Shell教程（三）：数组/Arrays、基本运算符

定义数组值：

一个数组变量和标量变量之间的差异可以解释如下。

说，你正试图表示各种学生为变量集的名字。每一个单个变量是一个标量变量，如下所示：

NAME01="Zara"

NAME02="Qadir"

NAME03="Mahnaz"

NAME04="Ayan"

NAME05="Daisy"

我们可以用一个单一的阵列来存储所有上述提及的名称。以下是最简单的方法创建一个数组变量分配一个值，其索引之一。这是表示，如下所示：

array_name[index]=value

array_name 是数组名，索引是在阵列中，你要设置的项目索引，值是你想要的值设置该项目。

作为一个例子，下面的命令：

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz www.yiibai.com"

NAME[3]="Ayan"

NAME[4]="Daisy"

如果您使用的是ksh shell在这里初始化数组的语法：

set -A array_name value1 value2 ... valuen

如果您使用的是bash shell中，这里是初始化数组的语法：

array_name=(value1 ... valuen)

访问数组值：

当您设置任何数组变量，并可访问它，如下所示：

${array_name[index]}

在这里，array_name是数组的名称，index是索引进行访问的值。下面是一个简单的例子：

#!/bin/sh

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz"

NAME[3]="Ayan"

NAME[4]="Daisy"

echo "First Index: ${NAME[0]}"

echo "Second Index: ${NAME[1]}"

This would producefollowing result:

$./test.sh

First Index: Zara

Second Index: Qadir

您可以访问数组中的所有项目通过以下方式之一：

${array_name[*]}

${array_name[@]}

array_name 是数组的名字你所感兴趣的以下是个最简单的例子：

#!/bin/sh

NAME[0]="Zara"

NAME[1]="Qadir"

NAME[2]="Mahnaz"

NAME[3]="Ayan"

NAME[4]="Daisy"

echo "First Method: ${NAME[*]}"

echo "Second Method: ${NAME[@]}"

这将产生以下结果：

$./test.sh

First Method: Zara Qadir Mahnaz Ayan Daisy

Second Method: Zara Qadir Mahnaz Ayan Daisy

有各种不同的运算符shell都支持。本教程是基于默认shell（Bourne），所以我们要涵盖所有重要的Bourne Shell运算符。

有以下的运算符，我们将要讨论的：

· 算术运算符。

· 关系运算符。

· 布尔运算符。

· 字符串运算符。

· 文件测试操作。

Bourne shell的最初并没有任何机制来执行简单的算术，但它使用外部程序，无论是awk或必须简单的程序expr。

下面是简单的例子，把两个数相加：

#!/bin/sh

val=`expr 2 + 2`

echo "Total value : $val"

这将产生以下结果：

Total value : 4

记下有以下几点：

· 运算符和表达式之间必须有空格，例如2+2是不正确的，因为它应该写成2 + 2。

· ``，称为倒逗号之间应包含完整的表达。

算术运算符：

算术运算符有以下Bourne Shell支持。

假设变量a=10,变量b=20：

算术运算符例子

运算符	描述	例子
+	Addition - Adds values on either side of the operator	`expr $a + $b` will give 30
-	Subtraction - Subtracts right hand operand from left hand operand	`expr $a - $b` will give -10
*	Multiplication - Multiplies values on either side of the operator	`expr $a * $b` will give 200
/	Division - Divides left hand operand by right hand operand	`expr $b / $a` will give 2
%	Modulus - Divides left hand operand by right hand operand and returns remainder	`expr $b % $a` will give 0
=	Assignment - Assign right operand in left operand	a=$b would assign value of b into a
==	Equality - Compares two numbers, if both are same then returns true.	[ $a == $b ] would return false.
!=	Not Equality - Compares two numbers, if both are different then returns true.	[ $a != $b ] would return true.

