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建包含命令的文件，而且此文件可成为新的命令。这些新的命令与在/bin目录下的系统命 <BR>令有相同的地位，它允许用户或组定制自己的环境（工作平台）。
<BR>
<BR> A shell allows execution of Unix commands, both synchronously（同步的） <BR>and
<BR>asynchronously（异步的）. The shell waits for synchronous commands to <BR>complete
<BR>before accepting more input; asynchronous commands continue to execute
<BR>in parallel with the shell while it reads and executes additional
<BR>commands. The "redirection" constructs（结构） permit fine-grained（细致的） <BR>control of
<BR>the input and output of those commands, and the shell allows control
<BR>over the contents of their environment. Unix shells also provide a
<BR>small set of built-in commands ("builtins") implementing functionality（功能） <BR>
<BR>impossible (e.g., `cd', `break', `continue', and `exec'), or
<BR>inconvenient (`history', `getopts', `kill', or `pwd', for example) to
<BR>obtain via separate utilities. Shells may be used interactively or
<BR>non-interactively: they accept input typed from the keyboard or from a
<BR>file. All of the shell builtins are described in subsequent sections.
<BR>
<BR>shell允许Unix命令同步或异步执行。对同步命令，shell在接受新的输入之前等待命令完 <BR>成。而对异步命令，则在读入输入的shell中与其它命令平行执行。重定向结构允许对这 <BR>些命令的输入和输出进行更细致的控制，而且shell允许控制环境的内容。Unix shell还 <BR>提供了一个小的内建命令的集合，用于实现不能（如：cd，break，continue，exec）或 <BR>不适合（history，getopts，kill，pwd等）通过单独的应用来获得的功能。Shell可以以 <BR>交互的形式或非交互的形式来使用：它们从键盘输入或者文件中接收输入数据。所有的 <BR>shell内建命令会在后序章节描述。
<BR>
<BR> While executing commands is essential（基本的）, most of the power (and
<BR>complexity) of shells is due to their embedded programming languages.
<BR>Like any high-level language, the shell provides variables, flow
<BR>control constructs, quoting（引用，加引号）, and functions.
<BR>
<BR>尽管执行命令是最基本的，但是shell更多的强大功能（以及复杂性）是来自其内嵌的编 <BR>程语言。象其它任何高级语言一样，shell提供了变量，流程控制结构，引用，还有函数 <BR>。
<BR>
<BR> Shells have begun offering features geared（使...适合） specifically for
<BR>interactive use rather than to augment（增强） the programming language. <BR>These
<BR>interactive features include job control, command line editing, history
<BR>and aliases. Each of these features is described in this manual.
<BR>
<BR>shell已经开始提供使之特别适合交互使用的特性而不是增强其编程语言。这些交互特性 <BR>包括作业控制，命令行编辑，历史和别名。这些特性在本指南中都将进行描述。
<BR>
<BR>File: bashref.info, Node: Definitions, Next: Basic Shell Features, Prev: <BR>Introduction, Up: Top
<BR>
<BR>Definitions（定义）
<BR>***********
<BR>
<BR> These definitions are used throughout the remainder of this manual.
<BR>
<BR>下面的这些定义将在本指南的其余部分使用到。
<BR>
<BR>`POSIX'
<BR> A family of open system standards based on Unix. Bash is
<BR> concerned with POSIX 1003.2, the Shell and Tools Standard.
<BR> 基于开放系统标准的规范集。Bash与POSIX 1003.2，Shell和Tools标准有关。
<BR>
<BR>`blank'（间隔符）
<BR> A space or tab character.（空格或制表符）
<BR>
<BR>`builtin'（内建命令）
<BR> A command that is implemented internally by the shell itself,
<BR> rather than by an executable program somewhere in the file system.
<BR> 由shell自身在内部实现的，而不是有文件系统中的某个可执行程序实现的命令。
<BR>
<BR>`control operator'（控制符）
<BR> A `word' that performs a control function. It is a `newline' or
<BR> one of the following: `||', `&&', `&', `;', `;;', `|', `(', or `)'.
