Ubuntu – Does killall (sometimes?) needs to be applied twice

command linekillkillallprocess

Why does killall (sometimes?) need to be applied twice? Here's an example where a "double" killall is needed to end the process.

When and why does a process needed to be killed "twice"? What happens under the bonnet?

Best Answer

I am going to state a application-specific possibility.

When you use killall program, a SIGTERM (signal 15) is sent to the program. The usual response to SIGTERM is that the program would exit gracefully.

Now as the SIGTERM is catchable, a program can have a signal handler for SIGTERM that would do some task upon receiving the first SIGTERM (first killall) and return to a state where the second SIGTERM would just terminate it (default action). This is highly dependent on the developer of the program of course and not a general case.

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Ubuntu – “kill ” not really killing the process, why

Processes can ignore some signals. If you send SIGKILL it will not be able to ignore it (and neither catch it to do cleanups). Try:

kill -9 {PID}

Learn more by reading the manual page:

man kill

Ubuntu – a process, and why doesn’t it get killed

From your output we see a "defunct", which means the process has either completed its task or has been corrupted or killed, but its child processes are still running or these parent process is monitoring its child process. To kill this kind of process, kill -9 PID doesn't work. You can try to kill them with this command but it will show this again and again.

Determine which is the parent process of this defunct process and kill it. To know this run the command:

$ ps -ef | grep defunct
    UID          PID     PPID       C    STIME      TTY          TIME              CMD
    1000       637      27872      0   Oct12      ?        00:00:04 [chrome] <defunct>
    1000      1808      1777       0    Oct04     ?        00:00:00 [zeitgeist-datah] <defunct>

Then kill -9 637 27872, then verify the defunct process is gone by ps -ef | grep defunct.

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Ubuntu – “kill ” not really killing the process, why

Ubuntu – a process, and why doesn’t it get killed

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