bash - 为什么这个 kill 在此 Bash 脚本中不起作用，而只在脚本之外起作用？

Question

下面是一个演示效果的人为示例，必须使用root运行。它在后台执行 ping 进程并尝试终止它。

#!/bin/bash

# Ensure that there is no ping process before we begin.
killall ping

sudo ping google.com > /dev/null &
PID=$!

sleep 0.5

kill $PID
echo "Exit code of kill $PID: $?"

# Check the running ping processes. There should be no ping
# process if the above `kill $PID` worked correctly.
ps aux | grep -v grep | grep ping

但是，脚本无法终止进程，即使 kill 的返回码为 0。以下是示例输出。

$ bash test.sh
Exit code of kill 16516: 0
root     16516  0.0  0.0  14956  2212 pts/2    S    13:22   0:00 sudo ping google.com
root     16518  1.0  0.0  13112  1292 pts/2    S    13:22   0:00 ping google.com

我注意到如果我取出 sudo 然后它就会正确地杀死它。为什么会这样？我怀疑 sudo 的子进程以某种方式搞砸了它。

更新 1:

更奇怪。如果我在脚本之后执行相同的 kill 命令，它就会起作用。

$ bash test.sh
Exit code of kill 16631: 0
root     16631  3.0  0.0  14956  2212 pts/2    S    13:29   0:00 sudo ping google.com
root     16633  0.0  0.0  13112  1292 pts/2    S    13:29   0:00 ping google.com
$ ps aux | grep -v grep | grep ping
root     16631  0.5  0.0  14956  2212 pts/2    S    13:29   0:00 sudo ping google.com
root     16633  0.0  0.0  13112  1292 pts/2    S    13:29   0:00 ping google.com
$ kill 16631
$ ps aux | grep -v grep | grep ping
$
$ kill 16631
-bash: kill: (16631) - No such process
$

Answer 1

发生这种情况是因为控制 sudo 进程不会传播来自其自己的进程组 ( source ) 的信号:

/*
 * Do not forward signals sent by a process in the command's process
 * group, as we don't want the command to indirectly kill itself.
 * For example, this can happen with some versions of reboot that
 * call kill(-1, SIGTERM) to kill all other processes.
 */
if (USER_SIGNALED(sc->siginfo) && sc->siginfo->si_pid != 0) {
    pid_t si_pgrp = getpgid(sc->siginfo->si_pid);
    if (si_pgrp != -1) {
    if (si_pgrp == ec->ppgrp || si_pgrp == ec->cmnd_pid)
        debug_return;
    } else if (sc->siginfo->si_pid == ec->cmnd_pid) {
        debug_return;
    }
}

当您在脚本外部执行命令时，您会在单独的进程组中运行它，因此信号会被中继。