The Kill Command in Linux is used to terminate the operation of a specified process. It is a common command for process management in Linux. Usually, the Ctrl+C key can be used to terminate a foreground process, but for a background process, we need to terminate with kill command. We need to use tools such as ps/pidof/pstree/top to get the process PID first, and then kill the process with kill command. The Kill Command terminates a process by sending a specified signal to it. By default, TERM signals numbered 15 are used. TERM signals terminate all processes that cannot capture the signal. For those processes that can capture the signal, kill the process by using a kill signal numbered 9.

1. Command format:

kill [parameter] [process number]

2. Command function:

3. Command parameters:

• - L signal, if no signal numbering parameter is added, the "-l" parameter will list all the signal names.
• - a When dealing with the current process, there is no restriction on the correspondence between the command name and the process number
• - p Specifies that the kill Command prints only the process number of the process concerned, without sending any signals.
• - s Designated Send Signal
• - u Designated User

Be careful:

1. The kill Command can be with or without a signal number option. If there is no signal number, the kill Command sends a termination signal (15), which can be captured by the process so that the process can clean up and release resources before exiting. kill can also be used to send a specific signal to a process. For example:
   kill -2 123
   Its effect is the same as pressing Ctrl+C key when running the process with PID 123 in the foreground. However, ordinary users can only use kill commands without signal parameters or - 9 signals at most.
2. kill can take a process ID number as a parameter. When killing signals to these processes, they must be the masters. If you try to revoke a process that has no permission to revoke or a process that does not exist, you get an error message.
3. Multiple processes can be signaled or terminated.
4. When kill successfully sends a signal, the shell displays the termination information of the process on the screen. Sometimes this information will not be displayed immediately, only when the Enter key is pressed to make the shell's command prompt appear again.
5. It should be noted that signals force processes to terminate, which often brings some side effects, such as data loss or terminal failure to return to normal state. Care must be taken when sending signals. Kill signals (9) are used only when absolutely necessary, because processes cannot capture them first. To undo all background jobs, type kill 0. Because some commands that run in the background will start multiple processes, it's troublesome to track and find the PID of all the processes to kill. At this point, kill 0 is an effective way to terminate all processes initiated by the current shell.

4. Use examples:

Example 1: List all signal names
Order:

kill -l

Output:

[root@localhost test6]# 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

Explain:

Example 2: Get the value of the specified signal
Order:
Output:

[root@localhost test6]# kill -l KILL
9
[root@localhost test6]# kill -l SIGKILL
9
[root@localhost test6]# kill -l TERM
15
[root@localhost test6]# kill -l SIGTERM
15
[root@localhost test6]#

Explain:

Example 3: Find the process with ps and kill it
Order:

kill 3268

Output:

[root@localhost test6]# ps -ef|grep vim 
root      3268  2884  0 16:21 pts/1    00:00:00 vim install.log
root      3370  2822  0 16:21 pts/0    00:00:00 grep vim
[root@localhost test6]# kill 3268 
[root@localhost test6]# kill 3268 
-bash: kill: (3268) - No process
[root@localhost test6]#

Explain:

Example 4: Kill the process completely
Order:

kill –9 3268 

Output:

[root@localhost test6]# ps -ef|grep vim 
root      3268  2884  0 16:21 pts/1    00:00:00 vim install.log
root      3370  2822  0 16:21 pts/0    00:00:00 grep vim
[root@localhost test6]# kill –9 3268 
[root@localhost test6]# kill 3268 
-bash: kill: (3268) - No process
[root@localhost test6]#

Explain:

Example 5: Kill all processes of a specified user
Order:

kill -9 $(ps -ef | grep peidalinux)
kill -u peidalinux

Output:

[root@localhost ~]# kill -9 $(ps -ef | grep peidalinux) 
[root@localhost ~]# kill -u peidalinux

Explain:

Example 6: The init process is not killable
Order:

kill -9 1

Output:

[root@localhost ~]# ps -ef|grep init
root         1     0  0 Nov02 ?        00:00:00 init [3]                  
root     17563 17534  0 17:37 pts/1    00:00:00 grep init
[root@localhost ~]# kill -9 1
[root@localhost ~]# kill -HUP 1
[root@localhost ~]# ps -ef|grep init
root         1     0  0 Nov02 ?        00:00:00 init [3]                  
root     17565 17534  0 17:38 pts/1    00:00:00 grep init
[root@localhost ~]# kill -KILL 1
[root@localhost ~]# ps -ef|grep init
root         1     0  0 Nov02 ?        00:00:00 init [3]                  
root     17567 17534  0 17:38 pts/1    00:00:00 grep init
[root@localhost ~]#

Explain: