Slackware systems often run hundreds or thousands of programs, each of which is referred to as a process. Managing these processes is an important part of system administration. So how exactly do we handle all of these seperate processes?

The first step in managing processes is figuring out what processes are currently running. The most popular and powerful tool for this is ps(1). Without any arguments, ps won't tell you much information. By default, it only tells you what processes are running in your currently active shell. If we want more information, we'll need to look deeper. darkstar:~$ ps PID TTY TIME CMD 12220 pts/4 00:00:00 bash 12236 pts/4 00:00:00 ps Here you can see what processes you are running in your currently active shell or terminal and only some information is included. The PID is the "Process ID"; every process is assigned a unique number. The TTY tells you what terminal device the process is attached to. Naturally, CMD is the command that was run. You might be a little confused by TIME though, since it seems to move so slowly. This isn't the amount of real time the process has been running, but rather the amount of CPU time the process has consumed. An idle process uses virtually no CPU time, so this value may not increase quickly. Viewing only our own processes isn't very much fun, so let's take a look at all the processes on the system with the -e argument. darkstar:~$ ps -e PID TTY TIME CMD 1 ? 00:00:00 init 2 ? 00:00:00 kthreadd 3 ? 00:00:00 migration/0 4 ? 00:00:00 ksoftirqd/0 7 ? 00:00:11 events/0 9 ? 00:00:01 work_on_cpu/0 11 ? 00:00:00 khelper 102 ? 00:00:02 kblockd/0 105 ? 00:01:19 kacpid 106 ? 00:00:01 kacpi_notify ... many more lines omitted ... The above example uses the standard ps syntax, but much more information can be discovered if we use BSD syntax. In order to do so, we must use the aux argument. This is distinct from the -aux argument, but in most cases the two arguments are equivilant. This is a decades-old relic. For more information, see the man page for ps. darkstar:~$ ps aux USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 1 0.0 0.0 3928 632 ? Ss Apr05 0:00 init [3] root 2 0.0 0.0 0 0 ? S Apr05 0:00 [kthreadd] root 3 0.0 0.0 0 0 ? S Apr05 0:00 [migration/0] root 4 0.0 0.0 0 0 ? S Apr05 0:00 [ksoftirqd/0] root 7 0.0 0.0 0 0 ? S Apr05 0:11 [events/0] root 9 0.0 0.0 0 0 ? S Apr05 0:01 [work_on_cpu/0] root 11 0.0 0.0 0 0 ? S Apr05 0:00 [khelper] ... many more lines omitted .... As you can see, BSD syntax offers much more information, including what user controls the process and what percentage of RAM and CPU the process is consuming when ps is run. To accomplish bits of this, on a per process basis, ps allows one or more process IDs (PIDs) to be provided in the command line, and has the '-o' flag to show a particular attribute of the PID. darkstar:~$ ps -o cmd -o etime $$ CMD ELAPSED /bin/bash 12:22 What this is displaying, is the PID's command name (cmd), and its elapsed time (etime). The PID in this example, is a shell variable for the PID of the current shell. So you can see, in this example, the shell process has existed for 12 minutes, 22 seconds. Using the pgrep(1) command, this can get more automatable. darkstar:~$ ps -o cmd -o rss -o vsz $(pgrep httpd) CMD RSS VSZ /usr/sbin/httpd -k restart 33456 84816 /usr/sbin/httpd -k restart 33460 84716 /usr/sbin/httpd -k restart 33588 84472 /usr/sbin/httpd -k restart 30424 81608 /usr/sbin/httpd -k restart 33104 84900 /usr/sbin/httpd -k restart 33268 85112 /usr/sbin/httpd -k restart 30640 82724 /usr/sbin/httpd -k restart 15168 67396 /usr/sbin/httpd -k restart 33180 84416 /usr/sbin/httpd -k restart 33396 84592 /usr/sbin/httpd -k restart 32804 84232 In this example, a subshell execution, using pgrep, is returning the PIDs of any process, whose command name includes 'httpd'. Then ps displaying the command name, resident memory size, and virtual memory size. Finally, ps can also create a process tree. This shows you which processes have children processes. Ending the parent of a child process also ends the child. We do this with the -ejH argument. darkstar:~$ ps -ejH ... many lines omitted ... 