Basic shell operations (Part 2)

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Operator

Arithmetic operator

• expr + , - , *, /, % Add , reduce , ride , except , Remainder
  
  Compound operation , Be careful `` Not single quotes
  
• “$((shipmentcounttype))”or“$[ Arithmetic expression ]”
  

String operators

• = Checks if two strings are equal , Relative return true
• != Detect whether two strings are not equal , Unequal return true
• -z Check if the string length is 0, by 0 return true
• -n Check whether the string length is not 0, Not for 0 return true
• $ Check if the string is empty , Not empty return true

In the process control meeting

Process control

if else Judge

grammar :

if [  Conditional judgment  ] 
  then 
     command  
fi
===============================
if [  Conditional judgment  ] 
  then 
     command  
else
	 command 
fi
==============================
if [  Conditional judgment  ] 
  then 
     command  
elif [  Conditional judgment  ]
  then
	 command 
else
	 command 
fi

Be careful Space , stay if And after [] Spaces should be added in

You can also write it as a sentence and add ; Such as if [ a=b ];then echoxxxxxxx;xxx;xxx;

case sentence

Every case The branch begins with a right parenthesis , Use two semicolons ;; Express break, End with esac Express

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

for loop

And in other languages for The cycle is similar

for  Variable  in item1 item2 ... itemN
do
     command 
	.....
done
==========================
for (( expression ))
do
     command 
	.....
done

while loop

while [  Conditional judgment  ] 
  do 
     command 
  done
====================
while (( expression ))
  do 
     command 
  done

function

 Function name (){
    
	 The body of the function 
	[return]
}

No addition return Will return the last sentence