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Unix System Overview
Unix System Overview
Introduction
All operating systems provide services for programs they run. Typical services include executing a new program, opening a file, reading a file, allocating a region of memory, getting the current time of day, and so on. The focus of this text is to describe the services provided by various versions of the UNIX operating system.
Describing the UNIX System in a strictly linear fashion, without any forward references to terms that haven’t been described yet, is nearly impossible (and would probably be boring). This chapter provides a whirlwind tour of the UNIX System from a programmer ’s perspective. We’ll give some brief descriptions and examples of terms and concepts that appear throughout the text. We describe these features in much more detail in later chapters. This chapter also provides an introduction to and overview of the services provided by the UNIX System for programmers new to this environment.
UNIX Architecture
In a strict sense, an operating system can be defined as the software that controls the hardware resources of the computer and provides an environment under which programs can run. Generally, we call this software the kernel, since it is relatively small and resides at the core of the environment. Figure 1.1 shows a diagram of the UNIX System architecture.
Figure 1.1 Architecture of the UNIX operating system
The interface to the kernel is a layer of software called the system calls (the shaded portion in Figure 1.1). Libraries of common functions are built on top of the system call interface, but applications are free to use both. (We talk more about system calls and library functions in Section 1.11.) The shell is a special application that provides an interface for running other applications.
In a broad sense, an operating system consists of the kernel and all the other software that makes a computer useful and gives the computer its personality. This other software includes system utilities, applications, shells, libraries of common functions, and so on.
For example, Linux is the kernel used by the GNU operating system. Some people refer to this combination as the GNU/Linux operating system, but it is more commonly referred to as simply Linux. Although this usage may not be correct in a strict sense, it is understandable, given the dual meaning of the phrase operating system. (It also has the advantage of being more succinct.)
Logging In
Login Name
When we log in to a UNIX system, we enter our login name, followed by our password. The system then looks up our login name in its password file, usually the file /etc/passwd. If we look at our entry in the password file, we see that it’s composed of seven colon-separated fields: the login name, encrypted password, numeric user ID (1000), numeric group ID (1000), a comment field, home directory (/home/fireway), and shell program (/bin/bash).
fireway:x:1000:1000:fireway,,,:/home/fireway:/bin/bash
All contemporary systems have moved the encrypted password to a different file. In Chapter 6, we’ll look at these files and some functions to access them.
Shells
Once we log in, some system information messages are typically displayed, and then we can type commands to the shell program. (Some systems start a window management program when you log in, but you generally end up with a shell running in one of the windows.) A shell is a command-line interpreter that reads user input and executes commands. The user input to a shell is normally from the terminal (an interactive shell) or sometimes from a file (called a shell script). The common shells in use are summarized in Figure 1.2.
| Name | Path | FreeBSD 8.0 | Linux 3.2.0 | Mac OS X 10.6.8 | Solaris 10 |
| Bourne shell | /bin/sh | ? | ? | copy of bash | ? |
| Bourne-again shell | /bin/bash | optional | ? | ? | ? |
| C shell | /bin/csh | link to tcsh | optional | link to tcsh | ? |
| Korn shell | /bin/ksh | optional | optional | ? | ? |
| TENEX C shell | /bin/tcsh | ? | optional | ? | ? |
Figure 1.2 Common shells used on UNIX systems
The system knows which shell to execute for us based on the final field in our entry in the password file.
The Bourne shell, developed by Steve Bourne at Bell Labs, has been in use since Version 7 and is provided with almost every UNIX system in existence. The control-flow constructs of the Bourne shell are reminiscent of Algol 68.
The C shell, developed by Bill Joy at Berkeley, is provided with all the BSD releases. Additionally, the C shell was provided by AT&T with System V/386 Release 3.2 and was also included in System V Release 4 (SVR4). (We’ll have more to say about these different versions of the UNIX System in the next chapter.) The C shell was built on the 6th Edition shell, not the Bourne shell. Its control flow looks more like the C language, and it supports additional features that weren’t provided by the Bourne shell: job control, a history mechanism, and command-line editing.
The Korn shell is considered a successor to the Bourne shell and was first provided with SVR4. The Korn shell, developed by David Korn at Bell Labs, runs on most UNIX systems, but before SVR4 was usually an extra-cost add-on, so it is not as widespread as the other two shells. It is upward compatible with the Bourne shell and includes those features that made the C shell popular: job control, command-line editing, and so on.
The Bourne-again shell is the GNU shell provided with all Linux systems. It was designed to be POSIX conformant, while still remaining compatible with the Bourne shell. It supports features from both the C shell and the Korn shell.
