Shell process control (emphasis), if judgment, case statement, let usage, for ((initial value; loop control condition; variable change)) and for variable in value 1 value 2 value 3..., while loop

1. Process control （ a key ）

1.1if Judge

1.1.1 Basic grammar

matters needing attention ：
①[ Conditional judgment ], There must be a space between the bracket and the conditional judgment
②if Space after

1.1.1.1 Single branch

if [  Conditional judgment  ];then
  Program  
fi
 perhaps  
if [  Conditional judgment  ] 
then
  Program  
fi

1.1.1.2 Multiple branches

if [  Conditional judgment  ] 
then
	 Program  
elif [  Conditional judgment  ] 
then
	 Program  
else
	 Program  
fi

1.1.2 Case study

1.1.2.1 Single branch

When it's written like this , When no parameters are passed in , The complains , So follow the following steps

In this way, there will be no error , Because if you don't enter parameters, neither side will be empty

1.1.2.2 With logic and && Logic or || Connect （ Single branch ）

1.1.2.2.1 Logic and &&

Don't use the words in brackets && It's about using -a（-and）

1.1.2.2.2 Logic or ||

Don't use the words in brackets || It's about using -o（-or）

1.1.2.3 Multiple branches

…

1.2case sentence

1.2.1 Basic grammar

case $ Variable name  in 
" value  1"）
	 If the value of the variable is equal to the value  1, Then execute the procedure  1 
;;
" value  2"）
	 If the value of the variable is equal to the value  2, Then execute the procedure  2 
;; 
	… Omit other branches … 
*） 
	 If none of the values of the variables are above , Then execute this procedure  
;;
esac

1.2.2 matters needing attention ：

（1）case Line ending must be a word “in”, Each pattern match must be in right parenthesis “）” end . （2） Double a semicolon “;;” Indicates the end of the command sequence , amount to java Medium break.
（3） final “*）” Represents the default mode , amount to java Medium default.

1.2.3 Case study cast_test.sh

1.3for loop

1.3.1 Basic grammar 1

for ((  Initial value ; Cycle control conditions ; Variable change  )) 
do 
	 Program  
done

1.3.2 Basic grammar 2

for  Variable  in  value  1  value  2  value  3… 
do 
	 Program  
done

1.3.3 Case study

1.3.3.1for (( Initial value ; Cycle control conditions ; Variable change ))

1.3.3.2“{}” stay linux shell It represents the generation sequence

{1…100} From 1 To 100, There are two points in the middle , Don't read it wrong

1.3.3.3for Variable in value 1 value 2 value 3…

1.3.3.2 stay for In circulation $* and [email protected] The difference between

When $* and [email protected] Without double quotation marks, the output result is lower ：

Their output results are consistent

When double quotation marks are added . give the result as follows ：

 When they are double quoted “” Inclusion time ,
$* All parameters will be taken as a whole , With “$1 $2 …$n” Form input   Output all parameters ;
[email protected] Separate parameters , With “$1” “$2”…“$n” Output all parameters in the form of .

1.4while loop

1.4.1 Basic grammar

while [  Conditional judgment  ] 
do 
	 Program  
done

1.4.2 from 1 Add to 100

1.4.3let usage

In the new shell Supported in let Of , His language is similar to high-level language