(1) Shell, like other programming languages, supports a variety of operators, including:
- arithmetic operators
- relational operator
- boolean operator
- string operator
- file test operator
Native bash does not support simple mathematical operations, but can be achieved through other commands, such as awk and expr, expr is the most commonly used .
expr is an expression evaluation tool that can be used to evaluate expressions .
For example, to add two numbers ( note the use of backticks ` instead of single quotes' ):
#!/bin/bash
val=`expr 2 + 2` (注意: 2和'+'号 之间要有空格，不然结果为 2+2， 而不是4)
echo "两数之和为 : $val"Execute the script and the output is as follows:
The sum of the two numbers is: 4
Two points of note:
- There should be spaces between expressions and operators , for example 2+2 is wrong, it must be written as 2 + 2, which is different from most programming languages we are familiar with.
- The complete expression should be enclosed by ` `, note that this character is not the usual single quote , below the Esc key.
(2) Arithmetic operators
The following table lists common arithmetic operators, assuming that variable a is 10 and variable b is 20:
| operator | illustrate | Example |
|---|---|---|
| + | addition | `expr $a + $b` results in 30. |
| - | subtraction | `expr $a - $b` results in -10. |
| * | multiplication | `expr $a \ * $b` results in 200. |
| / | division | `expr $b / $a` results in 2. |
| % | remainder | `expr $b % $a` results in 0. |
| = | assign | a=$b will assign the value of variable b to a. |
| == | equal. Used to compare two numbers, returns true if they are the same. | [ $a == $b ] returns false. |
| != | not equal. Used to compare two numbers, returns true if they are not the same. | [ $a != $b ] returns true. |
Note: Conditional expressions should be placed between square brackets and spaces . For example: [$a==$b] is wrong and must be written as [ $a == $b ] .
example
Examples of arithmetic operators are as follows:
#!/bin/bash
# author:ethan
a=10
b=20
val=`expr $a + $b`
echo "a + b : $val"
val=`expr $a - $b`
echo "a - b : $val"
val=`expr $a \* $b`
echo "a * b : $val"
val=`expr $b / $a`
echo "b / a : $val"
val=`expr $b % $a`
echo "b % a : $val"
if [ $a == $b ] (注意:$a前面和后面, $b前面和后面都需要加空格)
then
echo "a 等于 b"
fi
if [ $a != $b ]
then
echo "a 不等于 b"
fi
执行脚本，输出结果如下所示：
a + b : 30
a - b : -10
a * b : 200
b / a : 2
b % a : 0
a 不等于 bNotice:
- The multiplication sign (*) must be preceded by a backslash (\) to achieve multiplication ;
- if...then...fi is a conditional statement, which will be explained later.
- The expr syntax of the shell in MAC is: $((expression)), where the "*" in the expression does not need the escape symbol "\" .
(3) Relational operators
Relational operators only support numbers, not strings, unless the value of the string is a number .
