C.1.2.1 Listing processes with systems derived from System V

The System V ps command will normally only print the processes that are associated with the terminal on which the program is being run. To list all of the processes that are running on your computer, you must run the program with the -ef options. The options are:

For example:

sun.vineyard.net% /bin/ps -ef
     UID   PID  PPID  C    STIME TTY      TIME COMD
    root     0     0 64   Nov 16 ?        0:01 sched
    root     1     0 80   Nov 16 ?        9:56 /etc/init -
    root     2     0 80   Nov 16 ?        0:10 pageout
    root     3     0 80   Nov 16 ?       78:20 fsflush
    root   227     1 24   Nov 16 ?        0:00 /usr/lib/saf/sac -t 300
    root   269     1 18   Nov 16 console  0:00 /usr/lib/saf/ttymon -g -    root    97     1 80   Nov 16 ?        1:02 /usr/sbin/rpcbind
    root   208     1 80   Nov 16 ?        0:01 /usr/dt/bin/dtlogin
    root    99     1 21   Nov 16 ?        0:00 /usr/sbin/keyserv
    root   117     1 12   Nov 16 ?        0:00 /usr/lib/nfs/statd
    root   105     1 12   Nov 16 ?        0:00 /usr/sbin/kerbd
    root   119     1 27   Nov 16 ?        0:00 /usr/lib/nfs/lockd
    root   138     1 12   Nov 16 ?        0:00 /usr/lib/autofs/automoun    root   162     1 62   Nov 16 ?        0:01 /usr/lib/lpsched
    root   142     1 41   Nov 16 ?        0:00 /usr/sbin/syslogd
    root   152     1 80   Nov 16 ?        0:07 /usr/sbin/cron
    root   169   162  8   Nov 16 ?        0:00 lpNet
    root   172     1 80   Nov 16 ?        0:02 /usr/lib/sendmail -q1h
    root   199     1 80   Nov 16 ?        0:02 /usr/sbin/vold
    root   180     1 80   Nov 16 ?        0:04 /usr/lib/utmpd
    root   234   227 31   Nov 16 ?        0:00 /usr/lib/saf/listen tcp
 simsong 14670 14563 13 12:22:12 pts/11   0:00 rlogin next
    root   235   227 45   Nov 16 ?        0:00 /usr/lib/saf/ttymon
 simsong 14673 14535 34 12:23:06 pts/5    0:00 rlogin next
 simsong 14509     1 80 11:32:43 ?        0:05 /usr/dt/bin/dsdm
 simsong 14528 14520 80 11:32:51 ?        0:18 dtwm
 simsong 14535 14533 66 11:33:04 pts/5    0:01 /usr/local/bin/tcsh
 simsong 14529 14520 80 11:32:56 ?        0:03 dtfile -session dta003TF
    root 14467     1 11 11:32:23 ?        0:00 /usr/openwin/bin/fbconso simsong 14635 14533 80 11:48:18 pts/12   0:01 /usr/local/bin/tcsh
 simsong 14728 14727 65 15:29:20 pts/9    0:01 rlogin next
    root   332   114 80   Nov 16 ?        0:02 /usr/dt/bin/rpc.ttdbserv    root 14086   208 80   Dec 01 ?        8:26 /usr/openwin/bin/Xsun :0 simsong 13121 13098 80   Nov 29 pts/6    0:01 /usr/local/bin/tcsh
 simsong 15074 14635 20 10:48:34 pts/12   0:00 /bin/ps -ef

Table 27.2 describes the meaning of each field in this output.

Field in ps Output (System V)

C.1.2.2 Listing processes with Berkeley-derived versions of UNIX

With Berkeley UNIX, you can use the command:

% ps -auxww  

to display detailed information about every process running on your computer. The options specified in this command are:

For example:[1]

[1] Many Berkeley-derived versions also show a start time (START) between STAT and TIME.

