Fundamentals of shell programming (Chapter 9: loop)

Preface

    In life, we always do the same thing , Loop simulation is used in the program to do repetitive things , Any language has circular statements , stay shell Programming , Support 3 A circular grammar , Namely while、for、for in！

while Two kinds of grammar

1、 On the same line

while condition; do
    command
done

while And do On the same line , Use ; separate ,while Every time I judge condition Exit status code , If 0, Then execute the code of the loop body

2、 Not on the same line

while condition
do
    command
done

while And do Not in the same line , There is no need for ;

In a single line

while true; do echo 'hello world'; done

Use ; Just separate

Only judge the exit status code

while true; do 
    echo 'hello world'; 
done

because true The exit status code of this command is always 0, So the cycle will continue . Any command with exit status code can be placed in while Behind , They could be

1、 function

2、 A command

3、 Another script program

wait , As long as there is an exit status code , As a condition

Multiple conditions , Only recognize the last condition

while true; false; do 
    echo 'hello world'; 
done

The same line , Only recognize the exit status code of the last condition

while summary

1、 Almost if The standards are the same , The condition is to exit the status code , therefore test Commands are often used as conditions

2、while The utilization rate of is quite high , Please learn it

for in grammar

1、 On the same line

for item in hello world; do
    echo $item
done

2、 Not on the same line

for item in hello world
do
    echo $item
done

list

stay for in In the syntax ,in Followed by a list , And the list is blank （ Space 、 Line break 、 Tabulation 、 Any character ） A list of separated words , In circulation , The words in the list will be traversed one by one , So we can often use the return value of command substitution , Traversal , For example, traverse every file in the current working directory

for file in $(ls);do
    echo $file
done

Write in a line

for item in hello world;do echo $item; done

Replace with file name

for i in *.png; do
  echo $i
done

for in summary

1、 Remember one ,in Followed by a list of words separated by white space characters （ Be careful ：IFS You can modify the default separator ）

for grammar

    class C Language innovation is the ancestor of computer high-level language ,shell Programming is also learned C Linguistic for grammar

for (( i=0; i<5; i=i+1 )); do
  echo $i
done

    Grammar , Double brace , ha-ha , Can't hide ！！ In double parentheses , Accessing variable values , Even the dollar sign can be omitted ！！

Infinite loop

for ((;;))
do
    echo hello world
    sleep 5
done

Final summary

1、shell Programming , Support while、for in、for,3 There are two ways of circulation

2、 In fact, there is another kind until, Because it's not often used , This article will not mention

3、 No matter what language , Follow its standard syntax , Learn to get twice the result with half the effort ！！！！