Python如何终止在一个if条件内创建、另一if条件触发的subprocess进程

报错根因 #

• process为seek()函数内的局部变量，仅在触发新建进程逻辑时才会被定义，触发终止逻辑时该变量大概率未定义，直接调用方法会抛出NameError
• 未做进程状态校验：进程可能已经退出、或从未启动过，直接调用kill()/terminate()会抛出异常
• 代码存在拼写错误：new_lenght应为new_length，可能额外导致逻辑判断异常

修复步骤 #

1. 新增全局变量存储tcpdump进程实例，初始值设为None
2. 启动进程前判断是否已有运行中的进程，避免重复启动
3. 终止进程前校验进程状态，仅当进程处于运行状态时执行终止操作
4. 无需重复调用kill()和terminate()，二者选其一即可

修改后的完整代码 #

import netifaces 
import netaddr
import nmap
import re
import sys
import time
import subprocess
import os

hostList = []
gracePeriod = 1
process = None # 全局存储tcpdump进程实例

try:
    nm = nmap.PortScanner()         # instance of nmap.PortScanner
except nmap.PortScannerError:
    print('Nmap not found', sys.exc_info()[0])
    sys.exit(0)
except:
    print("Unexpected error:", sys.exc_info()[0])
    sys.exit(0)
    
def seek():
   curHosts =[]
   global wlan_iface, process
   ifaces=netifaces.interfaces() #Get all the avalialbe interfaces
   pattern = '^w' #Pattern maching for wlan interface
   
   for position in range(len(ifaces)):
       name = ifaces[position]
       match_result = re.match(pattern,str(name))
       if match_result:
           wlan_iface=name
   addrs = netifaces.ifaddresses(wlan_iface)
   ipinfo = addrs[netifaces.AF_INET][0]
   address = ipinfo['addr']
   wlan_iface1=str(wlan_iface)
   netmask = ipinfo['netmask']
   # Create ip object and get CIDR
   cidr = netaddr.IPNetwork('%s/%s' % (address, netmask))
   a=str(cidr)
   nm.scan(hosts = a, arguments = '-sn -T4')
   # executes a ping scan
   localtime = time.asctime(time.localtime(time.time()))
   print('============ {0} ============\n'.format(localtime))
   for host in nm.all_hosts():

       curHosts.append((host,gracePeriod))
   curHosts.remove((str(address),gracePeriod))
   old=sniff_old()
   old_length=len(old)
   updateHostList(curHosts)
   new=sniff_new()
   new_length=len(new) # 修正拼写错误
   if new_length>old_length:
       print("I will Record")
       # 仅当无运行中进程时才启动新进程
       if process is None or process.poll() is not None:
           process = subprocess.Popen(['sudo', 'tcpdump', '-l', '-i', wlan_iface1, '-w', f'{new[-1]}.pcap'], stdout=subprocess.PIPE, stderr=subprocess.DEVNULL)
   if new_length < old_length:
       print("I will Stop")
       # 仅当进程处于运行状态时才终止
       if process is not None and process.poll() is None:
           process.kill()
           process.wait()
           process = None # 重置状态
    
   return len(hostList)
   
def sniff_new():
    sniff_list=[]
    for host in hostList:      
       sniff_list.append(host[0])

    print(f"{sniff_list} NEW")
    return((sniff_list))
def sniff_old():
    sniff_list=[]
    for host in hostList:      
       sniff_list.append(host[0])
    old_sniff=(sniff_list)
    
    print(f"{old_sniff} OLD")
    return((old_sniff))
           
           
             
def updateHostList(curHosts):
    global hostList
    if hostList == []:
        hostList = curHosts
    else:
        hostList = [(x[0],x[1]-1) for x in hostList]
        


        # only the hosts that were new in this iteration
        newList = [(x[0],x[1]) for x in curHosts if not (any(x[0]==y[0] for y in hostList))]

        for host in newList:
            hostList.append(host)

        for host in hostList:
            if any(host[0] == y[0] for y in curHosts):
                hostList[hostList.index(host)] = (host[0],gracePeriod)

        for host in hostList:
            if host[1] <= 0:
                hostList.remove(host)
           
def beep():                         # no sound dependency
    print('\a')            
    
if __name__ == '__main__':
    old_count = new_count = seek()


    startCounter = gracePeriod
    
    # are there any new hosts?
    while True:
        startCounter -= 1
        time.sleep(1)               # increase to slow down the speed
        old_count = new_count
        new_count = seek()

内容的提问来源于stack exchange，提问作者jay soni