3660 3660 3660 tty1 00:00:00 bash 29947 29947 3660 tty1 00:00:00 startx 29963 29947 3660 tty1 00:00:00 xinit 29964 29964 29964 tty7 00:27:11 X 29972 29972 3660 tty1 00:00:00 sh 29977 29972 3660 tty1 00:00:05 xscreensaver 29988 29972 3660 tty1 00:00:04 xfce4-session 29997 29972 3660 tty1 00:00:16 xfwm4 29999 29972 3660 tty1 00:00:02 Thunar ... many more lines omitted ... As you can see, ps(1) is an incredibly powerful tool for determining not only what processes are currently active on your system, but also for learning lots of important information about them. As is the case with many of the applications, there is often several tools for the job. Similar to the ps -ejH output, but more terse, is pstree(1). It displays the process tree, a bit more visually. darkstar:~$ pstree init-+-atd |-crond |-dbus-daemon |-httpd---10*[httpd] |-inetd |-klogd |-mysqld_safe---mysqld---8*[{mysqld}] |-screen-+-4*[bash] | |-bash---pstree | |-2*[bash---ssh] | `-bash---irssi |-2*[sendmail] |-ssh-agent |-sshd---sshd---sshd---bash---screen `-syslogd

Managing processes isn't only about knowing which ones are running, but also about communicating with them to change their behavior. The most common way of managing a program is to terminate it. Thus, the tool for the job is named kill(1). Despite the name, kill doesn't actually terminate processes, but sends signals to them. The most common signal is a SIGTERM, which tells the process to finish up what it is doing and terminate. There are a variety of other signals that can be sent, but the three most common are SIGTERM, SIGHUP, and SIGKILL. What a process does when it receives a signal varies. Most programs will terminate (or attempt to terminate) whenever they receive any signal, but there are a few important differences. For starters, the SIGTERM signal informs the process that it should terminate itself at its earliest convenience. This gives the process time to finish up any important activities, such as writing information to the disk, before it closes. In contrast, the SIGKILL signal tells the process to terminate itself immediately, no questions asked. This is most useful for killing processes that are not responding and is sometimes called the "silver bullet". Some processes (particularly daemons) capture the SIGHUP signal and reload their configuration files whenever they receive it. In order to signal a process, we first need to know it's PID. You can get this easily with ps as we discused. In order to send different signals to a running process, you simply pass the signal number and -s as an argument. The -l argument lists all the signals you can choose and their number. You can also send signals by their name with -s. darkstar:~$ kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT ... many more lines omitted ... darkstar:~$ kill 1234 # SIGTERM darkstar:~$ kill -s 9 1234 # SIGKILL darkstar:~$ kill -s 1 1234 # SIGHUP darkstar:~$ kill -s HUP 1234 # SIGHUP Sometimes you may wish to terminate all running processes with a certain name. You can kill processes by name with killall(1). Just pass the same arguments to killall that you would pass to kill. darkstar:~$ killall bash # SIGTERM darkstar:~$ killall -s 9 bash # SIGKILL darkstar:~$ killall -s 1 bash # SIGHUP darkstar:~$ killall -s HUP bash # SIGHUP

