4.8. Processes and Daemons

FreeBSD is a multi-tasking operating system. Each program running at any one time is called a process. Every running command starts at least one new process and there are a number of system processes that are run by FreeBSD.

Each process is uniquely identified by a number called a process ID (PID). Similar to files, each process has one owner and group, and the owner and group permissions are used to determine which files and devices the process can open. Most processes also have a parent process that started them. For example, the shell is a process, and any command started in the shell is a process which has the shell as its parent process. The exception is a special process called init(8) which is always the first process to start at boot time and which always has a PID of 1.

Some programs are not designed to be run with continuous user input and disconnect from the terminal at the first opportunity. For example, a web server responds to web requests, rather than user input. Mail servers are another example of this type of application. These types of programs are known as daemons. The term daemon comes from Greek mythology and represents an entity that is neither good nor evil, and which invisibly performs useful tasks. This is why the BSD mascot is the cheerful-looking daemon with sneakers and a pitchfork.

There is a convention to name programs that normally run as daemons with a trailing d. For example, BIND is the Berkeley Internet Name Domain, but the actual program that executes is named. The Apache web server program is httpd and the line printer spooling daemon is lpd. This is only a naming convention. For example, the main mail daemon for the Sendmail application is sendmail, and not maild.

4.8.1. Viewing Processes

To see the processes running on the system, use ps(1) or top(1). To display a static list of the currently running processes, their PIDs, how much memory they are using, and the command they were started with, use ps(1). To display all the running processes and update the display every few seconds in order to interactively see what the computer is doing, use top(1).

By default, ps(1) only shows the commands that are running and owned by the user. For example:

% ps
 PID TT  STAT    TIME COMMAND
8203  0  Ss   0:00.59 /bin/csh
8895  0  R+   0:00.00 ps

The output from ps(1) is organized into a number of columns. The PID column displays the process ID. PIDs are assigned starting at 1, go up to 99999, then wrap around back to the beginning. However, a PID is not reassigned if it is already in use. The TT column shows the tty the program is running on and STAT shows the program's state. TIME is the amount of time the program has been running on the CPU. This is usually not the elapsed time since the program was started, as most programs spend a lot of time waiting for things to happen before they need to spend time on the CPU. Finally, COMMAND is the command that was used to start the program.

A number of different options are available to change the information that is displayed. One of the most useful sets is auxww, where a displays information about all the running processes of all users, u displays the username and memory usage of the process' owner, x displays information about daemon processes, and ww causes ps(1) to display the full command line for each process, rather than truncating it once it gets too long to fit on the screen.

The output from top(1) is similar:

% top
last pid:  9609;  load averages:  0.56,  0.45,  0.36              up 0+00:20:03  10:21:46
107 processes: 2 running, 104 sleeping, 1 zombie
CPU:  6.2% user,  0.1% nice,  8.2% system,  0.4% interrupt, 85.1% idle
Mem: 541M Active, 450M Inact, 1333M Wired, 4064K Cache, 1498M Free
ARC: 992M Total, 377M MFU, 589M MRU, 250K Anon, 5280K Header, 21M Other
Swap: 2048M Total, 2048M Free

  PID USERNAME    THR PRI NICE   SIZE    RES STATE   C   TIME   WCPU COMMAND
  557 root          1 -21  r31   136M 42296K select  0   2:20  9.96% Xorg
 8198 dru           2  52    0   449M 82736K select  3   0:08  5.96% kdeinit4
 8311 dru          27  30    0  1150M   187M uwait   1   1:37  0.98% firefox
  431 root          1  20    0 14268K  1728K select  0   0:06  0.98% moused
 9551 dru           1  21    0 16600K  2660K CPU3    3   0:01  0.98% top
 2357 dru           4  37    0   718M   141M select  0   0:21  0.00% kdeinit4
 8705 dru           4  35    0   480M    98M select  2   0:20  0.00% kdeinit4
 8076 dru           6  20    0   552M   113M uwait   0   0:12  0.00% soffice.bin
 2623 root          1  30   10 12088K  1636K select  3   0:09  0.00% powerd
 2338 dru           1  20    0   440M 84532K select  1   0:06  0.00% kwin
 1427 dru           5  22    0   605M 86412K select  1   0:05  0.00% kdeinit4

The output is split into two sections. The header (the first five or six lines) shows the PID of the last process to run, the system load averages (which are a measure of how busy the system is), the system uptime (time since the last reboot) and the current time. The other figures in the header relate to how many processes are running, how much memory and swap space has been used, and how much time the system is spending in different CPU states. If the ZFS file system module has been loaded, an ARC line indicates how much data was read from the memory cache instead of from disk.

