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Shell 基本運(yùn)算符
2021-08-31 09:54 更新
有各種不同的運(yùn)算符shell都支持。本教程是基于默認(rèn)shell（Bourne），所以我們要涵蓋所有重要的Bourne Shell運(yùn)算符。
有以下的運(yùn)算符，我們將要討論的：
- 算術(shù)運(yùn)算符。
- 關(guān)系運(yùn)算符。
- 布爾運(yùn)算符。
- 字符串運(yùn)算符。
- 文件測(cè)試操作。
Bourne shell的最初并沒(méi)有任何機(jī)制來(lái)執(zhí)行簡(jiǎn)單的算術(shù)，但它使用外部程序，無(wú)論是awk或必須簡(jiǎn)單的程序expr。
下面是簡(jiǎn)單的例子，把兩個(gè)數(shù)相加：
#!/bin/sh
val=`expr 2 + 2`
echo "Total value : $val"這將產(chǎn)生以下結(jié)果：
Total value : 4記下有以下幾點(diǎn)：
- 運(yùn)算符和表達(dá)式之間必須有空格，例如2+2是不正確的，因?yàn)樗鼞?yīng)該寫成2 + 2。
- ``，稱為倒逗號(hào)之間應(yīng)包含完整的表達(dá)。
算術(shù)運(yùn)算符：
算術(shù)運(yùn)算符有以下Bourne Shell支持。
假設(shè)變量a=10,變量b=20：
| 運(yùn)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. |
這是非常重要的，這里要注意，所有的條件式將放在方括號(hào)內(nèi)，他們身邊有一個(gè)空格，例如 [ $a == $b ]是正確的，為[$a==$b] 是不正確的。
所有的算術(shù)計(jì)算，使用長(zhǎng)整數(shù)。
關(guān)系運(yùn)算符：
Bourne Shell的支持，關(guān)系運(yùn)算符的具體數(shù)值。這些運(yùn)算符不能使用字符串值，除非它們的值是數(shù)字。
例如，運(yùn)算符將努力檢查10和20之間的關(guān)系，以及在“10”和“20”，但不是“10”和“21”之間。
假設(shè)變量a=10,變量b=20：
| 運(yùn)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. |
這里要注意，所有的條件式將放在方括號(hào)內(nèi)，他們周圍有一個(gè)空格，這是非常重要的，例如 [ $a <= $b ]是正確的， [$a <= $b]是不正確的。
布爾運(yùn)算：
布爾運(yùn)算符有以下Bourne Shell的支持。
假設(shè)變量一個(gè)變量b=10，然后變量b=20：
| 運(yùn)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. |
字符串運(yùn)算符：
有下列字符串運(yùn)算由Bourne Shell支持。
假設(shè)變量a=“abc”和變量b=“efg”：
| 運(yùn)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. |
文件測(cè)試操作：
有以下是操作測(cè)試Unix文件相關(guān)聯(lián)的各種屬性。
假設(shè)一個(gè)的變量文件保存現(xiàn)有文件名“test”，其大小為100字節(jié)，有讀，寫和執(zhí)行權(quán)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:
以下鏈接將在C Shell運(yùn)算符給出簡(jiǎn)單的用法。
Korn Shell 運(yùn)算符:
以下鏈接將在Korn Shell運(yùn)算符給出簡(jiǎn)單的用法
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