Shell 基本运算符

摘要: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...

作者:LoveCode标签:Shell,基本运算符,运算符,变量,算术,例子,布尔,操作,关系,示例

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

 

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