So far we've learned how to look at the active processes for a moment in time, but what if we want to monitor them for an extended period? top(1) allows us to do just that. It displays an ordered list of the processes on your system, along with vital information about them, and updates periodically. By default, processes are ordered by their CPU percentage and updates occur every three seconds. darkstar:~$ top top - 16:44:15 up 26 days, 5:53, 5 users, load average: 0.08, 0.03, 0.03 Tasks: 122 total, 1 running, 119 sleeping, 0 stopped, 2 zombie Cpu(s): 3.4%us, 0.7%sy, 0.0%ni, 95.5%id, 0.1%wa, 0.0%hi, 0.2%si, 0.0%st Mem: 3058360k total, 2853780k used, 204580k free, 154956k buffers Swap: 0k total, 0k used, 0k free, 2082652k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1 root 20 0 3928 632 544 S 0 0.0 0:00.99 init 2 root 15 -5 0 0 0 S 0 0.0 0:00.00 kthreadd 3 root RT -5 0 0 0 S 0 0.0 0:00.82 migration/0 4 root 15 -5 0 0 0 S 0 0.0 0:00.01 ksoftirqd/0 7 root 15 -5 0 0 0 S 0 0.0 0:11.22 events/0 9 root 15 -5 0 0 0 S 0 0.0 0:01.19 work_on_cpu/0 11 root 15 -5 0 0 0 S 0 0.0 0:00.01 khelper 102 root 15 -5 0 0 0 S 0 0.0 0:02.04 kblockd/0 105 root 15 -5 0 0 0 S 0 0.0 1:20.08 kacpid 106 root 15 -5 0 0 0 S 0 0.0 0:01.92 kacpi_notify 175 root 15 -5 0 0 0 S 0 0.0 0:00.00 ata/0 177 root 15 -5 0 0 0 S 0 0.0 0:00.00 ata_aux 178 root 15 -5 0 0 0 S 0 0.0 0:00.00 ksuspend_usbd 184 root 15 -5 0 0 0 S 0 0.0 0:00.02 khubd 187 root 15 -5 0 0 0 S 0 0.0 0:00.00 kseriod 242 root 20 0 0 0 0 S 0 0.0 0:03.37 pdflush 243 root 15 -5 0 0 0 S 0 0.0 0:02.65 kswapd0 The man page has helpful details on how to interact with top such as changing its delay interval, the order processes are displayed, and even how to terminate processes right from within top itself.

Ok, so we've learned many different ways of viewing the active processes on our system and means of signalling them, but what if we want to run a process periodically? Fortunately, Slackware includes just the thing, crond(8). cron runs processes for every user on the schedule that user demands. This makes it very useful for processes that need to be run periodically, but don't require full daemonization, such as backup scripts. Every user gets their own entry in the cron database, so non-root users can periodically run processes too. In order to run programs from cron, you'll need to use the crontab(1). The man page lists a variety of ways to do this, but the most common method is to pass the -e argument. This will lock the user's entry in the cron database (to prevent it from being overwritten by another program), then open that entry with whatever text editor is specified by the VISUAL environment variable. On Slackware systems, this is typically the vi editor. You may need to refer to the chapter on vi before continuing. The cron database entries may seem a little archaic at first, but they are highly flexible. Each uncommented line is processed by crond and the command specified is run if all the time conditions match. darkstar:~$ crontab -e # Keep current with slackware 30 02 * * * /usr/local/bin/rsync-slackware64.sh 1>/dev/null 2>&1 As mentioned before, the syntax for cron entries is a little difficult to understand at first, so let's look at each part individually. From left to right, the different sections are: Minute, Hour, Day, Month, Week Day, and Command. Any asterisk * entry matches every minute, hour, day, and so on. So from the example above, the command is "/usr/local/bin/rsync-slackware64.sh 1>/dev/null 2>&1", and it runs every weekday or every week of every month at 2:30 a.m. crond will also e-mail the local user with any output the command generates. For this reason, many tasks have their output redirected to /dev/null, a special device file that immediately discards everything it receives. In order to make it easier for you to remember these rules, you might wish to paste the following commented text at the top of your own cron entries. # Redirect everything to /dev/null with: # 1>/dev/null 2>&1 # # MIN HOUR DAY MONTH WEEKDAY COMMAND By default, Slackware includes a number of entries and comments in root's crontab. These entries make it easier to setup periodic system tasks by creating a number of directories in /etc corresponding to how often the tasks should run. Any script placed within these directories will be run hourly, daily, weekly, or monthly. The names should be self-explanatory: /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly.