8.1. 运算符

赋值

变量赋值

初始化或更改变量的值

=

通用赋值运算符，适用于算术和字符串赋值。

var=27 category=minerals # No spaces allowed after the "=".

不要将 "=" 赋值运算符与 = 测试运算符 混淆。 # = as a test operator if [ "$string1" = "$string2" ] then command fi # if [ "X$string1" = "X$string2" ] is safer, #+ to prevent an error message should one of the variables be empty. # (The prepended "X" characters cancel out.)

算术运算符

+

加

-

减

*

乘

/

除

**

求幂

# Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" # 5 * 5 * 5 echo "z = $z" # z = 125

%

取模，或 mod (返回整数除法运算的余数)

bash$ expr 5 % 3 2

5/3 = 1，余数为 2

此运算符用途广泛，其中包括在特定范围内生成数字（参见示例 9-11 和 示例 9-15）和格式化程序输出（参见 示例 27-16 和 示例 A-6）。它甚至可以用于生成素数（参见 示例 A-15）。取模运算在数值计算中非常常见。

示例 8-1. 最大公约数

#!/bin/bash # gcd.sh: greatest common divisor # Uses Euclid's algorithm # The "greatest common divisor" (gcd) of two integers #+ is the largest integer that will divide both, leaving no remainder. # Euclid's algorithm uses successive division. # In each pass, #+ dividend <--- divisor #+ divisor <--- remainder #+ until remainder = 0. # The gcd = dividend, on the final pass. # # For an excellent discussion of Euclid's algorithm, see #+ Jim Loy's site, http://www.jimloy.com/number/euclids.htm. # ------------------------------------------------------ # Argument check ARGS=2 E_BADARGS=85 if [ $# -ne "$ARGS" ] then echo "Usage: `basename $0` first-number second-number" exit $E_BADARGS fi # ------------------------------------------------------ gcd () { dividend=$1 # Arbitrary assignment. divisor=$2 #! It doesn't matter which of the two is larger. # Why not? remainder=1 # If an uninitialized variable is used inside #+ test brackets, an error message results. until [ "$remainder" -eq 0 ] do # ^^^^^^^^^^ Must be previously initialized! let "remainder = $dividend % $divisor" dividend=$divisor # Now repeat with 2 smallest numbers. divisor=$remainder done # Euclid's algorithm } # Last $dividend is the gcd. gcd $1 $2 echo; echo "GCD of $1 and $2 = $dividend"; echo # Exercises : # --------- # 1) Check command-line arguments to make sure they are integers, #+ and exit the script with an appropriate error message if not. # 2) Rewrite the gcd () function to use local variables. exit 0

+=

加等于（变量递增一个常量） [1]

let "var += 5"结果为var被递增了5.

-=

减等于（变量递减一个常量）

*=

乘等于（变量乘以一个常量）

let "var *= 4"结果为var被乘以了4.

/=

除等于（变量除以一个常量）

%=

模等于（变量除以一个常量的余数）

算术运算符经常出现在 expr 或 let 表达式中。

示例 8-2. 使用算术运算

#!/bin/bash # Counting to 11 in 10 different ways. n=1; echo -n "$n " let "n = $n + 1" # let "n = n + 1" also works. echo -n "$n " : $((n = $n + 1)) # ":" necessary because otherwise Bash attempts #+ to interpret "$((n = $n + 1))" as a command. echo -n "$n " (( n = n + 1 )) # A simpler alternative to the method above. # Thanks, David Lombard, for pointing this out. echo -n "$n " n=$(($n + 1)) echo -n "$n " : $[ n = $n + 1 ] # ":" necessary because otherwise Bash attempts #+ to interpret "$[ n = $n + 1 ]" as a command. # Works even if "n" was initialized as a string. echo -n "$n " n=$[ $n + 1 ] # Works even if "n" was initialized as a string. #* Avoid this type of construct, since it is obsolete and nonportable. # Thanks, Stephane Chazelas. echo -n "$n " # Now for C-style increment operators. # Thanks, Frank Wang, for pointing this out. let "n++" # let "++n" also works. echo -n "$n " (( n++ )) # (( ++n )) also works. echo -n "$n " : $(( n++ )) # : $(( ++n )) also works. echo -n "$n " : $[ n++ ] # : $[ ++n ] also works echo -n "$n " echo exit 0

位运算符

<<

位左移（乘以2每个移位位置）

<<=

左移等于

let "var <<= 2"结果为var左移了2位（乘以4)

>>

位右移（除以2每个移位位置）

>>=

右移等于（<<= 的逆运算）

&

按位与

&=

按位与等于

|

按位或

|=

按位或等于

~

按位非

^

按位异或

^=

按位异或等于

逻辑（布尔）运算符

!

非

if [ ! -f $FILENAME ] then ...

&&

与

if [ $condition1 ] && [ $condition2 ] # Same as: if [ $condition1 -a $condition2 ] # Returns true if both condition1 and condition2 hold true... if [[ $condition1 && $condition2 ]] # Also works. # Note that && operator not permitted inside brackets #+ of [ ... ] construct.

&& 也可在 and list 中使用，具体取决于上下文，用于连接命令。

||

或

if [ $condition1 ] || [ $condition2 ] # Same as: if [ $condition1 -o $condition2 ] # Returns true if either condition1 or condition2 holds true... if [[ $condition1 || $condition2 ]] # Also works. # Note that || operator not permitted inside brackets #+ of a [ ... ] construct.

Bash 测试使用逻辑运算符链接的每个语句的 退出状态。

示例 8-3. 使用 && 和 || 的复合条件测试

#!/bin/bash a=24 b=47 if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] then echo "Test #1 succeeds." else echo "Test #1 fails." fi # ERROR: if [ "$a" -eq 24 && "$b" -eq 47 ] #+ attempts to execute ' [ "$a" -eq 24 ' #+ and fails to finding matching ']'. # # Note: if [[ $a -eq 24 && $b -eq 24 ]] works. # The double-bracket if-test is more flexible #+ than the single-bracket version. # (The "&&" has a different meaning in line 17 than in line 6.) # Thanks, Stephane Chazelas, for pointing this out. if [ "$a" -eq 98 ] || [ "$b" -eq 47 ] then echo "Test #2 succeeds." else echo "Test #2 fails." fi # The -a and -o options provide #+ an alternative compound condition test. # Thanks to Patrick Callahan for pointing this out. if [ "$a" -eq 24 -a "$b" -eq 47 ] then echo "Test #3 succeeds." else echo "Test #3 fails." fi if [ "$a" -eq 98 -o "$b" -eq 47 ] then echo "Test #4 succeeds." else echo "Test #4 fails." fi a=rhino b=crocodile if [ "$a" = rhino ] && [ "$b" = crocodile ] then echo "Test #5 succeeds." else echo "Test #5 fails." fi exit 0

&& 和 || 运算符也用于算术上下文中。

bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0)) 1 0 1 0

杂项运算符

,

逗号运算符

逗号运算符将两个或多个算术运算链接在一起。所有运算都会被求值（可能带有副作用。[2]

let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1" ^^^^^^ # t1 = 11 # Here t1 is set to the result of the last operation. Why? let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". echo "t2 = $t2 a = $a" # t2 = 5 a = 9

逗号运算符主要用于 for 循环。请参见 示例 11-13。