Shell_ 06_ Judgment and circulation

Judge

if

Conditional statements ： Judge according to the condition and execute the corresponding command

Command format ：

if [  Conditions 1 ];then
 Procedures section 1
elif [  Conditions 2 ];then
 Procedures section 2
else
 Procedures section 3
fi

1）if and fi Always in pairs （ Represents the beginning and end of a judgment statement ）;

2） Judgement statements can be nested ;

3） When a judgment statement contains multiple levels of the same if or elif when , Then only the status code returned by the first condition is 0 The statement will be executed （ If the levels are different , You can run multiple ）

1） Conditions established , function then Followed by the program segment （ conversely , function else Followed by the program segment ）

// Judge whether it is true according to the status code returned by the condition （0 To establish , Others are failures ）

2） A space must be added on the left and right sides of the condition , Otherwise, the condition cannot be recognized ;

3） You can also execute the specified command as a condition , But the running results will be displayed on the terminal

Such as ： Judging statements by conditions , Judge whether the current user's home directory and the specified file exist

1） To write test5.sh Script files

2） call test5.sh Script files

Such as ： Judging statements by conditions , Check users who have a home directory but do not exist and users who do not have a home directory

1） To write test6.sh Script files

2） call test6.sh Script files

case

Content judgment statement ： Judge according to the variable content and execute the corresponding command

Command format ：

case ${ Variable name } in

“ Variable content 1”）

 Procedures section 1
;;
“ Variable content 2”）
 Procedures section 2
;;
“ Variable content N”）
 Procedures section N
;;
*）
;;
esac

1） The program segment of each variable content needs two semicolons “;;” Represents the end of the program paragraph ;

2）case and esac Always in pairs （ Represents the beginning and end of content judgment ）;

3） The variable can be ： Defining variables 、 The input variable 、 Built-in variables 、 Accept variables ;

4） If the variable content contains more than one content , Use between content “|” Separate ;

Such as ： Set up hello3.sh Script files are only input hello Only then respond

1） To write hello3.sh Script files

2） call hello3.sh Script files

Such as ： Judge sentences by content , Different content shows different responses

1） To write test26.sh Script files

2） call test26.sh Script files

loop

Cycle category

for do done

for loop ： Specify the number of cycles

Command format 1：

for  Formal variable  in  Content 1  Content 2  Content N
do
 Procedures section 
done

1） Content 1~N Will bring in formal variables in turn ;

2） Content 1~N Array variables or variable forms can be used to replace ;

3） When the content contains single quotation marks or spaces , Use double quotation marks

// You can also modify IFS Variable , Modify the field separator

4） When the cycle is over , The formal variable saves the last brought in content by default ;

5） stay done Add output redirection after , You can import the cycle results into the specified file ;

Such as ： adopt for Loop to realize the output of different contents of variables

1） To write test3.sh Script files ;

2） call test3.sh Script files

Such as ： establish dir_perm.sh File implementation determines the permissions of all files in the directory

1） To write dir_perm.sh Script files ;

2） call dir_perm.sh Script files

Internal field separator （Internal Field Separator）：Shell Characters used to separate each field in

1） Variable bit of internal character separator ：IFS

2） The default value is ： Space

3） Assignment format ：IFS= character

// If there are multiple characters as separators , It can be written together during assignment

Such as ： Will a newline 、 The colon 、 The format of semicolon and double quotation mark as separator is ：IFS=$‘\n’：;”

Such as ： adopt IFS Variable implementation for Multiple newline inputs in the loop

1） To write test7.sh Script files

2） call test7.sh Script files

Command format 2：

for （（  Initial value ; The loop condition ; The assignment operation  ））
do
 Procedures section 
done

1） The format is similar to C Linguistic for loop , And variables can be called ;

2） There can be multiple initial values and assignment operations , But there can only be one cycle condition ;

Such as ： establish sum1_input.sh file , Calculation 1 Add to the sum of the input numbers

1） To write sum1_input.sh Script files

2） call sum1_input.sh Script files

Such as ： stay for Define multiple initial values and assignment operations in the loop

1） To write test8.sh Script files

2） call test8.sh Script files

while do done

while loop ： Don't exit the loop until the condition doesn't hold

Command format ：

while [  Judge the condition  ]
do
 Procedures section 
done

（1） If multiple judgment conditions are specified

1） Only judge whether to continue the cycle according to the returned status code of the last judgment condition ;

2） Each judgment condition needs to occupy a separate line ;

3） Multiple judgment conditions will be executed in turn ;

（2） stay done Add output redirection after , You can import the cycle results into the specified file ;

Such as ： adopt while Cycle output value

1） To write test9.sh Script files

2） call test9.sh Script files

until do done

until loop ： Do not exit the cycle until the condition is established

Command format ：

until [  Judge the condition  ]
do
 Procedures section 
done

（1） If multiple judgment conditions are specified

1） Only judge whether to continue the cycle according to the returned status code of the last judgment condition ;

2） Each judgment condition needs to occupy a separate line ;

3） Multiple judgment conditions will be executed in turn ;

（2） stay done Add output redirection after , You can import the cycle results into the specified file ;

Control cycle

Nested loop

Nested loop （Nested Loop）： Any type of command can be called in the program segment of the loop （ Contains loops ）

Such as ： establish choice_what.sh Output three numbers randomly from the array , And can't repeat

1） To write choice_what.sh Script files

2） call choice_what.sh Script files

Such as ： Through internal circulation /etc/passwd Formatted output of file

1） To write test10.sh Script files ;

2） call test10.sh Script files

Out of the loop

Jump out of the loop into ：break command 、continue command

（1）break command ： Jump out of the currently executing loop , And end the cycle

1） Command format ：break N

//N Specify the loop level to jump out （ If there is no , The default is 1）

Such as ： adopt break command , Jump out of for loop

1） To write test11.sh Script files

2） call test11.sh Script files

（2）continue command ： Jump out of this cycle , Start the next cycle

1） Command format ：continue N

//N Specify the loop level to jump out （ If there is no , The default is 1）

Such as ： adopt continue Out of the loop

1） To write test12.sh Script files

2） call test12.sh Script files