kill

You can use the built-in shell command kill to send a signal to any process you created—not just the currently running job. kill takes as an argument the process ID, job number, or command name of the process to which you want to send the signal. By default, kill sends the TERM ("terminate") signal, which usually has the same effect as the INT signal you send with CTRL-C. But you can specify a different signal by using the signal name (or number) as an option, preceded by a dash.

kill is so named because of the nature of the default TERM signal, but there is another reason, which has to do with the way UNIX handles signals in general. The full details are too complex to go into here, but the following explanation should suffice.

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Most signals cause a process that receives them to die; therefore, if you send any one of these signals, you "kill" the process that receives it. However, programs can be set up to Section 8.4 specific signals and take some other action. For example, a text editor would do well to save the file being edited before terminating when it receives a signal such as INT, TERM, or QUIT. Determining what to do when various signals come in is part of the fun of UNIX systems programming.

Here is an example of kill. Say you have an alice process in the background, with process ID 150 and job number 1, which needs to be stopped. You would start with this command:

$ kill %1

If you were successful, you would see a message like this:

[1]+  Terminated              alice

If you don't see this, then the TERM signal failed to terminate the job. The next step would be to try QUIT:

$ kill -QUIT %1

If that worked, you would see this message:

[1]+  Exit 131                alice

The 131 is the exit status returned by alice.[11] But if even QUIT doesn't work, the "last-ditch" method would be to use KILL:

$ kill -KILL %1

This produces the message:

[1]+  Killed                  alice

It is impossible for a process to Section 8.4 a KILL signal—the operating system should terminate the process immediately and unconditionally. If it doesn't, then either your process is in one of the "funny states" we'll see later in this chapter, or (far less likely) there's a bug in your version of UNIX.

Here's another example.

Task 8-1

Write a script called killalljobs that kills all background jobs.

The solution to this task is simple, relying on jobs -p:

kill "$@" $(jobs -p)

You may be tempted to use the KILL signal immediately, instead of trying TERM (the default) and QUIT first. Don't do this. TERM and QUIT are designed to give a process the chance to "clean up" before exiting, whereas KILL will stop the process, wherever it may be in its computation. Use KILL only as a last resort!

You can use the kill command with any process you create, not just jobs in the background of your current shell. For example, if you use a windowing system, then you may have several terminal windows, each of which runs its own shell. If one shell is running a process that you want to stop, you can kill it from another window—but you can't refer to it with a job number because it's running under a different shell. You must instead use its process ID.