12.14 stay Unix Start the daemon on the system
problem
You want to write one as a Unix Or class Unix The daemon running on the system runs the program .
Solution
Creating a correct daemon requires a precise sequence of system calls and control over the details . The following code shows how to define a daemon , It can be easily stopped after starting .
# daemon.py
import os
import sys
import atexit
import signal
def daemonize(pidfile, *, stdin='/dev/null',
stdout='/dev/null',
stderr='/dev/null'):
if os.path.exists(pidfile):
raise RuntimeError('Already running')
# First fork (detaches from parent)
try:
if os.fork() > 0:
raise SystemExit(0) # Parent exit
except OSError as e:
raise RuntimeError('fork #1 failed.')
os.chdir('/')
os.umask(0)
os.setsid()
# Second fork (relinquish session leadership)
try:
if os.fork() > 0:
raise SystemExit(0)
except OSError as e:
raise RuntimeError('fork #2 failed.')
# Flush I/O buffers
sys.stdout.flush()
sys.stderr.flush()
# Replace file descriptors for stdin, stdout, and stderr
with open(stdin, 'rb', 0) as f:
os.dup2(f.fileno(), sys.stdin.fileno())
with open(stdout, 'ab', 0) as f:
os.dup2(f.fileno(), sys.stdout.fileno())
with open(stderr, 'ab', 0) as f:
os.dup2(f.fileno(), sys.stderr.fileno())
# Write the PID file
with open(pidfile,'w') as f:
print(os.getpid(),file=f)
# Arrange to have the PID file removed on exit/signal
atexit.register(lambda: os.remove(pidfile))
# Signal handler for termination (required)
def sigterm_handler(signo, frame):
raise SystemExit(1)
signal.signal(signal.SIGTERM, sigterm_handler)
def main():
import time
sys.stdout.write('Daemon started with pid {}\n'.format(os.getpid()))
while True:
sys.stdout.write('Daemon Alive! {}\n'.format(time.ctime()))
time.sleep(10)
if __name__ == '__main__':
PIDFILE = '/tmp/daemon.pid'
if len(sys.argv) != 2:
print('Usage: {} [start|stop]'.format(sys.argv[0]), file=sys.stderr)
raise SystemExit(1)
if sys.argv[1] == 'start':
try:
daemonize(PIDFILE,
stdout='/tmp/daemon.log',
stderr='/tmp/dameon.log')
except RuntimeError as e:
print(e, file=sys.stderr)
raise SystemExit(1)
main()
elif sys.argv[1] == 'stop':
if os.path.exists(PIDFILE):
with open(PIDFILE) as f:
os.kill(int(f.read()), signal.SIGTERM)
else:
print('Not running', file=sys.stderr)
raise SystemExit(1)
else:
print('Unknown command {!r}'.format(sys.argv[1]), file=sys.stderr)
raise SystemExit(1)
To start this daemon , The user needs to use the following command ：
bash % daemon.py start
bash % cat /tmp/daemon.pid
2882
bash % tail -f /tmp/daemon.log
Daemon started with pid 2882
Daemon Alive! Fri Oct 12 13:45:37 2012
Daemon Alive! Fri Oct 12 13:45:47 2012
…
Daemons can run completely in the background , So this command will immediately return . however , You can view the related as above pid Files and logs . To stop this daemon , Use ：
bash % daemon.py stop
bash %