这是非常重要的，这里要注意，所有的条件式将放在方括号内，他们身边有一个空格，例如 [ $a == $b ]是正确的，为[$a==$b] 是不正确的。

所有的算术计算，使用长整数。

关系运算符：

Bourne Shell的支持，关系运算符的具体数值。这些运算符不能使用字符串值，除非它们的值是数字。

例如，运算符将努力检查10和20之间的关系，以及在“10”和“20”，但不是“10”和“21”之间。

假设变量a=10,变量b=20：

关系运算符

运算符	描述	示例
-eq	Checks if the value of two operands are equal or not, if yes then condition becomes true.	[ $a -eq $b ] is not true.
-ne	Checks if the value of two operands are equal or not, if values are not equal then condition becomes true.	[ $a -ne $b ] is true.
-gt	Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true.	[ $a -gt $b ] is not true.
-lt	Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true.	[ $a -lt $b ] is true.
-ge	Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true.	[ $a -ge $b ] is not true.
-le	Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true.	[ $a -le $b ] is true.

这里要注意，所有的条件式将放在方括号内，他们周围有一个空格，这是非常重要的，例如 [ $a <= $b]是正确的， [$a <= $b]是不正确的。

布尔运算：

布尔运算符有以下Bourne Shell的支持。

假设变量一个变量b=10，然后变量b=20：

布尔运算示例

运算符	描述	示例
!	This is logical negation. This inverts a true condition into false and vice versa.	[ ! false ] is true.
-o	This is logical OR. If one of the operands is true then condition would be true.	[ $a -lt 20 -o $b -gt 100 ] is true.
-a	This is logical AND. If both the operands are true then condition would be true otherwise it would be false.	[ $a -lt 20 -a $b -gt 100 ] is false.

字符串运算符：

有下列字符串运算由Bourne Shell支持。

假设变量a=“abc”和变量b=“efg”：

关系运算例子

运算符	描述	例子
=	Checks if the value of two operands are equal or not, if yes then condition becomes true.	[ $a = $b ] is not true.
!=	Checks if the value of two operands are equal or not, if values are not equal then condition becomes true.	[ $a != $b ] is true.
-z	Checks if the given string operand size is zero. If it is zero length then it returns true.	[ -z $a ] is not true.
-n	Checks if the given string operand size is non-zero. If it is non-zero length then it returns true.	[ -z $a ] is not false.
str	Check if str is not the empty string. If it is empty then it returns false.	[ $a ] is not false.

文件测试操作：

有以下是操作测试Unix文件相关联的各种属性。

假设一个的变量文件保存现有文件名“test”，其大小为100字节，有读，写和执行权限：

文件测试操作例子

操作符	描述	示例
-b file	Checks if file is a block special file if yes then condition becomes true.	[ -b $file ] is false.
-c file	Checks if file is a character special file if yes then condition becomes true.	[ -b $file ] is false.
-d file	Check if file is a directory if yes then condition becomes true.	[ -d $file ] is not true.
-f file	Check if file is an ordinary file as opposed to a directory or special file if yes then condition becomes true.	[ -f $file ] is true.
-g file	Checks if file has its set group ID (SGID) bit set if yes then condition becomes true.	[ -g $file ] is false.
-k file	Checks if file has its sticky bit set if yes then condition becomes true.	[ -k $file ] is false.
-p file	Checks if file is a named pipe if yes then condition becomes true.	[ -p $file ] is false.
-t file	Checks if file descriptor is open and associated with a terminal if yes then condition becomes true.	[ -t $file ] is false.
-u file	Checks if file has its set user id (SUID) bit set if yes then condition becomes true.	[ -u $file ] is false.
-r file	Checks if file is readable if yes then condition becomes true.	[ -r $file ] is true.
-w file	Check if file is writable if yes then condition becomes true.	[ -w $file ] is true.
-x file	Check if file is execute if yes then condition becomes true.	[ -x $file ] is true.
-s file	Check if file has size greater than 0 if yes then condition becomes true.	[ -s $file ] is true.
-e file	Check if file exists. Is true even if file is a directory but exists.	[ -e $file ] is true.

C Shell 操作符:

以下链接将在C Shell运算符给出简单的用法。

C Shell 运算符

Korn Shell 运算符:

以下链接将在Korn Shell运算符给出简单的用法

Korn Shell 运算符

from: http://www.yiibai.com/shell/what_is_shell.html#