Shell programming core technology II

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“ Shell Programming core technology 《 One 》” Students who haven't read this article , Please go to the official account to make up your classes ~ All right. , Start today's article sharing .

1

Use of special symbols

for example ： Input x=(echo abc) The instruction sets the string abc Deposit in x variable , Enter echo x Instructions , You'll see

Another thing to note is the sequence , such as We want to get from 1~10 Get a sequence , You can enter echo {1..10} Command to print 1~10 The number of .

just so so Use seq 1 10, They are equivalent .

2

Variable type

stay Shell Variable types are not distinguished in , This and Python It's like , All variables are basic types , Only do dynamic parsing at runtime . Where the string 、 Numbers 、 Boolean is commonly used .

First, the string , Strings are often used for something like pinching 、 Decapitation 、 Replacement operation , You can practice by yourself after class , There is no more specific demonstration here .

Then Boolean types , The basic form of Boolean is true and false, It is important to note that in Shell There is a special usage in , If a command returns 0, It means that this process is working normally .

for example ： Input ls /tmp/hello.txt ;echo $? Instructions , You can see that the return value is 0, Indicates that the process is running correctly , If other values are returned, it means that the process is running incorrectly .

for example ： Input ls /tmp/hello.txtaaa ;echo $? Instructions Run a nonexistent file , Output is 1 Indicates a running error .

3

Judge

Judgment mainly includes arithmetic judgment , Logical judgment of and or not , Some more Shell There are three aspects of judgment built in . First, let's look at arithmetic judgment , Usually, arithmetic judgment can be used to compare the relationship between two variables , For example, the size of two numbers is compared , String matching relationship , wait .

In addition to simple conditional judgment ,Shell It also supports complex and 、 or 、 Illogical judgment .

besides ,Shell Some built-in judgments are also provided , such as ：

• -e file Indicates if the file exists , The result is true ;
• -d file Indicates if the file is a subdirectory , The result is true ;
• -f file Indicates if the file is an ordinary file , The result is true ;
• -r file Indicates if the file is readable , The result is true ;
• -s file Indicates if the length of the file is not 0, The result is true ;
• -w file Indicates if the file is writable , The result is true ;
• -x file Indicates if the file is executable , The result is true .

Because arrays are not often used , No more details here , If you are interested, you can practice by yourself after class .

4

Logic control

After learning the relevant knowledge of variables , We continue to learn logic control , Some logic can be designed based on data , As shown below ：

• conditional if;
• Branch judgment case、select, Different treatments are carried out according to different conditions ;
• loop for、while、until;
• break and continue, Exit loop .

You can see the whole logic control and Python It's very similar .

5

if Judge

Take a look first if conditional ,if First, check whether the judgment condition is true , If so, execute then Logic within a statement block ,else Execute the logic that the judgment is not tenable , also elif…if…, It is similar to Python, When the conditions are not met, judge the next condition .

6

for loop

And then there was for loop ,for The cycle is Shell There are two uses in . The first usage is similar Java or Python , from 1~10 Cycle through , It can be used at this time for((i=0;i<10;i++)) Realization , This usage is a judgment of the exact number of cycles based on the cardinality .

The second usage is for Traversal cycle , You can use for…in… Sentence block .

for example ： Input for i in (seq 1 3 10) Instructions , And then in do Print in the statement block echo i Value ,do Statement block to done end .

You can see the print 1、4、7、10, This is for Use of loop traversal .

7

while loop

And finally while loop , and for The loop is very similar ,while First, judge the conditions , The condition holds in do Execute the operation in the statement block .

for example ： Definition i=0, Then input while ((i<3));do((i=i+1));sleep 1;echo $i;done Instructions .

You can see , Output results every 1 Print one per second i Value , and i The value of increases gradually , Until it's equal to 3 The condition is no longer satisfied , Exit loop .

and while There is also a very common function , Namely adopt while read line Loop through every line of the file .

for example ： Input while read line;do echo $line;done< /tmp/hello.txt Instructions , It will print out every line of information in the file .

besides , It is also possible to use pipes , Input cat /tmp/hello.txt | while read line;do echo $line;done Instructions , The output effect is the same .

8

Out of control

And there are some complex conditions that need to exit in time , At this time, we need to master the statements that control exit , such as ：

• return The function returns ;
• exit The script process exits ;
• break Exit the current loop ;
• continue Jump out of current loop , Enter next cycle .

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