The following table lists commonly used relational operators, assuming that variable a is 10 and variable b is 20:
| operator | illustrate | Example |
|---|---|---|
| -eq | Checks whether two numbers are equal, and returns true if they are equal. | [ $a -eq $b ] returns false. |
| -born | Check if two numbers are equal, return true if they are not equal. | [ $a -ne $b ] returns true. |
| -gt | Checks whether the number on the left is greater than the number on the right, and if so, returns true. | [ $a -gt $b ] returns false. |
| -lt | Checks whether the number on the left is less than the number on the right, and if so, returns true. | [ $a -lt $b ] returns true. |
| -give | Check if the number on the left is greater than or equal to the number on the right, and if so, return true. | [ $a -ge $b ] returns false. |
| -the | Checks whether the number on the left is less than or equal to the number on the right, and if so, returns true. | [ $a -le $b ] returns true. |
example
Examples of relational operators are as follows:
#!/bin/bash
# author:ethan
a=10
b=20
if [ $a -eq $b ]
then
echo "$a -eq $b : a 等于 b"
else
echo "$a -eq $b: a 不等于 b"
fi
if [ $a -ne $b ]
then
echo "$a -ne $b: a 不等于 b"
else
echo "$a -ne $b : a 等于 b"
fi
if [ $a -gt $b ]
then
echo "$a -gt $b: a 大于 b"
else
echo "$a -gt $b: a 不大于 b"
fi
if [ $a -lt $b ]
then
echo "$a -lt $b: a 小于 b"
else
echo "$a -lt $b: a 不小于 b"
fi
if [ $a -ge $b ]
then
echo "$a -ge $b: a 大于或等于 b"
else
echo "$a -ge $b: a 小于 b"
fi
if [ $a -le $b ]
then
echo "$a -le $b: a 小于或等于 b"
else
echo "$a -le $b: a 大于 b"
fi
执行脚本，输出结果如下所示：
10 -eq 20: a 不等于 b
10 -ne 20: a 不等于 b
10 -gt 20: a 不大于 b
10 -lt 20: a 小于 b
10 -ge 20: a 小于 b
10 -le 20: a 小于或等于 b
(4) Boolean operators
The following table lists common Boolean operators, assuming variable a is 10 and variable b is 20:
| operator | illustrate | Example |
|---|---|---|
| ! | NOT operation, returns false if the expression is true, otherwise returns true. | [ ! false ] returns true. |
| -O | OR operation, returns true if one of the expressions is true. | [ $a -lt 20 -o $b -gt 100 ] returns true. |
| -a | AND operation returns true when both expressions are true. | [ $a -lt 20 -a $b -gt 100 ] returns false. |
example
Examples of Boolean operators are as follows:
#!/bin/bash
# author:ethan
a=10
b=20
if [ $a != $b ]
then
echo "$a != $b : a 不等于 b"
else
echo "$a != $b: a 等于 b"
fi
if [ $a -lt 100 -a $b -gt 15 ]
then
echo "$a 小于 100 且 $b 大于 15 : 返回 true"
else
echo "$a 小于 100 且 $b 大于 15 : 返回 false"
fi
if [ $a -lt 100 -o $b -gt 100 ]
then
echo "$a 小于 100 或 $b 大于 100 : 返回 true"
else
echo "$a 小于 100 或 $b 大于 100 : 返回 false"
fi
if [ $a -lt 5 -o $b -gt 100 ]
then
echo "$a 小于 5 或 $b 大于 100 : 返回 true"
else
echo "$a 小于 5 或 $b 大于 100 : 返回 false"
fi
执行脚本，输出结果如下所示：
10 != 20 : a 不等于 b
10 小于 100 且 20 大于 15 : 返回 true
10 小于 100 或 20 大于 100 : 返回 true
10 小于 5 或 20 大于 100 : 返回 false(5) Logical operators
The following describes Shell's logical operators, assuming that variable a is 10 and variable b is 20:
| operator | illustrate | Example |
|---|---|---|
| && | logical AND | [[ $a -lt 100 && $b -gt 100 ]] 返回 false |
