[CLUG] FW: Cork digest, Vol 1 #325 - 4 msgs

[Dermot McGahon]

>>>>> "Bryan" == Roycroft, Bryan <BRoycroft at mcom.cit.ie> writes:

[-]

    Bryan> what I wanted to know was just what the "HUP" option stood
    Bryan> for, as I had an understanding of what kill did, what i've
    Bryan> gleaned is that the it is basically the level which is
    Bryan> passed to the command, ie if it should shutdown or
    Bryan> restart. Also in some examples its followed by a file name,
    Bryan> it seems to be the location of the executable that is being
    Bryan> restarted. Is this always necessary or can the OS figure
    Bryan> out what to restart from the process context, as the advice
    Bryan> thats in the column just refer to just the process id or is
    Bryan> the HUP the PID and the filename the actual executable to
    Bryan> execute, getting the HUP id.

You /could/ look at it as being a kind of level although it's really a
little broader than that.

A signal is a very primitive form of interprocess
communication. Mostly you send signals from a shell or terminal
process to an application process. There are many different signals
and some of them are architecture dependant. See man signal for more
information:

       SIGHUP        1        A      Hangup detected on controlling terminal
                                     or death of controlling process
       SIGINT        2        A      Interrupt from keyboard
       SIGQUIT       3        C      Quit from keyboard
       SIGILL        4        C      Illegal Instruction
       SIGABRT       6        C      Abort signal from abort(3)
       SIGFPE        8        C      Floating point exception
       SIGKILL       9       AEF     Kill signal
       SIGSEGV      11        C      Invalid memory reference
       SIGPIPE      13        A      Broken pipe: write to pipe with no readers
       SIGALRM      14        A      Timer signal from alarm(2)
       SIGTERM      15        A      Termination signal
       SIGUSR1   30,10,16     A      User-defined signal 1
       SIGUSR2   31,12,17     A      User-defined signal 2
       SIGCHLD   20,17,18     B      Child stopped or terminated
       SIGCONT   19,18,25            Continue if stopped
       SIGSTOP   17,19,23    DEF     Stop process
       SIGTSTP   18,20,24     D      Stop typed at tty
       SIGTTIN   21,21,26     D      tty input for background process
       SIGTTOU   22,22,27     D      tty output for background process


The kill command is used to send a signal to a process. If you don't
specify a specific signal then SIGTERM is used by default. If you
specify something like kill -9 then signal 9 is sent to the
process. Signal nice as you can see from the table above is the KILL
signal and you can use the numbers and the names interchangebly.

You should /not/ send a KILL signal to a process unless you absolutely
have to. Processes can have files open, pipes open, memory allocated
and then asked to terminate with a TERM signal the signal handler
written as part of the application/daemon closes all used resources
and basically does whatever housekeeping needs to be done before
exiting. When you send a KILL signal to a process it exits immediately
without doing this necessary housekeeping. See man kill for more
information.

The usual way to use kill is kill -signal PID. I'm not sure what you
mean about specifying files ? After you start an executable file
running then you have a process. You then send a signal to the process
id (PID). The PID can be found from the ps command.

Regards,

Dermot.
--