Shell 基本運算符
有各種不同的運算符shell都支持。本教程是基於默認shell（Bourne），所以我們要涵蓋所有重要的Bourne Shell運算符。
有以下的運算符，我們將要討論的：
算術運算符。
關係運算符。
布爾運算符。
字符串運算符。
文件測試操作。
Bourne shell的最初並沒有任何機制來執行簡單的算術，但它使用外部程序，無論是awk或必須簡單的程序expr。
下面是簡單的例子，把兩個數相加：
#!/bin/sh val=`expr 2 + 2` echo "Total value : $val"
這將產生以下結果：
Total value : 4
記下有以下幾點：
運算符和表達式之間必須有空格，例如2+2是不正確的，因爲它應該寫成2 + 2。
``，稱爲倒逗號之間應包含完整的表達。
算術運算符：
算術運算符有以下Bourne Shell支持。
假設變量a=10,變量b=20：
算術運算符例子
運算符
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，這裏要注意，所有的條件式將放在方括號內，他們身邊有一個空格，例如 [ $a == $b ]是正確的，爲[$a==$b] 是不正確的。
所有的算術計算，使用長整數。
關係運算符：
Bourne Shell的支持，關係運算符的具體數值。這些運算符不能使用字符串值，除非它們的值是數字。
例如，運算符將努力檢查10和20之間的關係，以及在「10」和「20」，但不是「10」和「21」之間。
假設變量a=10,變量b=20：
關係運算符
運算符
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.
這裏要注意，所有的條件式將放在方括號內，他們周圍有一個空格，這是非常重要的，例如 [ $a <= $b ]是正確的， [$a <= $b]是不正確的。
布爾運算：
布爾運算符有以下Bourne Shell的支持。
假設變量一個變量b=10，然後變量b=20：
布爾運算示例
運算符
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.
字符串運算符：
有下列字符串運算由Bourne Shell支持。
假設變量a=「abc」和變量b=「efg」：
關係運算例子
運算符
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.
文件測試操作：
有以下是操作測試Unix文件相關聯的各種屬性。
假設一個的變量文件保存現有文件名「test」，其大小爲100字節，有讀，寫和執行權限：
文件測試操作例子
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:
以下鏈接將在C Shell運算符給出簡單的用法。
C Shell 運算符
Korn Shell 運算符:
以下鏈接將在Korn Shell運算符給出簡單的用法
Korn Shell 運算符