8.1. Operators

Chapter 8. Operations and Related Topics

8.1. Operators

1、Example 8-1. Greatest common divisor最大公约数的算法不懂，里面的练习：判断命令行参数是否是整数不懂，记在这里。

2、发现一个问题，bash的算术操作符只能进行整数的运算，要进行其它的复杂点的运算：如开方、小数等怎么办呢？

3、Example 8-2. Using Arithmetic Operations

这个例子有不少地方不明白，记在这里以后进一步解决：

我知道 let "z = 2 + 3" 相当于 (( z = 2 + 3 ))，根据：

Similar to the let command, the ((...)) construct permits arithmetic expansion and evaluation.

1）、

10 : $((n = $n + 1))       11 #  ":" necessary because otherwise Bash attempts       12 #+ to interpret "$((n = $n + 1))" as a command.

2)、      20 n=$(($n + 1))

3)、

      23 : $[ n = $n + 1 ]

      24 #  ":" necessary because otherwise Bash attempts

      25 #+ to interpret "$[ n = $n + 1 ]" as a command.

      26 #  Works even if "n" was initialized as a string.

4)、

      29 n=$[ $n + 1 ]

      30 #  Works even if "n" was initialized as a string.

      31 #* Avoid this type of construct, since it is obsolete and

nonportable.

      32 #  Thanks, Stephane Chazelas.

5)、      44 : $(( n++ ))       # : $(( ++n )) also works.

6）、      47 : $[ n++ ]         # : $[ ++n ]] also works

4、bash不能处理浮点数：

Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings.

5、

if [ $condition1 -a $condition2 ] 相当于 if [[ $condition1 && $condition2 ]]

，但是&&不能用在[...]结构中，这些有什么规律吗？真不好掌握啊。

6、Example 8-3. Compound Condition Tests Using && and ||

6 if [ "$a" -eq 24 ] && [ "$b" -eq 47 ] 17 #  Note:  if [[ $a -eq 24 && $b -eq 24 ]]  works. 20 #    (The "&&" has a different meaning in line 17 than in line 6.)

我不懂&&在6行和17行有什么区别？

7、不懂 $((...))的用法

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

英语疑问:

1、Modulo turns up surprisingly often in various numerical recipes.

2、erroneous

An operation that takes a variable outside these limits will give an erroneous result.

3、flip, relevant, fly

"Bit flipping" is more relevant to compiled languages, such

as C and C++, which run fast enough to permit its use on the fly.

4、miscellaneous operators

5、evaluated

All the operations are evaluated (with possible side effects), but only the last operation is returned.