<BR> 具有控制功能的一个word，可以是一换行符或下面的字符中的某一个：||，&&，&， <BR>;，;;，(，)。
<BR>
<BR>`exit status'（退出码）
<BR> The value returned by a command to its caller.
<BR> 命令返回给其调用者的值。
<BR>
<BR>`field'（字段）
<BR> A unit of text that is the result of one of the shell expansions.
<BR> After expansion, when executing a command, the resulting fields
<BR> are used as the command name and arguments.
<BR> 是某个shell展开结果的一段文字。展开后，在执行命令时，结果字段被用于作为命 <BR>令名和命令变元。
<BR>
<BR>`filename'（文件名）
<BR> A string of characters used to identify a file.（用于标识一个文件的字符串 <BR>）
<BR>
<BR>`job'（作业）
<BR> A set of processes comprising a pipeline, and any processes
<BR> descended（从...传下来） from it, that are all in the same process <BR>group.
<BR> 组成管道的进程集合，或任何从其中分离出的在同一进程组的任何进程。
<BR>
<BR>`job control'（作业控制）
<BR> A mechanism by which users can selectively stop (suspend) and
<BR> restart (resume) execution of processes.
<BR> 一种机制，可允许用户选择停止（暂停）进程和重新启动（重新执行）进程继续执 <BR>行。
<BR>
<BR>`metacharacter'（元字符）
<BR> A character that, when unquoted, separates words. A metacharacter
<BR> is a `blank' or one of the following characters: `|', `&', `;',
<BR> `(', `)', `<', or `>'.
<BR> 在未被引号括起时作为分离单词的字符。元字符可以是一个间隔符或下面的某一个 <BR>字符：|，&，;，(，)，<，>。
<BR>
<BR>`name' （名）
<BR> A `word' consisting solely of letters, numbers, and underscores,
<BR> and beginning with a letter or underscore. `Name's are used as
<BR> shell variable and function names. Also referred to as an
<BR> `identifier'.
<BR> 仅由字母，数字，以及下划线组成的word，且以字母或下划线开头。name被用作 <BR>shell变量和函数名称。也叫标识符（identifier）。
<BR>
<BR>`operator'（操作符）
<BR> A `control operator' or a `redirection operator'. *Note
<BR> Redirections::, for a list of redirection operators.
<BR> 控制符或重定向符。
<BR>
<BR>`process group'（进程组）
<BR> A collection of related processes each having the same process group <BR>ID.
<BR> 具有相同进程组ID的相关进程集合。
<BR>
<BR>`process group ID'（进程组ID）
<BR> A unique identifer that represents a `process group' during its <BR>lifetime.
<BR> 用于在进程组生命期间表示进程组的唯一的标识符。
<BR>
<BR>`reserved word'（保留字）
<BR> A `word' that has a special meaning to the shell. Most reserved
<BR> words introduce shell flow control constructs, such as `for' and
<BR> `while'.
<BR> 对shell具有特殊含义的word。多数保留字引入shell流程控制结构，如for和while <BR>。
<BR>
<BR>`return status'（返回码）
<BR> A synonym（同义字） for `exit status'.（退出码的同义字）
<BR>
<BR>`signal'（信号）
<BR> A mechanism by which a process may be notified by the kernal of an
<BR> event occurring in the system.
<BR> 一种核心通知进程在系统中有事件发生的机制。
<BR>
<BR>`special builtin'（特殊内建命令）
<BR> A shell builtin command that has been classified as special by the
<BR> POSIX.2 standard.
<BR> 根据POSIX.2标准分类的特殊的shell内建命令。
<BR>
<BR>`token'（标记）
<BR> A sequence of characters considered a single unit by the shell.
<BR> It is either a `word' or an `operator'.
<BR> 被shell作为一个独立实体的字符序列。可以是word或操作符。
<BR>
<BR>`word'
<BR> A `token' that is not an `operator'.（非操作符的标记。）
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