Python守护进程在Ubuntu上无法在后台运行

在Ubuntu系统中，使用以下命令在终端中运行Python守护进程时，它可以正常运行： python /opt/my-daemon.py foreground

但是，当我使用 "start" 命令调用守护进程时，它却失败了。这是为什么？ python /opt/my-daemon.py start

这是我在/etc/rc.local文件中调用命令的方式： python /opt/my-daemon.py start &

以下是代码示例：

1.daemon.py

#!/usr/bin/env python
import sys, os, time, atexit
from signal import SIGTERM

class Daemon:

    """
    A generic daemon class.

    Usage: subclass the Daemon class and override the run() method
    """

    def __init__(self, pidfile,
                 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 Richard 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, e:
            sys.stderr.write("fork #1 failed: %d (%s)\n" % (e.errno,
                                                           e.strerror))
            sys.exit(1)
        # Decouple from parent environment
        os.chdir("/")
        os.setsid()
        os.umask(0)

        # Do second fork
        try:
            pid = os.fork()
            if pid > 0:
                # Exit from second parent
                sys.exit(0)
        except OSError, 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 = file(self.stdin, 'r')
        so = file(self.stdout, 'a+')
        se = file(self.stderr, 'a+', 0)
        os.dup2(si.fileno(), sys.stdin.fileno())
        os.dup2(so.fileno(), sys.stdout.fileno())
        os.dup2(se.fileno(), sys.stderr.fileno())

        # Write pidfile
        atexit.register(self.delpid)
        pid = str(os.getpid())
        file(self.pidfile, 'w+').write("%s\n" % pid)

    def delpid(self):
        os.remove(self.pidfile)

    def start(self):
        """
        Start the daemon
        """
        # Check for a pidfile to see if the daemon already runs
        try:
            pf = file(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 = file(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, 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().
        """


2.my-daemon.py

```python
import sys, time
from daemon import Daemon
import MySQLdb #MySQL libraries
#Database parameters
config = {"host":"localhost",...}
try:
    conn = MySQLdb.connect(config['host'],...
class MyDaemon(Daemon):
    def run(self):
        while True:
            time.sleep(2)
                        #{Do processes, connect to the database, etc....}
                        ...
if __name__ == "__main__":
    daemon = MyDaemon('/tmp/daemon-example.pid')
    if len(sys.argv) == 2:
        if 'start' == sys.argv[1]:
            daemon.start()
        elif 'stop' == sys.argv[1]:
            daemon.stop()
        elif 'restart' == sys.argv[1]:
            daemon.restart()
        elif 'foreground' == sys.argv[1]: #This runs the daemon in the foreground
            daemon.run()
        else:
            print "Unknown command"
            sys.exit(2)
        sys.exit(0)
    else:
        print "usage: %s start|foreground|stop|restart" % sys.argv[0]
        sys.exit(2)

解决方案

解决方法很简单，只需要将run方法作为start方法，并移除foreground参数即可。

def start(self):
    while True:
        time.sleep(2)

然后，在rc.local文件中，使用以下命令启动脚本：

python /opt/my-daemon.py start &

这样，守护进程就会在启动时执行，并且隐藏在后台。