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Signals
The kill command is used to “kill” processes. This allows us to terminate programs that need killing. Here's an example:
[me@linuxbox ~]$ xlogo & [1] 28401
[me@linuxbox ~]$ kill 28401
[1]+ Terminated xlogo
[me@linuxbox ~]$ xlogo & [1] 28401
[me@linuxbox ~]$ kill 28401
[1]+ Terminated xlogo
We first launch xlogo in the background. The shell prints the jobspec and the PID of the background process. Next, we use the kill command and specify the PID of the process we want to terminate. We could have also specified the process using a jobspec (for ex- ample, “%1”) instead of a PID.
While this is all very straightforward, there is more to it than that. The kill command doesn't exactly “kill” processes, rather it sends them signals. Signals are one of several ways that the operating system communicates with programs. We have already seen sig- nals in action with the use of Ctrl-c and Ctrl-z. When the terminal receives one of these keystrokes, it sends a signal to the program in the foreground. In the case of Ctrl- c, a signal called INT (Interrupt) is sent; with Ctrl-z, a signal called TSTP (Terminal Stop). Programs, in turn, “listen” for signals and may act upon them as they are received. The fact that a program can listen and act upon signals allows a program to do things like save work in progress when it is sent a termination signal.