FreeBSD Handbook
Prev | Chapter 4 UNIX Basics | Next

# 4.7 Processes

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.

To see the processes on the system, use ps(1) and 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`. To display all the running processes and update the display every few seconds so that you can interactively see what the computer is doing, use `top`.

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

    % ps
      PID  TT  STAT      TIME COMMAND
      298  p0  Ss     0:01.10 tcsh
     7078  p0  S      2:40.88 xemacs mdoc.xsl (xemacs-21.1.14)
    37393  p0  I      0:03.11 xemacs freebsd.dsl (xemacs-21.1.14)
    48630  p0  S      2:50.89 /usr/local/lib/netscape-linux/navigator-linux-4.77.bi
    72210  p0  R+     0:00.00 ps
      390  p1  Is     0:01.14 tcsh
     7059  p2  Is+    1:36.18 /usr/local/bin/mutt -y
     6688  p3  IWs    0:00.00 tcsh
    10735  p4  IWs    0:00.00 tcsh
    20256  p5  IWs    0:00.00 tcsh
      262  v0  IWs    0:00.00 -tcsh (tcsh)
      270  v0  IW+    0:00.00 /bin/sh /usr/X11R6/bin/startx -- -bpp 16
      280  v0  IW+    0:00.00 xinit /home/nik/.xinitrc -- -bpp 16
      284  v0  IW     0:00.00 /bin/sh /home/nik/.xinitrc
      285  v0  S      0:38.45 /usr/X11R6/bin/sawfish

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.

ps(1) supports a number of different options to change the information that is displayed. One of the most useful sets is `auxww`. `a` displays information about all the running processes of all users. `u` displays the username of the process' owner, as well as memory usage. `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. A sample session looks like this:

    % top
    last pid: 72257;  load averages:  0.13,  0.09,  0.03    up 0+13:38:33  22:39:10
    47 processes:  1 running, 46 sleeping
    CPU states: 12.6% user,  0.0% nice,  7.8% system,  0.0% interrupt, 79.7% idle
    Mem: 36M Active, 5256K Inact, 13M Wired, 6312K Cache, 15M Buf, 408K Free
    Swap: 256M Total, 38M Used, 217M Free, 15% Inuse

      PID USERNAME PRI NICE  SIZE    RES STATE    TIME   WCPU    CPU COMMAND
    72257 nik       28   0  1960K  1044K RUN      0:00 14.86%  1.42% top
     7078 nik        2   0 15280K 10960K select   2:54  0.88%  0.88% xemacs-21.1.14
      281 nik        2   0 18636K  7112K select   5:36  0.73%  0.73% XF86_SVGA
      296 nik        2   0  3240K  1644K select   0:12  0.05%  0.05% xterm
    48630 nik        2   0 29816K  9148K select   3:18  0.00%  0.00% navigator-linu
      175 root       2   0   924K   252K select   1:41  0.00%  0.00% syslogd
     7059 nik        2   0  7260K  4644K poll     1:38  0.00%  0.00% mutt
    ...

The output is split into two sections. The header (the first five 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 (47 in this case), how much memory and swap space has been used, and how much time the system is spending in different CPU states.

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 at the moment. In this example, Netscape® has required almost 30 MB of RAM, but is currently only using 9 MB.

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

Prev | Home | Next
Mounting and Unmounting File Systems | Up | Daemons, Signals, and Killing Processes