Sending Signals to Processes with kill, killall, and pkill

The Linux kernel allows many signals to be sent to processes. Use man 7 signals for a complete overview of all the available signals. Three of these signals work for all processes:
■ The signal SIGTERM (15) is used to ask a process to stop.
■ The signal SIGKILL (9) is used to force a process to stop.
■ The SIGHUP (1) signal is used to hang up a process. The effect is that the process will reread its configuration files, which makes this a useful signal to use after making modifications to a process configuration file.

To send a signal to a process, the kill command is used. The most common use is the need to stop a process, which you can do by using the kill command followed by the PID of the process. This sends the SIGTERM signal to the process, which normally causes the process to cease its activity. Sometimes the kill command does not work because the process you want to kill is busy. In that case, you can use kill -9 to send the SIGKILL signal to the process. Because the SIGKILL signal cannot be ignored, it forces the process to stop, but you also risk losing data while using this command. In general, it is a bad idea to use kill -9 :
■ You risk losing data.
■ Your system may become unstable if other processes depend on the process you have just killed.
TIP Use kill -l to show a list of available signals that can be used with kill .

[root@rhel7 ~]# kill -l
 ) SIGHUP       ) SIGINT       ) SIGQUIT      ) SIGILL       ) SIGTRAP
 ) SIGABRT      ) SIGBUS       ) SIGFPE       ) SIGKILL     ) SIGUSR1
) SIGSEGV     ) SIGUSR2     ) SIGPIPE     ) SIGALRM     ) SIGTERM
) SIGSTKFLT   ) SIGCHLD     ) SIGCONT     ) SIGSTOP     ) SIGTSTP
) SIGTTIN     ) SIGTTOU     ) SIGURG      ) SIGXCPU     ) SIGXFSZ
) SIGVTALRM   ) SIGPROF     ) SIGWINCH    ) SIGIO       ) SIGPWR
) SIGSYS      ) SIGRTMIN    ) SIGRTMIN+  ) SIGRTMIN+  ) SIGRTMIN+
) SIGRTMIN+  ) SIGRTMIN+  ) SIGRTMIN+  ) SIGRTMIN+  ) SIGRTMIN+
) SIGRTMIN+  ) SIGRTMIN+ ) SIGRTMIN+ ) SIGRTMIN+ ) SIGRTMIN+
) SIGRTMIN+ ) SIGRTMIN+ ) SIGRTMAX- ) SIGRTMAX- ) SIGRTMAX-
) SIGRTMAX- ) SIGRTMAX- ) SIGRTMAX-  ) SIGRTMAX-  ) SIGRTMAX-
) SIGRTMAX-  ) SIGRTMAX-  ) SIGRTMAX-  ) SIGRTMAX-  ) SIGRTMAX-
) SIGRTMAX-  ) SIGRTMAX
[root@rhel7 ~]# 

There are some commands that are related to kill:  killall  and  pkill . The  pkill  command is a bit easier to use because it takes the name rather than the PID of the process as an argument. You can use the  killall  command if multiple processes using the same name need to be killed simultaneously.   Using  killall  was particularly common when Linux environments were multiprocessing instead of multithreading. In a multiprocessing environment where a server starts several commands, all with the same name, it is not easy to stop these commands one by one based on their individual PID. Using  killall  enables you to terminate all these processes simultaneously.