Below the header is a series of columns containing similar information to the output from ps(1), such as the PID, username, amount of CPU time, and the command that started the process. By default, top(1) also displays the amount of memory space taken by the process. This is split into two columns: one for total size and one for resident size. Total size is how much memory the application has needed and the resident size is how much it is actually using now.

top(1) automatically updates the display every two seconds. A different interval can be specified with -s.

4.8.2. Killing Processes

One way to communicate with any running process or daemon is to send a signal using kill(1). There are a number of different signals; some have a specific meaning while others are described in the application's documentation. A user can only send a signal to a process they own and sending a signal to someone else's process will result in a permission denied error. The exception is the root user, who can send signals to anyone's processes.

The operating system can also send a signal to a process. If an application is badly written and tries to access memory that it is not supposed to, FreeBSD will send the process the Segmentation Violation signal (SIGSEGV). If an application has been written to use the alarm(3) system call to be alerted after a period of time has elapsed, it will be sent the Alarm signal (SIGALRM).

Two signals can be used to stop a process: SIGTERM and SIGKILL. SIGTERM is the polite way to kill a process as the process can read the signal, close any log files it may have open, and attempt to finish what it is doing before shutting down. In some cases, a process may ignore SIGTERM if it is in the middle of some task that can not be interrupted.

SIGKILL can not be ignored by a process. Sending a SIGKILL to a process will usually stop that process there and then. [1].

Other commonly used signals are SIGHUP, SIGUSR1, and SIGUSR2. Since these are general purpose signals, different applications will respond differently.

For example, after changing a web server's configuration file, the web server needs to be told to re-read its configuration. Restarting httpd would result in a brief outage period on the web server. Instead, send the daemon the SIGHUP signal. Be aware that different daemons will have different behavior, so refer to the documentation for the daemon to determine if SIGHUP will achieve the desired results.

Procedure 4.1. Sending a Signal to a Process

This example shows how to send a signal to inetd(8). The inetd(8) configuration file is /etc/inetd.conf, and inetd(8) will re-read this configuration file when it is sent a SIGHUP.

  1. Find the PID of the process to send the signal to using pgrep(1). In this example, the PID for inetd(8) is 198:

    % pgrep -l inetd
    198  inetd -wW
  2. Use kill(1) to send the signal. Because inetd(8) is owned by root, use su(1) to become root first.

    % su
    Password:
    # /bin/kill -s HUP 198

    Like most UNIX® commands, kill(1) will not print any output if it is successful. If a signal is sent to a process not owned by that user, the message kill: PID: Operation not permitted will be displayed. Mistyping the PID will either send the signal to the wrong process, which could have negative results, or will send the signal to a PID that is not currently in use, resulting in the error kill: PID: No such process.

    Why Use /bin/kill?:

    Many shells provide kill as a built in command, meaning that the shell will send the signal directly, rather than running /bin/kill. Be aware that different shells have a different syntax for specifying the name of the signal to send. Rather than try to learn all of them, it can be simpler to specify /bin/kill.

When sending other signals, substitute TERM or KILL with the name of the signal.

Important:

Killing a random process on the system is a bad idea. In particular, init(8), PID 1, is special. Running /bin/kill -s KILL 1 is a quick, and unrecommended, way to shutdown the system. Always double check the arguments to kill(1) before pressing Return.



[1] There are a few tasks that can not be interrupted. For example, if the process is trying to read from a file that is on another computer on the network, and the other computer is unavailable, the process is said to be uninterruptible. Eventually the process will time out, typically after two minutes. As soon as this time out occurs the process will be killed.

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