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kill-终止进程.md

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kill command

The kill command sends a signal to the specified pid process. If the signal to be sent is not specified, then by default the signal is SIGTERM, which terminates the process. To display all available signals, the -l option can be used to obtain the Linux signal list. Commonly used signals include HUP, INT, KILL, STOP, CONT, and 0. Signals can be specified in three ways: by number, for example, -9; with the SIG prefix, for example, -SIGKILL; or without the SIG prefix, for example, -KILL. Negative PID values are used to indicate process group ID. If a process group ID is passed, then all processes in that group will receive the signal. PID -1 is special, as it indicates all processes except for two: the kill process itself and init, specifically PID 1, which is the parent process of all processes on the system. Specifying -1 as the target will send the signal to all processes except these two.

Syntax

kill pid ...
kill {-signal | -s signal} pid ...
kill {-l | --list[=signal] | -L | --table}

Parameters

  • -signal, -s signal: The name, abbreviation, or number of the signal to be sent, preceded by a hyphen, for example, -SIGTERM, -TERM, -15, etc. To view a list of available signals, use kill -l.
  • pid: Numeric process ID. If uncertain about a specific process's PID, it can be listed using the ps command, for example, ps -aux. Typically, it is used in conjunction with a pipe and grep, for example, to view nginx-related processes, ps -aux | grep nginx.
  • -l, --list[=signal]: Lists available signal names. Use -l or --list to list all signal names. Using --list = signal can convert numbers into their signal names.
  • -L, --table: Lists available signal names and numbers in a table format.

Signals

Certain signals may not be used because they may not be supported by the system or have been interrupted. For details, refer to the actual system's kill -l command. Every system has signals 1, 3, 9, and 15, which are the most common. Termination signals are used not only to close locked applications but also to prevent software from performing unauthorized tasks. This means that some termination signals are a secure aspect. Additionally, the kill command not only stops or terminates processes, but also pauses, resumes, and restarts processes.

