Murder Most Fowl - page 2
Dealing with SIGHUPMan page authors tend to wobble between addressing end users and ace programmers. That's why you see statements like "the
dolist is executed as long as the last command in list returns a non-zero exit status." Which is as helpful as saying "send the process a SIGHUP". But not to worry, for today we shall peel off the mask of mystery that covers these deep dark subjects.
This falls under the heading of signals and process control. For us ace admins and users, our primary concerns are starting, stopping, and restarting services, and stopping runaway or hung processes with as little disruption as possible. Because signal-handling varies on different operating systems and different command shells, we'll stick to Linux and the bash shell.
Signals are used to communicate with daemons and processes. Any active task is a process, while daemons are background services that lurk in wait to respond to certain events, or to run scheduled tasks. A program must have some sort of signal handler programmed into it to trap and respond to signals. The
signal man page describes the various signals and what they do. Signals are sent by the
kill -l displays a list of signals and their numbers.
All daemons and processes have a Process ID (PID), as this
ps command shows:
$ ps aux USER PID %CPU %MEM TTY STAT COMMAND root 1 0.0 0.1 ? S init  105 7783 0.0 0.2 ? Ss /usr/bin/dbus-daemon --system hal 7796 0.0 0.7 ? Ss /usr/sbin/hald postfix 7957 0.0 0.2 ? S qmgr -l -t fifo -u -c nagios 8371 0.0 0.2 ? SNs /usr/sbin/nagios /etc/nagios/nagios.cfg
This output is slimmed down, you'll see more lines and columns on your system. If something is sucking up all your CPU or memory you'll see what it is in the %CPU and %MEM columns. A quicker way to find a runaway process is with the
top command, because by default the processes using the most CPU are displayed on top. We can play with this a bit with the
$ yes carla is teh awesum
This repeats "carla is teh awesum" at high speed until you stop it. It should drive your CPU usage into the red zone:
$ top ... PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 12144 carla 25 0 31592 17m 13m R 93.4 3.5 0:50.26 konsole 22236 carla 15 0 2860 468 400 S 4.3 0.1 0:00.97 yes
Interestingly, credit for hammering the CPU goes to Konsole, not
yes is running inside Konsole. If you drop to a "real" console (Ctrl+alt+f2) you'll see
yes with the big numbers.
There are a number of ways to stop
yes. If you go back to the shell it's running in, just hit CTRL+c. Or you can stop it with the
kill command in a second shell, either by PID or by name:
$ kill 22236 $ killall yes
CTRL+c sends a SIGINT (2), or a terminate interrupt from the keyboard.
killall both send a SIGTERM (15) by default. SIGTERM (15) can be caught and either ignored, or interpreted in a different way, so when it works unpredictably, you can blame the process you're trying to kill.
Killing a parent process will usually, but not always, kill its children as well. How do you know what the child processes are? Use the
-f flag with
$ ps axf 22371 ? R 2:35 _ konsole [kdeinit] 22372 pts/3 Ss 0:00 | _ /bin/bash 24322 pts/3 S+ 0:00 | | _ yes carla is teh awesum 22381 pts/4 Rs 0:00 | _ /bin/bash 24323 pts/4 R+ 0:00 | | _ ps axf
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