You should override this method when you subclass Daemon. It will be called after the process has been
+        daemonized by start() or restart().
+        """
+
+    def quit(self):
+        """
+        You should override this method when you subclass Daemon. It will be called before the process is stopped.
+        """