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Full Discussion: Need help with how to search a file for a variable string and delete that line
Top Forums UNIX for Beginners Questions & Answers Need help with how to search a file for a variable string and delete that line Post 303031333 by bakunin on Tuesday 26th of February 2019 12:02:32 PM
Old 02-26-2019
Quote:
Originally Posted by newbie_01
I think I should start adding that in all of the scripts especially for SIGKILL=9 when we have to do a kill -9.
Expanding on what Don Cragun has already explained: The difference between "9" and all other signals a process can get is that "9" (like in kill -9) is not handled by the process itself. Itis handled by the system (that is: a part of the kernel).

When you issue the command kill -n <PID> (for n being some number - see kill -l for a list of all legal values) this generates a "signal" which is then passed to the process and handled by it. In principle a process can react however it wants (or even not at all) to a specific signal but there are conventions and therefore expected reactions of a process to a certain signal. i.e. kill -1 usually causes a process to re-read its configuration file(s) and then resume operation using this new configuration.

The ultimate signal you can send to a process is "15" which means "terminate immediately". A well-behaved process will then terminate, no matter what it is doing. Still, "terminating" here means not just "exit" but to perform every cleanup possible: it there are temporary files open, then close them and remove them, if there are network connections open close them and free up the sockets, etc..

This is the main difference between signal 15 and 9: with 9 the process will not be notified but immediately (and in the most brutal way possible) removed by the OS. It will simply have no time to clean up, in fact it isn't even aware of it being killed.

Therefore, when you want to stop a process, always try kill -15 at least once (better: two to three times) and only if the process is proven not to be able to terminate itsself any more try a kill -9 - but ONLY then. Your system will remain much more stable because processes will have the chance to clean up (and well-behaved UNIX processes do that!).

I hope this helps.

bakunin
This User Gave Thanks to bakunin For This Post:
Don Cragun
 

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LEARN ABOUT PHP

kill

KILL(1)                                                            User Commands                                                           KILL(1)

NAME

       kill - send a signal to a process

SYNOPSIS

       kill [options] <pid> [...]

DESCRIPTION

       The  default  signal  for kill is TERM.  Use -l or -L to list available signals.  Particularly useful signals include HUP, INT, KILL, STOP,
       CONT, and 0.  Alternate signals may be specified in three ways: -9, -SIGKILL or -KILL.  Negative PID values may be  used  to  choose  whole
       process  groups;  see  the  PGID  column  in ps command output.  A PID of -1 is special; it indicates all processes except the kill process
       itself and init.

OPTIONS

       <pid> [...]
              Send signal to every <pid> listed.

       -<signal>
       -s <signal>
       --signal <signal>
              Specify the signal to be sent.  The signal can be specified by using name or number.  The behavior of signals is explained  in  sig-
              nal(7) manual page.

       -l, --list [signal]
              List signal names.  This option has optional argument, which will convert signal number to signal name, or other way round.

       -L, --table
              List signal names in a nice table.

       NOTES  Your shell (command line interpreter) may have a built-in kill command.  You may need to run the command described here as /bin/kill
              to solve the conflict.

EXAMPLES

       kill -9 -1
              Kill all processes you can kill.

       kill -l 11
              Translate number 11 into a signal name.

       kill -L
              List the available signal choices in a nice table.

       kill 123 543 2341 3453
              Send the default signal, SIGTERM, to all those processes.

SEE ALSO

       kill(2), killall(1), nice(1), pkill(1), renice(1), signal(7), skill(1)

STANDARDS

       This command meets appropriate standards. The -L flag is Linux-specific.

AUTHOR

       Albert Cahalan <albert@users.sf.net> wrote kill in 1999 to replace a bsdutils one that was not standards  compliant.   The  util-linux  one
       might also work correctly.

REPORTING BUGS

       Please send bug reports to <procps@freelists.org>

procps-ng                                                          October 2011                                                            KILL(1)
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