很多时候，会有人建议你，如果kill杀不掉一个进程，就用kill -9. 为什么？
kill是Linux下常见的命令。其man手册的功能定义如下：
kill – send a signal to a process
明朗了，其实kill就是给某个进程id发送了一个信号。默认发送的信号是SIGTERM，而kill -9发送的信号是SIGKILL，即exit。exit信号不会被系统阻塞，所以kill -9能顺利杀掉进程。当然你也可以使用kill发送其他信号给进程。
经常使用的killall呢？
killall – kill processes by name
即，通过指定进程名的方式杀死进程。
KILL(1) Linux User's Manual KILL(1)
NAME
kill - send a signal to a process
SYNOPSIS
kill [ -signal | -s signal ] pid ...
kill [ -L | -V, --version ]
kill -l [ signal ]
DESCRIPTION
The default signal for kill is TERM. Use -l or -L to list avail‐
able signals. Particularly useful signals include HUP, INT,
KILL, STOP, CONT, and 0. Alternate signals may be specified in
three ways: -9 -SIGKILL -KILL. Negative PID values may be used
to choose whole process groups; see the PGID column in ps command
output. A PID of -1 is special; it indicates all processes except
the kill process itself and init.
SIGNALS
The signals listed below may be available for use with kill.
When known constant, numbers and default behavior are shown.
Name Num Action Description
0 0 n/a exit code indicates if a signal may be sen
t
ALRM 14 exit
HUP 1 exit
INT 2 exit
KILL 9 exit cannot be blocked
PIPE 13 exit
POLL exit
PROF exit
TERM 15 exit
USR1 exit
USR2 exit
VTALRM exit
STKFLT exit might not be implemented
PWR ignore might exit on some systems
WINCH ignore
CHLD ignore
URG ignore
TSTP stop might interact with the shell
TTIN stop might interact with the shell
TTOU stop might interact with the shell
STOP stop cannot be blocked
CONT restart continue if stopped, otherwise ignore
ABRT 6 core
FPE 8 core
ILL 4 core
QUIT 3 core
SEGV 11 core
TRAP 5 core
SYS core might not be implemented
EMT core might not be implemented
BUS core core dump might fail
XCPU core core dump might fail
XFSZ core core dump might fail
NOTES
Your shell (command line interpreter) may have a built-in kill
command. You may need to run the command described here as
/bin/kill to solve the conflict.
EXAMPLES
kill -9 -1
Kill all processes you can kill.
kill -l 11
Translate number 11 into a signal name.
kill -L
List the available signal choices in a nice table.
kill 123 543 2341 3453
Send the default signal, SIGTERM, to all those processes.
SEE ALSO
pkill(1), skill(1), kill(2), renice(1), nice(1), signal(7), kil‐
lall(1).
STANDARDS
This command meets appropriate standards. The -L flag is Linux-
specific.
AUTHOR
Albert Cahalan <[email protected]> wrote kill in 1999 to
replace a bsdutils one that was not standards compliant. The
util-linux one might also work correctly.
Please send bug reports to <[email protected]>
Linux November 21, 1999 KILL(1)
kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP
6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1
11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM
16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP
21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR
31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3
38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8
43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13
48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12
53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7
58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
63) SIGRTMAX-1 64) SIGRTMAX