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：
| 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：
| 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：
| 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”：
| 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：
| 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。
Korn Shell 运算符:
以下链接将在Korn Shell运算符给出简单的用法