| || | logical OR | [[ $a -lt 100 || $b -gt 100 ]] returns true |
example
Examples of logical operators are as follows:
#!/bin/bash
# author:ethan
a=10
b=20
if [[ $a -lt 100 && $b -gt 100 ]]
then
echo "返回 true"
else
echo "返回 false"
fi
if [[ $a -lt 100 || $b -gt 100 ]]
then
echo "返回 true"
else
echo "返回 false"
fi
执行脚本，输出结果如下所示：
返回 false
返回 true(6) String operators
The following table lists common string operators, assuming that variable a is "abc" and variable b is "efg":
| operator | illustrate | Example |
|---|---|---|
| = | Checks if two strings are equal, returns true for equality. | [ $a = $b ] returns false. |
| != | Checks whether two strings are equal, and returns true if they are not equal. | [ $a != $b ] returns true. |
| -with | Check if the string length is 0, if 0 returns true . | [ -z $a ] returns false. |
| -n | Check if the string length is 0, if not return true . | [ -n $a ] returns true. |
| str | Check if the string is empty, return true if not . | [ $a ] returns true. |
example
Examples of string operators are as follows:
#!/bin/bash
# author:ethan
a="abc"
b="efg"
if [ $a = $b ]
then
echo "$a = $b : a 等于 b"
else
echo "$a = $b: a 不等于 b"
fi
if [ $a != $b ]
then
echo "$a != $b : a 不等于 b"
else
echo "$a != $b: a 等于 b"
fi
if [ -z $a ]
then
echo "-z $a : 字符串长度为 0"
else
echo "-z $a : 字符串长度不为 0"
fi
if [ -n $a ]
then
echo "-n $a : 字符串长度不为 0"
else
echo "-n $a : 字符串长度为 0"
fi
if [ $a ]
then
echo "$a : 字符串不为空"
else
echo "$a : 字符串为空"
fi
执行脚本，输出结果如下所示：
abc = efg: a 不等于 b
abc != efg : a 不等于 b
-z abc : 字符串长度不为 0
-n abc : 字符串长度不为 0
abc : 字符串不为空(七)文件测试运算符
文件测试运算符用于检测 Unix 文件的各种属性。
属性检测描述如下：
| 操作符 | 说明 | 举例 |
|---|---|---|
| -b file | 检测文件是否是块设备文件，如果是，则返回 true。 | [ -b $file ] 返回 false。 |
| -c file | 检测文件是否是字符设备文件，如果是，则返回 true。 | [ -c $file ] 返回 false。 |
| -d file | 检测文件是否是目录，如果是，则返回 true。 | [ -d $file ] 返回 false。 |
| -f file | 检测文件是否是普通文件（既不是目录，也不是设备文件），如果是，则返回 true。 | [ -f $file ] 返回 true。 |
| -g file | 检测文件是否设置了 SGID 位，如果是，则返回 true。 | [ -g $file ] 返回 false。 |
| -k file | 检测文件是否设置了粘着位(Sticky Bit)，如果是，则返回 true。 | [ -k $file ] 返回 false。 |
| -p file | 检测文件是否是有名管道，如果是，则返回 true。 | [ -p $file ] 返回 false。 |
| -u file | 检测文件是否设置了 SUID 位，如果是，则返回 true。 | [ -u $file ] 返回 false。 |
| -r file | 检测文件是否可读，如果是，则返回 true。 | [ -r $file ] 返回 true。 |
| -w file | 检测文件是否可写，如果是，则返回 true。 | [ -w $file ] 返回 true。 |
| -x file | 检测文件是否可执行，如果是，则返回 true。 | [ -x $file ] 返回 true。 |
| -s file | 检测文件是否为空（文件大小是否大于0），不为空返回 true。 | [ -s $file ] 返回 true。 |
| -e file | 检测文件（包括目录）是否存在，如果是，则返回 true。 | [ -e $file ] 返回 true。 |
实例
变量 file 表示文件"/Users/youzan-ethan/documents/shell/shell_5.sh"，它的大小为100字节，具有 rwx 权限。下面的代码，将检测该文件的各种属性：
#!/bin/bash
# author:ethan
file="/Users/youzan-ethan/documents/shell/shell_5.sh"
if [ -r $file ]
then
echo "文件可读"
else
echo "文件不可读"
fi
if [ -w $file ]
then
echo "文件可写"
else
echo "文件不可写"
fi
if [ -x $file ]
then
echo "文件可执行"
else
echo "文件不可执行"
fi
if [ -f $file ]
then
echo "文件为普通文件"
else
echo "文件为特殊文件"
fi
if [ -d $file ]
then
echo "文件是个目录"
else
echo "文件不是个目录"
fi
if [ -s $file ]
then
echo "文件不为空"
else
echo "文件为空"
fi
if [ -e $file ]
then
echo "文件存在"
else
echo "文件不存在"
fi
执行脚本，输出结果如下所示：
文件可读
文件可写
文件可执行
文件为普通文件
文件不是个目录
文件不为空
文件存在