Shell 基本运算符

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.