Linux with Operating System Concepts

s
is the number of seconds and 
t
is the number of tenths of seconds. For instance, 
–d 1
would change the delay from 3 s to 1 while 
–d 0.5
would change the delay to every 
half second. You could provide an even shorter interval if desired (for instance, 
–d 0.001
) 
but that is probably unnecessary and also resource consuming. The value supplied must 
be positive or 0.
The top program’s output is illustrated in Figure 4.4. The first thing you might notice 
when running top is that it fills the entire terminal window no matter what size the termi-
nal window is. Along the top of the output are system statistics. They include the current 
number of users, the average CPU load over several intervals, the number of tasks (pro-
cesses, services, threads) and summaries of their statuses (e.g., running, sleeping, stopped), 
CPU statistics (percentage of time in user versus system modes, percentage of low priority 
processes, idle processes, processes waiting on I/O, and others), memory utilization, and 
swap space (virtual memory) utilization.
The lower portion of the output consists of statistics on the running processes. The 
default is to order the processes by CPU time. The information shown for each process is 
the process’ ID, the process’ priority and niceness values (nice is discussed below), CPU 
statistics, memory statistics and swap space statistics, and finally the process’ name. Other 
information, such as the parent process’ ID, the user who launched the process given as 
FIGURE 4.3 
Resource utilization as shown through system monitor.

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Managing Processes
◾
141
real name, UID, user name and group name, the terminal window responsible for launch-
ing the process, is available by issuing different options or commands.
The interaction with top, aside from refreshing itself based on an interval, is through 
single character input. The more useful controls for top are given in Table 4.1. In addition 
to these keystrokes, pressing the enter key will automatically update the image.
Many of the commands shown in Table 4.1 can also be specified from the command 
line as options when top is launched. These include –H to show threads, 
–
i to show idle 
processes and 
–u username
to show processes started by 
username
. The user can specify 
–n 
#
where 
#
is an integer to indicate the number of refreshes that top should perform 
before terminating. For instance, 
top –n 10
will only update itself nine times after the 
FIGURE 4.4 
Example of top program output.
TABLE 4.1 
Keystroke Commands for Top Instruction