% ps -auxww
USER       PID %CPU %MEM   SZ  RSS TT STAT   TIME COMMAND
simsong   1996 62.6  0.6 1136 1000 q8 R      0:02 ps auxww
root       111  0.0  0.0   32   16 ?  I      1:10 /etc/biod 4
daemon     115  0.0  0.1  164  148 ?  S      2:06 /etc/syslog
root       103  0.0  0.1  140  116 ?  I      0:44 /etc/portmap
root       116  0.0  0.5  860  832 ?  I     12:24 /etc/mountd -i -s
root       191  0.0  0.2  384  352 ?  I      0:30 /usr/etc/bin/lpd
root        73  0.0  0.3  528  484 ?  S <    7:31 /usr/etc/ntpd -n
root         4  0.0  0.0    0    0 ?  I      0:00 tpathd
root         3  0.0  0.0    0    0 ?  R      0:00  idleproc
root         2  0.0  0.0 4096    0 ?  D      0:00  pagedaemon
root       239  0.0  0.1  180  156 co I      0:00  std.9600 console 
root         0  0.0  0.0    0    0 ?  D      0:08  swapper
root       178  0.0  0.3  700  616 ?  I      6:31 /etc/snmpd
root       174  0.0  0.1  184  148 ?  S      5:06 /etc/inetd
root       168  0.0  0.0   56   44 ?  I      0:16 /etc/cron
root       132  0.0  0.2  452  352 co I      0:11 /usr/etc/lockd
jdavis     383  0.0  0.1  176   96 p0 I      0:03 rlogin hymie
ishii     1985  0.0  0.1  284  152 q1 S      0:00 /usr/ucb/mail bl
root     26795  0.0  0.1  128   92 ?  S      0:00 timed
root     25728  0.0  0.0  136   56 t3 I      0:00 telnetd
jdavis     359  0.0  0.1  540  212 p0 I      0:00 -tcsh (tcsh)
root       205  0.0  0.1  216  168 ?  I      0:04 /usr/local/cap/atis
kkarahal 16296  0.0  0.4 1144  640 ?  I      0:00 emacs 
root       358  0.0  0.0  120   44 p0 I      0:03 rlogind
root     26568  0.0  0.0    0    0 ?  Z      0:00 <exiting>
root     10862  0.0  0.1  376  112 ?  I      0:00 rshd

The fields in this output are described in Table 27.3. Individual STAT characters are described in Tables C-3, C-4, and C-5.

[2] If this happens, follow up to be sure you don't have an intruder.

NOTE: Because command arguments are stored in the process's own memory space, a process can change what appears on its command line. If you suspect that a process may not be what it claims to be, type:

	% ps -c

This causes ps to print the name of the command stored in the kernel. This approach is substantially faster than the standard ps, and is more suitable for use with scripts that run periodically. Unfortunately, the ps -c display does not include the arguments of each command that is running.

C.1.3.1 Process identification numbers (PID)

Every process is assigned a unique number called the process identifier, or PID. The first process to run, called init, is given the number 1. Process numbers can range from 1 to 65535.[3] When the kernel runs out of process numbers, it recycles them. The kernel guarantees that no two active processes will ever have the same number.

[3] Some versions of UNIX may allow process numbers in a range different from 1 to 65535.

C.1.3.2 Process real and effective UID

Every UNIX process has two user identifiers: a real UID and an effective UID.

The real UID (RUID) is the actual user identifier (UID) of the person who is running the program. It is usually the same as the UID of the actual person who is logged into the computer, sitting in front of the terminal (or workstation).

The effective UID (EUID) identifies the actual privileges of the process that is running.

Normally, the real UID and the effective UID are the same. That is, normally you have only the privileges associated with your own UID. Sometimes, however, the real and effective UID can be different. This occurs when a user runs a special kind of program, called a SUID program, which is used to accomplish a specific function (such as changing the user's password). SUID programs are described in Chapter 4, Users, Groups, and the Superuser.

C.1.3.3 Process priority and niceness

Although UNIX is a multitasking operating system, most computers that run UNIX can run only a single process at a time.[4] Every fraction of a second, the UNIX operating system rapidly switches between many different processes, so that each one gets a little bit of work done within a given amount of time. A tiny but important part of the UNIX kernel called the process scheduler decides which process is allowed to run at any given moment and how much CPU time that process should get.

[4] Multiprocessor computers can run as many processes at a time as they have processors.

To calculate which process it should run next, the scheduler computes the priority of every process. The process with the lowest priority number (or the highest priority) runs. A process's priority is determined with a complex formula that includes what the process is doing and how much CPU time the process has already consumed. A special number, called the nice number or simply the nice, biases this calculation: the lower a process's nice number, the higher its priority, and the more likely that it will be run.

On most versions of UNIX, nice numbers are limited from -20 to +20. Most processes have a nice of 0. A process with a nice number of +19 will probably not run until the system is almost completely idle; likewise, a process with a nice number of -19 will probably preempt every other user process on the system.

Sometimes you will want to make a process run slower. In some cases, processes take more than their "fair share" of the CPU, but you don't want to kill them outright. An example is a program that a researcher has left running overnight to perform mathematical calculations that isn't finished the next morning. In this case, rather than killing the process and forcing the researcher to restart it later from the beginning, you could simply cut the amount of CPU time that the process is getting and let it finish slowly during the day. The program /etc/renice lets you change a process's niceness.

