UNIX Tutorial 12. Viewing processes  

Commands covered in this section:   ps, grep, top

The basic unit of execution in UNIX (and many other operating systems) is called a process. In the "Redirecting input/output" section of the tutorial, you were asked to think of a process as a program you run from the shell. This is not far from the truth in most cases. What you think of as a program often consists of a single process. Your login shell and all the commands you've used in this tutorial run as single processes. In some cases, however, what you think of as a single program (e.g., a web or database server) actually consists of multiple processes communicating with each other. In this section, you will learn how to list the processes currently running on the system using the ps and top commands. In the next section, you'll learn how to manipulate processes.

Exercise 12.1

Use the ps command to see the processes associated with the current shell. 
          % ps
            PID TTY      TIME CMD
          22441 pts/7    0:00 tcsh
Until you've learned how to run processes in the background (covered in the next section), the output of ps is not terribly interesting, since there are no other processes besides the shell itself to display. However, by adding some options to ps, we can make things a bit more interesting. The "-e" option tells ps to list all processes running on the system, not just those associated with the current shell. The "-f" option creates a full listing, which includes more information about each process.

Exercise 12.2

Use ps -ef to get a full listing of all processes on the system. Since there are generally many processes to be listed, you'll want to pipe the output into a pager, such as more. 
          % ps -ef | more
               UID   PID  PPID  C    STIME TTY  TIME CMD
              root     0     0  0   Sep 18 ?    0:17 sched
              root     1     0  0   Sep 18 ?    0:54 /etc/init -
              root     2     0  0   Sep 18 ?    0:00 pageout
              root     3     0  0   Sep 18 ?    6:15 fsflush
              root   418     1  0   Sep 18 ?    0:00 /usr/lib/saf/sac -t 300
            daemon   156     1  0   Sep 18 ?    0:00 /usr/lib/nfs/statd
               .      .      .  .      .   .     .      .
               .      .      .  .      .   .     .      .
               .      .      .  .      .   .     .      .
The ps command under most versions of BSD requires different arguments to produce a full listing. Instead of "ps -ef", you would enter "ps aux". Linux and many versions of UNIX support both the standard UNIX arguments (with a "-") and the BSD-style arguments (without a "-"). The output of ps varies somewhat between UNIX versions, but you will generally see something similar to the output shown above. Table 12.1 shows the meaning of each field displayed in the full listing for ps in the last exercise.

Table 12.1

  field     description  
  UID     the user who owns the process  
  PID     the process id, a unique identifier assigned to each process  
  PPID     the parent process id, the process that spawned the current process  
  C     this field is obsolete  
  STIME     the start time for the current process  
  TTY     the controlling terminal for the current process  
  TIME     the amount of CPU time accumulated by the current process  
  CMD     the command used to invoke the process  
In some cases, you may want to restrict the listing to a few processes -- perhaps all processes belonging to a given user, or all occurrences of a certain program. You can use the grep command to do the filtering. It prints lines matching a specified pattern in its input while discarding lines that don't match.

Exercise 12.3

Create a pipeline using ps -ef to get a full listing of all processes and grep look for all occurrences of the pine e-mail program. 
          % ps -ef | grep pine
            boris 12089  7171  0 22:27:36 pts/2    0:00 grep pine
          sherman  5183  5178  0 20:14:09 pts/3    0:03 pine
          peabody 25996 25985  0 08:18:04 pts/14   0:01 pine
          natasha 27244 17083  0 17:43:27 pts/15   0:02 pine
            rocky  8009  8879  0 21:09:36 pts/6    0:00 pine
The top program also lists processes, but adds the ability to sort by various criteria (CPU utilization by default) and provides a continuously updated display. It also gives a useful summary of the status of the system, including memory and CPU utilization.

Exercise 12.4

Use top to display the processes using the most CPU time. To exit top when you're done, press 'q'. 
          % top
          load averages:  0.01,  0.03,  0.05   22:46:49
          81 processes:  80 sleeping, 1 on cpu
          CPU states: 99.4% idle,  0.4% user,  0.2% kernel,  0.0% iowait,  0.0% swap
          Memory: 256M real, 27M free, 61M swap in use, 451M swap free

          PID USERNAME THR PRI NICE  SIZE   RES STATE   TIME    CPU COMMAND
        13066 boris      1  33    0 1472K 1384K cpu     0:00  0.16% top
          164 root       6   7    0 7952K 4088K sleep  25:32  0.15% automountd
        13072 www        4  24    0 5928K 3288K sleep   0:00  0.09% httpd
         8084 www        4  33    0 7840K 6216K sleep   3:22  0.05% httpd
         7837 www        4  33    0 7840K 6264K sleep   2:54  0.05% httpd
           .   .         .   .    .   .     .     .       .     .     .
           .   .         .   .    .   .     .     .       .     .     .
           .   .         .   .    .   .     .     .       .     .     .
Note that top also displays additional fields not shown by ps, including the scheduling priority and "nice" level. We'll talk about these in the next section.

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