Ten minutes Easy Learning Series: 2020-3-25_SHELL learning basic process control _

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• Process control is divided into three types: sequence, selection, cycle

1) order:

This nothing to say it, as a function of the previous chapter basis, top-down line by line explanation , demand: How to cook instant noodles to complete
the process of programming abstractions of the human behavior, the human thing to do with each describe the process, Therefore, the following pseudo-code:

#定义变量
面饼，调料，水，承载器皿
#制作方法 (这个就是函数了，选择不同的方法，二选一，也可以掰一半，一半泡一半煮-_-#)
function 泡() {
制作过程xxxxx
}
function 煮() {
#定义变量
火
制作过程xxxxx
}

2) Select:

Branch statement, and if the conditions are met xx, xx execution; yy meet the conditions, execution yy ...
I do not speak complex changes, they talk about a fixed format, later wrote as I write this fixed format, certainly right
shell script fixed selection process is as follows :

if [  ];then
    xxxxx #做什么事儿，自己填内容
elif [];then #不是必要的格式，增加条件分支
    xxxxx #做什么事儿，自己填内容
else 
    xxxxx #做什么事儿，自己填内容
fi  

for example:

if [ 3 -gt 2 ];then    #如果3>2,打印我最帅
    echo "我最帅"
elif [ 2 -lt 3 ];then  #如果2<3,打印我最帅，if...elif 和 if...if的区别我在后续流程进阶中讲
    echo "我最帅"
else 
    echo "我也最帅"       #如果上述条件都不满足，我也最帅
fi  

3) cycle:

Childhood teacher asked 1 + ... + 100 is equal to the number, a lot of people know that 5050, there are several ways to calculate?
Attended school knew two kinds:
one is superimposed silly Baji, not to mention 1 + ... + 100, I pain free eggs may calculate 1 + ... + ∞;
One is Mr. genius mathematician Gaosi 1 + 100 + 99 = 101 = 101,2 algorithm of this quality.
Distinguish the difference between the two algorithms, data structures, if the university learned to know first algorithm complexity is O (n), the second is O (1)
to learn more about Big O count method, please see the data structure textbooks University (Yan Wei Min version)
with the most stupid way is to count the cycle:

Do not learn grammar, science fixed format, for and while two kinds of cycles , until I advanced to the follow-up talk anymore.

for i in {1..200}    #1到200，打印出每个值，循环200次呗
do
  echo $i  
done

i=0
while [ $i -lt 201 ]
do 
  echo $i
  let i++  #只要i比201小，那么我就让i自增1，直到i=201的时候跳出循环，即完成1-200的循环
done

for example:

#!/bin/bash
i=0
sum=0               #两个变量初始化为0
while [ $i -lt 5 ]  #如果i小于5
do
  echo $i           #那么打印i值
  let sum+=$i       #让sum增加当前i值
  let i++           #i自增1
done
echo $sum           #sum是i的累加值

Code does not require understanding, understanding fixed format written shell procedure, follow-up article I will open the Advanced