 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
  • SIGHUP: The SIGHUP signal is used to disconnect a process from its parent process, and it can also be used to restart a process, which is useful for daemons with memory leaks.
  • SIGINT: This signal is equivalent to pressing Ctrl+C. On some systems, delete + break will send the same signal to the process, causing it to interrupt and stop, but the process itself can ignore this signal.
  • SIGQUIT: Similar to SIGINT, but because the QUIT character is usually controlled by Ctrl+\, when a process exits upon receiving SIGQUIT, a core file is generated, in this sense similar to a program error signal.
  • SIGILL: When a process performs an erroneous, prohibited, or unknown operation, the system sends a SIGILL signal to the process, representing an illegal operation.
  • SIGTRAP: This signal is used for debugging purposes. It is sent to a process when it performs an operation or meets a condition that the debugger is waiting for.
  • SIGABRT: This termination signal is an abort signal. Typically, a process sends this termination signal to itself.
  • SIGBUS: When a process receives a SIGBUS signal, it is because the process has caused a bus error. Typically, these bus errors occur when a process attempts to use a false physical address or when the process's memory alignment settings are incorrect.
  • SIGFPE: Processes that attempt to divide by zero terminate using the SIGFPE signal.
  • SIGKILL: The SIGKILL signal forces a process to immediately stop executing. The program cannot ignore this signal, and this process cannot be cleaned up either.
  • SIGUSR1: This represents a user-defined condition, and users can set this signal by programming a command in sigusr1.c.
  • SIGSEGV: This signal is sent to a process when there is a segmentation fault in the application.
  • SIGUSR2: This represents a user-defined condition.
  • SIGPIPE: This signal is sent to a process when it attempts to write to one end of a pipe that is not connected to a reader. The reader is the process that is reading data from the end of the pipe.
  • SIGALRM: The SIGALRM signal is sent when a real-time or clock timer expires.
  • SIGTERM: This signal requests that a process stop running. This signal can be ignored by the program itself, and the process has time to shut down gracefully. When a program shuts down normally, it means that it has time to save its progress and release resources, in other words, it is not forced to stop.
  • SIGCHLD: When a parent process loses its child process, the parent process receives the SIGCHLD signal, which cleans up the resources used by the child process. A child process refers to a process started by another process called the parent process.
  • SIGCONT: To make a process continue running after being paused by a SIGTSTP or SIGSTOP signal, the SIGCONT signal needs to be sent to the paused process. This is the CONTiNUE SIGNAL, and it is very useful for executing background tasks in Unix job control.
  • SIGSTOP: This signal causes the operating system to pause the execution of a process, and the process itself cannot ignore this signal.
  • SIGTSTP: This signal is similar to pressing Ctrl+Z. It sends a request to the terminal containing the process to temporarily stop, and the process itself can ignore this signal.
  • SIGTTIN: The process receives this signal when it attempts to read data from a tty terminal.
  • SIGTTOU: When a process attempts to perform a write operation on a tty terminal, the process receives this signal.
  • SIGURG: The SIGURG signal is sent to the process when there is urgent data to be read or when the data is very large.
  • SIGXCPU: When a process uses the CPU after the allocated time, the system sends this signal to the process. The behavior of SIGXCPU is like a warning, giving the process time to save its progress (if possible) and close it before the system terminates it using SIGKILL.
  • SIGXFSZ: When a program attempts to violate the size limitation of a file system, the system sends the SIGXFSZ signal to the process.
  • SIGVTALRM: The SIGVTALRM signal is sent when the CPU time used by the process expires.
  • SIGPROF: The SIGPROF signal is sent when the process exceeds the CPU time used on behalf of the process by the system.
  • SIGWINCH: The process will receive this signal when it is in a terminal that is changing its size.
  • SIGIO: An alias for SIGPOLL, or at least behaves very much like SIGPOLL.
  • SIGPWR: If there is a power failure, the system will send this signal to the process if it is still running.
  • SIGSYS: A process that provides invalid arguments for a system call will receive this signal.
  • SIGRTMIN*: This is a group of signals that vary between systems, they are marked as SIGRTMIN + 1, SIGRTMIN + 2, SIGRTMIN + 3, ..., typically up to a maximum of 15, these are user-defined signals that must be programmed in the source code of the Linux kernel.
  • SIGRTMAX*: This is a group of signals that vary between systems, they are marked as SIGRTMAX-1, SIGRTMAX-2, SIGRTMAX-3, ..., typically up to a maximum of 14, these are user-defined signals that must be programmed in the source code of the Linux kernel.
  • SIGEMT: Indicates an implementation-defined hardware fault.
  • SIGINFO: Sometimes, the terminal may send a status request to the process, in which case the process will also receive this signal.
  • SIGLOST: A process that tries to access a locked file receives this signal.
  • SIGPOLL: When a process causes an asynchronous I/O event, the SIGPOLL signal is sent to the process.

Example

To display all available signals and view detailed information about a signal, the man command can be used, for example for the signal 7 SIGBUS you can use man 7 signal.

kill -l

List the available signal names and numbers in a table.

kill -L

Terminate the session, reload the configuration file, and smoothly restart.

kill -1 111

Notify the process to close, allowing it to shut down on its own in a safe, clean manner. If the kill command is followed directly by the process's pid, the default option is -15. This signal can be ignored and the process can continue. It can be blocked and ignored.

kill -15 111

Forcefully interrupt the current program's execution, similar to pressing Ctrl+C to terminate a process. This signal can be blocked and ignored.

kill -2 111

Exit the process, similar to pressing Ctrl+\ to terminate a process. This signal can be blocked and ignored.

kill -3 111

Forcefully terminate the process. The -9 signal unconditionally terminates a process, cannot be caught or ignored, and the receiving process cannot perform any cleanup upon receiving this signal. This signal cannot be blocked or ignored, but it's generally not recommended to use kill -9. It's better to try using -15 and -2 to give the target process a chance to clean up its resources. Don't use a scythe to prune the flowers in a flowerpot.

kill -9 111

Pause the process. The process itself cannot ignore this pause signal.

kill -19 111

Resume the process. The resume signal must be sent to a paused process to be effective.

kill -18 111

Daily Question

https://github.com/WindrunnerMax/EveryDay

References

https://www.computerhope.com/unix/ukill.htm
https://www.linux.org/threads/kill-commands-and-signals.8881/
https://www.geeksforgeeks.org/kill-command-in-linux-with-examples/