This appendix provides technical background on how the UNIX operating system manages processes. The information presented in this chapter is important to understand if you are concerned with the details of system administration or are simply interested in UNIX internals, but we felt that it was too technical to present early in this book.
UNIX is a multitasking operating system. Every task that the computer is performing at any moment - every user running a word processor program, for example - has a process . The process is the operating system's fundamental tool for controlling the computer.
Nearly everything that UNIX does is done with a process. One process displays the word login: on the user's terminal and reads the characters that the user types to log into the system. Another process controls the line printer. On a workstation, a special process called the "window server" displays text in windows on the screen. Another process called the "window manager" lets the user move those windows around.
At any given moment, the average UNIX operating system might be running anywhere from ten to several hundred different processes; large mainframes might be running several thousand. UNIX runs at least one process for every user who is logged in, another process for every program that every user is running, and another process for every hard-wired terminal that is waiting for a new user to log in. UNIX also uses a variety of special processes for system functions.
A process is an abstraction of control that has certain special properties associated with it. These include a private stack, values of registers, a program counter, an address space containing program code and data, and so on. The underlying hardware and operating system software manage the contents of registers in such a way that each process views the computer's resources as its "own" while it is running. With a single processor, only one process at a time is actually running, with the operating system swapping processes from time to time to give the illusion that they are all running concurrently. Multi-processor computers can naturally run several processes with true synchronicity.
Every UNIX process has a program that it is running, even if that program is part of the UNIX operating system (a special program). Programs are usually referred to by the names of the files in which they are kept. For example, the program that lists files is called /bin/ls and the program that runs the line printer may be called /usr/lib/lpd.
A process can run a program that is not stored in a file in either of two ways:
Normally, processes run a single program and then exit. However, a program can cause another program to be run. In this case, the same process starts running another program.
The ps command gives you a snapshot of all of the processes running at any given moment. ps tells you who is running programs on your system, as well as which programs the operating system is spending its time executing.
Most system administrators routinely use the ps command to see why their computers are running so slowly; system administrators should also regularly use the command to look for suspicious processes. (Suspicious processes are any processes that you don't expect to be running. Methods of identifying suspicious processes are described in detail in earlier chapters.)
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:
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)
% ps -auxww
to display detailed information about every process running on your computer. The options specified in this command are:
% 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.
% 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.
The kernel maintains a set of properties for every UNIX process. Most of these properties are denoted by numbers. Some of these numbers refer to processes, while others determine what privileges the processes have.
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. 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.
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 .
Although UNIX is a multitasking operating system, most computers that run UNIX can run only a single process at a time. 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.
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. 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.
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.
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.