我想以10秒的时间限制运行tcpdump命令.

timeout 10 sudo tcpdump -i eth0 -nn 'host 192.168.1.176'

它并没有停止.为什么超时命令在这里对tcpdump无效？

解决方法:

问题在于超时会以您的用户权限运行. sudo进程将特权升级为root(或另一个用户),因此不允许超时将SIGTERM发送给子进程.可以用strace来显示(我以#开头的注释,以及为了可读性而使用的空行)：

user$strace timeout 1 sudo sleep 5
# lots of irrelevant stuff
# here, timeout sets up the timer to get a signal when the child should be terminated
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
timer_create(CLOCK_REALTIME, {sigev_value={sival_int=1889673072, sival_ptr=0x560c70a21f70}, sigev_signo=SIGALRM, sigev_notify=SIGEV_SIGNAL}, [0]) = 0
timer_settime(0, 0, {it_interval={tv_sec=0, tv_nsec=0}, it_value={tv_sec=1, tv_nsec=0}}, NULL) = 0
wait4(12320, 0x7ffdfeb0ef0c, 0, NULL)   = ? ERESTARTSYS (To be restarted if SA_RESTART is set)

# the signal arrives
--- SIGALRM {si_signo=SIGALRM, si_code=SI_TIMER, si_timerid=0, si_overrun=0, si_value={int=1889673072, ptr=0x560c70a21f70}} ---

# timeout tries to kill the child
kill(12320, SIGTERM)                    = -1 EPERM (Operation not permitted)
# and gets EPERM!

解决方法是也以root特权运行超时.以下将按预期工作：

user$sudo timeout 1 sleep 5

当然,如果您已经是root用户,则在命令行中的sudo之前或之后放置超时1都没有关系.

root$sudo timeout 1 sleep 5
root$timeout 1 sudo sleep 5