Shell_ 05_ operator

Operator

Operators are divided into ： Arithmetic operator 、 Relational operator 、 Logical operators 、 String operators and file test operators

1） When using operators , You must add spaces on the left and right sides of the operator to separate other variables

Command operator

Association system command execution ： There is a dependency between the two commands （ And 、 or ）

	Command execution
command 1 && command 2	If command 1 Successful execution , Then start to execute the command 2
	If command 1 An error occurred during execution , Do not execute the order 2
command 1 || command 2	If command 1 Successful execution , Do not execute the order 2
	If command 1 An error occurred during execution , Then start to execute the command 2

1） Whether the command is successfully executed depends on “$?” Is the variable 0 To judge

Such as ： stay /root/test/abc Create files under folders test and test2

// Regardless of /abc Does the folder exist , Can be established

Arithmetic operator

Suppose there are variables a and b, The values are 10 and 20

Operator	explain	give an example
+	Add	`expr $a + $b` The result is 30.
-	Subtraction	`expr $a - $b` The result is -10.
*	Multiplication	`expr $a \* $b` The result is 200.
/	division	`expr $b / $a` The result is 2.
%	Remainder	`expr $b % $a` The result is 0.
=	assignment	a=$b Will put the variable b The value is assigned to a
= =	Used to compare two numbers Same returns true（ really ）	[ $a == $b ] return false（ false ）
!=	Used to compare two numbers If not, return to true	[ $a != $b ] return true

1）bash It doesn't support arithmetic itself , It can be done by awk、expr and bc To achieve

2） Through the bracket operation form ：[ $ Variable 1 Operator $ Variable 2 ]

// If you assign the result of the operation to other variables , It is also necessary to add $

expr command ： Record the result of expression calculation

Command format ：expr value 1 Operator value 2

1） Only integer level operations can be performed

Such as ： Use expr Do four calculations

//“*” Operators need to pass “\” Transference

Bracket operators

Judging symbols ： brackets “[ ]”

Command format ：[ Detection expression ]

1）[ ] Belongs to operator , When using, you need to add spaces at the beginning and end

2） Variables in brackets , It is best to use double quotation marks

3） Constants in brackets , It's best to use a single / In double quotation marks

Test parameters	meaning	analysis
1. About file type detection
-e File path	Test files （ Including directory ） Whether there is If it is , Then return to true	[ -e $file ] return true
-d File path	Check if the file is a directory If it is , Then return to true	[ -d $file ] return false
-f File path	Check if the file is a normal file If it is , Then return to true	[ -f $file ] return true
-s File path	Check if the file is empty （ file size ） Not empty return true	[ -s $file ] return true
-p File path	Check whether the file is a pipeline file If it is , Then return to true	[ -p $file ] return false
-b File path	Check whether the file is a block device file If it is , Then return to true	[ -b $file ] return false
-c File path	Detect whether the file is a character device file If it is , Then return to true	[ -c $file ] return false
-S File path	Check whether the file is Socket file If it is , Then return to true	[ -c $file ] return false
2. About file permission detection
-r File path	Check whether the file is readable If it is , Then return to true	[ -r $file ] return true
-w File path	Check whether the file is writable If it is , Then return to true	[ -w $file ] return true
-x File path	Check if the file is executable If it is , Then return to true	[ -x $file ] return true
-u File path	Check if the file is set SUID position If it is , Then return to true	[ -u $file ] return false
-g File path	Check if the file is set SGID position If it is , Then return to true	[ -g $file ] return false
-k File path	Check whether the file is set with the adhesive bit (Sticky Bit) If it is , Then return to true	[ -k $file ] return false
3. About the detection of document comparison
-nt （new than）	Check whether the first file is newer than the second If new , Then return to true	[ $file1 -nt $file2 ]
-ot （old than）	Check whether the first file is older than the second If it's old , Then return to true	[ $file1 -ot $file2 ]
-ef	Check whether the two files are the same If the same file , Then return to true	[ $file1 -ef $file2 ]
4. About integer detection
-eq （eaqul）	Check whether two numbers are equal Equal return true	[ $a -eq $b ] return false.
-ne （not equal）	Check whether two numbers are equal Unequal return true	[ $a -ne $b ] return true.
-gt （great than）	Check whether the number on the left is greater than that on the right If it is , Then return to true	[ $a -gt $b ] return false.
-ge	Check whether the number on the left is equal to or greater than the number on the right If it is , Then return to true	[ $a -ge $b ] return false.
-lt （less than）	Check if the number on the left is less than the number on the right If it is , Then return to true	[ $a -lt $b ] return true.
-le	Check whether the number on the left is less than or equal to the number on the right If it is , Then return to true	[ $a -le $b ] return true.
5. About string detection
-z	Check if the string length is 0 by 0 return true	[ -z $c ] return false
-n	Check if the string length is 0 Not for 0 return true	[ -n $c ] return true
\>	Check whether the left string is larger than the right string Greater than return true（ according to ASCII）	[ $c \> $d ] return true
\<	Check whether the left string is smaller than the right string Less than return true（ according to ASCII）	[ $c \< $d ] return false
= =	Checks if two strings are equal Equal return true	[ $c = = $d ] return false
6. About multiple condition detection
-a （ And operation ）	Both expressions are true To return to true	[ $a -lt 20 -a $b -gt 100 ] return false
-o （ Or operations ）	There is an expression for true Then return to true	[ $a -lt 20 -o $b -gt 100 ] return true
! （ Non operation ）	Expression for true Then return to false	[ ! false ] return true

1） Suppose there are variables a and b, The values are 10 and 20

2） Suppose there are variables c and d, The values are ‘abc’ and ‘efg’

3） The authority is judged as or ： Only one of the three identities has , Think true

test command ： The file / Data for specified detection

Command format ：test Detection expression

1） The use method and effect are equivalent to the bracket judgment symbol

Such as ： Compare sizes of integers

//test And brackets do not directly print test results , Need to use “$?” see

Double parenthesis operation

Double parentheses can realize advanced mathematical expressions in Inner parentheses

1） Format ：（（ Mathematical expression ））

2） A mathematical expression can be any mathematical assignment or comparison expression ;

3） More used for for loop , The implementation is similar to C Linguistic for Circular format ;

Operator	explain
++	Self increasing operation
–	Self built operation
!	Logical negation
~	Position inversion
**	Power operation
<<	Left displacement
>>	Right displacement
&	Bit Boolean and
|	Bit Boolean or
&&	And operation
||	Or operations