The TENEX C shell is an enhanced version of the C shell. It borrows several features, such as command completion, from the TENEX operating system (developed in 1972 at Bolt Beranek and Newman). The TENEX C shell adds many features to the C shell and is often used as a replacement for the C shell.
The shell was standardized in the POSIX 1003.2 standard. The specification was based on features from the Korn shell and Bourne shell.
The default shell used by different Linux distributions varies. Some distributions use the Bourne-again shell. Others use the BSD replacement for the Bourne shell, called dash (Debian Almquist shell, originally written by Kenneth Almquist and later ported to Linux). The default user shell in FreeBSD is derived from the Almquist shell. The default shell in Mac OS X is the Bourne-again shell. Solaris, having its heritage in both BSD and System V, provides all the shells shown in Figure 1.2. Free ports of the shells are available on the Internet.
Throughout the text, we will use parenthetical notes such as this to describe historical notes and to compare different implementations of the UNIX System. Often the reason for a particular implementation technique becomes clear when the historical reasons are described.
Throughout this text, we’ll show interactive shell examples to execute a program that we’ve developed. These examples use features common to the Bourne shell, the Korn shell, and the Bourne-again shell.
Files and Directories
Input and Output
Programs and Processes
Error Handling
User Identification
User ID
The user ID from our entry in the password file is a numeric value that identifies us to the system. This user ID is assigned by the system administrator when our login name is assigned, and we cannot change it. The user ID is normally assigned to be unique for every user. We’ll see how the kernel uses the user ID to check whether we have the appropriate permissions to perform certain operations.
We call the user whose user ID is 0 either root or the superuser. The entry in the password file normally has a login name of root, and we refer to the special privileges of this user as superuser privileges. As we’ll see in Chapter 4, if a process has superuser privileges, most file permission checks are bypassed. Some operating system functions are restricted to the superuser. The superuser has free rein over the system.
Client versions of Mac OS X ship with the superuser account disabled; server versions ship with the account already enabled. Instructions are available on Apple’s Web site describing how to enable it. See http://support.apple.com/kb/HT1528.
Group ID
Our entry in the password file also specifies our numeric group ID. This, too, is assigned by the system administrator when our login name is assigned. Typically, the password file contains multiple entries that specify the same group ID. Groups are normally used to collect users together into projects or departments. This allows the sharing of resources, such as files, among members of the same group. We’ll see in Section 4.5 that we can set the permissions on a file so that all members of a group can access the file, whereas others outside the group cannot.
There is also a group file that maps group names into numeric group IDs. The group file is usually /etc/group.
The use of numeric user IDs and numeric group IDs for permissions is historical. With every file on disk, the file system stores both the user ID and the group ID of a file’s owner. Storing both of these values requires only four bytes, assuming that each is stored as a two-byte integer. If the full ASCII login name and group name were used instead, additional disk space would be required. In addition, comparing strings during permission checks is more expensive than comparing integers.
Users, however, work better with names than with numbers, so the password file maintains the mapping between login names and user IDs, and the group file provides the mapping between group names and group IDs. The ls -l command, for example, prints the login name of the owner of a file, using the password file to map the numeric user ID into the corresponding login name.
Early UNIX systems used 16-bit integers to represent user and group IDs. Contemporary UNIX systems use 32-bit integers.
Example
The program in Figure 1.9 prints the user ID and the group ID.
/*** 文件名: intro/uidgid.c* 内容：prints the user ID and the group ID* 时间: 2016年 09月 16日 星期五 22:40:34 CST* 作者：firewaywei*/#include "apue.h"intmain(void){printf("uid = %d, gid = %d\n", getuid(), getgid());exit(0);}
Figure 1.9 Print user ID and group ID
We call the functions getuid and getgid to return the user ID and the group ID. Running the program yields
$ ./uidgid
uid = 1000, gid = 1000
$ su
密码：
# ./uidgid
uid = 0, gid = 0
Supplementary Group IDs
In addition to the group ID specified in the password file for a login name, most versions of the UNIX System allow a user to belong to other groups. This practice started with 4.2BSD, which allowed a user to belong to up to 16 additional groups. These supplementary group IDs are obtained at login time by reading the file /etc/group and finding the first 16 entries that list the user as a member. As we shall see in the next chapter, POSIX requires that a system support at least 8 supplementary groups per process, but most systems support at least 16.
Signals
Time Values
System Calls and Library Functions
Summary
参考
《Advanced Programming in the UNIX Envinronment, 2013》Chapter 1. UNIX System Over view
Unix System Overview
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