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Learning the Unix Operating System

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6.3 Cancelling a Process

You may decide that you shouldn't have put a process in the background. Or you decide that the process is taking too long to execute. You can cancel a background process if you know its process ID.

6.3.1 kill

The kill command aborts a process. The command's format is:

kill PID(s)

kill terminates the designated process IDs (shown under the PID heading in the ps listing). If you do not know the process ID, do a ps first to display the status of your processes.

In the following example, the " sleep n " command simply causes a process to "go to sleep" for n number of seconds. We enter two commands, sleep and who , on the same line, as a background process.


(sleep 60; who)&

[1] 21087


20055    4    0:10  sh
21087    4    0:01  sh
21088    4    0:00  sleep
21089    4    0:02  ps

kill 21088

% tom     tty2   Aug 30  11:27
grace   tty4   Aug 30  12:24
tim     tty5   Aug 30  07:52
dale    tty7   Aug 30  14:34

We decided that 60 seconds was too long a time to wait for the output of who . The ps listing showed that sleep had the process ID number 21088, so we used this PID to kill the sleep process. You should see a message like "terminated" or "killed"; if you don't, use another ps command to be sure the process has been killed.

The who command is executed immediately, since it is no longer waiting on sleep ; it lists the users logged into the system.

6.3.2 Problem checklist

The process didn't die when I told it to.

Some processes can be hard to kill. If a normal kill of these processes is not working, enter " kill -9 PID ". This is a sure kill and can destroy almost anything, including the shell that is interpreting it.

In addition, if you've run an interpreted program (like a shell script), you may not be able to kill all dependent processes by killing the interpreter process that got it all started; you may need to kill them individually. However, killing a process that is feeding data into a pipe will generally kill any processes receiving that data.

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