What does kill mean?

The kill command in Linux is used to terminate the running of a specified process (terminate a process). Kill is a common command for process management under Linux. kill sends a specific signal (signal) to the process whose ID is pid and performs specific actions based on the signal. If not specified, the default is to send a termination (TERM) signal.

Usually, to terminate a foreground process, you can use the Ctrl C key. However, for a background process, you must use the kill command to terminate it. We need to use ps first. Use tools such as /pidof/pstree/top to obtain the process PID, and then use the kill command to kill the process. The kill command ends the corresponding process by sending a specified signal to the process.

1. Command format:

kill[parameter][process number]

2. Command function :

Send the specified signal to the corresponding process. Failure to specify a model will send SIGTERM (15) to terminate the specified process. If the program cannot be terminated, the "-KILL" parameter can be used, and the signal it sends is SIGKILL(9), which will force the end of the process. Use the ps command or jobs command to view the process number. The root user will affect the user's process, and non-root users can only affect their own process.

3. Command parameters:

-l signal, if the signal number parameter is not added, using the "-l" parameter will list all signal names

-a When processing the current process, the corresponding relationship between the command name and the process number is not limited

-p specifies that the kill command only prints the process number of the relevant process without sending any signal

-s specifies that the signal is sent

-u Specified user

Note:

1. The kill command can have a signal number option or not. If there is no signal number, the kill command will issue a termination signal (15), which can be captured by the process so that the process can clean up and release resources before exiting. You can also use kill to send specific signals to a process. For example:

kill -2 123

Its effect is equivalent to pressing the Ctrl C key when the process with PID 123 is running in the foreground. However, ordinary users can only use the kill command without the signal parameter or with up to -9 signals.

2. Kill can take the process ID number as a parameter. When using kill to send signals to these processes, you must be the owner of these processes. If you try to cancel a process that you do not have permission to cancel or a process that does not exist, you will get an error message.

3. You can send signals to multiple processes or terminate them.

4. When kill successfully sends the signal, the shell will display the process termination information on the screen. Sometimes this message does not appear immediately until the shell's command prompt appears again when the Enter key is pressed.

5. It should be noted that the signal causes the process to be forcibly terminated, which often brings some side effects, such as data loss or the terminal being unable to return to normal state. Care must be taken when sending signals, and the kill signal(9) should only be used as a last resort, since the process cannot catch it in the first place. To cancel all background jobs, enter kill 0. Because some commands running in the background will start multiple processes, it is very troublesome to track and find the PIDs of all processes to be killed. At this time, it is an effective method to use kill 0 to terminate all processes started by the current shell.

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