For example, suppose that Mike left a program running before he went home. Now it's late at night, and Mike's program is taking up most of the computer's CPU time:

% ps aux | head -5 
USER     PID %CPU  %MEM  VSIZE  RSIZE   TT STAT TIME   COMMAND  
mike     211 70.0   6.7  2.26M  1.08M   01  R   4:01   cruncher 
mike     129  8.2  15.1  7.06M  2.41M   01  S   0:48   csh  
donna    212  7.0   7.3  2.56M  1.16M   p1  S   1:38   csh 
michelle 290  4.0  11.9  14.4M  1.91M   03  R   19:00  rogue % 

You could slow down Mike's program by renicing it to a higher nice number.

For security reasons, normal users are only allowed to increase the nice numbers of their own processes. Only the superuser can lower the nice number of a process or raise the nice number of somebody else's process. (Fortunately, in this example, we know the superuser password!)

% /bin/su 
password: another39
# /etc/renice +4 211 
211: old priority 0, new priority 4  
# ps u211 
USER PID %CPU  %MEM  VSIZE   RSIZE   TT  STAT  TIME  COMMAND  
mike  211  1.5  6.7  2.26M   1.08M   01  R  N  4:02  cruncher

The N in the STAT field indicates that the cruncher process is now running at a lower priority (it is "niced"). Notice that the process's CPU consumption has already decreased. Any new processes that are spawned by the process with PID 211 will inherit this new nice value, too.

You can also use /etc/renice to lower the nice number of a process to make it finish faster.[5] Although setting a process to a lower priority won't speed up the CPU or make your computer's hard disk transfer data faster, the negative nice number will cause UNIX to run a particular process more than it runs others on the system. Of course, if you ran every process with the same negative priority, there wouldn't be any apparent benefit.

[5] Only root can renice a process to make it faster. Normal processes can't even change themselves back to what they were (if they've been niced down). Normal users can't even raise the priority of their processes to the value at which they were started.

Some versions of the renice command allow you to change the nice of all processes belonging to a user or all processes in a process group (described in the next section). For instance, to speed up all of Mike's processes, you might type:

# renice -2 -u mike

Remember, processes with a lower nice number run faster.

Note that because of the UNIX scheduling system, renicing several processes to lower numbers is likely to increase paging activity if there is limited physical memory, and therefore adversely impact overall system performance.

What do process priority and niceness have to do with security? If an intruder has broken into your system and you have contacted the authorities and are tracing the phone call, slowing the intruder down with a priority of +10 or +15 will limit the damage that the intruder can do without hanging up the phone (and losing your chance to catch the intruder). Of course, any time that an intruder is on a system, exercise extreme caution.

Also, running your own shell with a higher priority may give you an advantage if the system is heavily loaded. The easiest way to do so is by typing:

# renice -5 $$

The shell will replace the $$ with the PID of the shell's process.

C.1.3.4 Process groups and sessions

With Berkeley-derived versions of UNIX, including SVR4, each process is assigned a process ID (PID), a process group ID, and a session ID. Process groups and sessions are used to implement job control.

For each process, the PID is a unique number, the process group ID is the PID of the process group leader process, and the session ID is the PID of the session leader process. When a process is created, it inherits the process group ID and the session ID of its parent process. Any process may create a new process group by calling setpgrp() and may create a new session by calling the UNIX system call setsid(). All processes that have the same process group ID are said to be in the same process group.

Each UNIX process group belongs to a session group. This is used to help manage signals and orphaned processes. Once a user has logged in, the user may start multiple sets of processes, or jobs, using the shell's job-control mechanism. A job may have a single process, such as a single invocation of the ls command. Alternatively, a job may have several processes, such as a complex shell pipeline. For each of these jobs, there is a process group. UNIX also keeps track of the particular process group which is controlling the terminal. This can be set or changed with ioctl() system calls. Only the controlling process group can read or write to the terminal.

A process could become an orphan if its parent process exits but it continues to run. Historically, these processes would be inherited by the init process but would remain in their original process group. If a signal were sent by the controlling terminal (process group), then it would go to the orphaned process, even though it no longer had any real connection to the terminal or the rest of the process group.

To counter this, POSIX defines an orphaned process group. This is a process group where the parent of every member is either not a member of the process group's session, or is itself a member of the same process group. Orphaned process groups are not sent terminal signals when they are generated. Because of the way in which new sessions are created, the initial process in the first process group is always an orphan (its ancestor is not in the session). Command interpreters are usually spawned as session leaders so they ignore TSTP signals from the terminal.