I finally managed to convert the process of my script in daemon, and after hours of testing I discovered that the script, converted in daemon, doesn't execute operations like:
out_file = open("test.txt","w")
out_file.write("")
out_file.close()
My script works this way:
class daemon: # taken here https://web.archive.org/web/20160305151936/http://www.jejik.com/articles/2007/02/a_simple_unix_linux_daemon_in_python/
"""A generic daemon class.
Usage: subclass the daemon class and override the run() method."""
def __init__(self, pidfile):
self.pidfile = pidfile
self.main = main
def daemonize(self):
"""Deamonize class. UNIX double fork mechanism."""
try:
pid = os.fork()
if pid > 0:
# exit first parent
sys.exit(0)
except OSError as err:
sys.stderr.write('fork #1 failed: {0}\n'.format(err))
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 as err:
sys.stderr.write('fork #2 failed: {0}\n'.format(err))
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())
# write pidfile
atexit.register(self.delpid)
pid = str(os.getpid())
with open(self.pidfile,'w+') as f:
f.write(pid + '\n')
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:
with open(self.pidfile,'r') as pf:
pid = int(pf.read().strip())
except IOError:
pid = None
if pid:
self.restart()
# Start the daemon
self.daemonize()
self.run()
def stop(self):
"""Stop the daemon."""
# Get the pid from the pidfile
try:
with open(self.pidfile,'r') as pf:
pid = int(pf.read().strip())
except IOError:
pid = None
if not pid:
message = "pidfile {0} does not exist. " + \
"Daemon not running?\n"
sys.stderr.write(message.format(self.pidfile))
return # not an error in a restart
# Try killing the daemon process
try:
while 1:
os.kill(pid, signal.SIGTERM)
time.sleep(0.1)
except OSError as err:
e = str(err.args)
if e.find("No such process") > 0:
if os.path.exists(self.pidfile):
os.remove(self.pidfile)
else:
print (str(err.args))
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()."""
while True:
self.main()
def main():
# several and various operations
if __name__ == "__main__":
d = daemon('/tmp/daemon-example.pid')
d.start()
Is there a way to allow my daemon operations like opening files and writing files? And, for curiosity, why a daemon can't do I/O operations?
The os.chdir('/') change working directory of the program to the /. So every relative path is relative to /. If you call open("test.txt", "w") as you've provided in the example it raises PermissionError because you don't have rights to write in the /test.txt unless you are running the program as the root.
You have those options: