Python实现在Linux系统下更改当前进程运行用户

#!/usr/bin/env python
import time,os,pwd,sys,signal
logfile="/tmp/d2.log"
#step one, get the username you want to running with
try:
    user=sys.argv[1]
except:
    user=raw_input('Please input a username in this machine you want to run this program: ')
if user=="":sys.exit(-1)
try:
    uid=pwd.getpwnam(user)
    uid=uid.pw_uid
except:
    print "Uer not exists!"
    sys.exit(-1)
#step two:Generation of daemon
pid=os.fork()
if(pid):sys.exit(0)
os.setsid()
os.chdir("/")
os.umask(0)
#step three :fork again
pid=os.fork()
if(pid==0):
    os.setuid(uid)
    os.setsid()
    os.chdir("/")
    os.umask(0)
log=open(logfile,'a')
log.write('Daemon start up at %s\n'%(time.strftime('%Y:%m:%d',time.localtime(time.time()))))
log.close()
def reload(a,b):
  log=open(logfile,'a')
  log.write('Daemon reload at %s\n'%(time.strftime('%Y:%m:%d',time.localtime(time.time()))))
  log.close()
while True:
  signal.signal(signal.SIGHUP,reload)
  time.sleep(2)

[root@home ~]# ps aux|grep python
root   1139 0.0 0.5  5288 2372 ?    Ss  22:40  0:00 python ./d2.py nginx
nginx   1140 0.0 0.5  5288 2360 ?    S  22:40  0:00 python ./d2.py nginx
root   1151 0.0 0.1  2336  648 pts/0  S+  22:50  0:00 grep python