10: Shell operator
Method (1) numerical operations
What if I need a numerical calculation, you can use any one of three ways: using declare declare a variable type, since all variables default type is string, so long as we declare a variable as integer arithmetic can not Yet? Use the command to declare the variable type declaration can be achieved. Command is as follows:
[Root @ localhost ~] # declare [+/-] [options]
Variable name options:
-: type attribute set to a variable
+: Type attribute variable canceled
-a: variable declared as an array type
-i: variable declared as an integer type (integer)
-r: speak variable declared as read-only variables. Note that, once the read-only variables, we can not modify the value of a variable, the variable can not be deleted, even not canceled by + r read-only attribute.
-x: Declaring a variable as environment variable
-p: Specifies the type of display variable is declared
Example 1: declare variable type computes
[root@localhost ~]# aa=11
[root@localhost ~]# bb=22
# Assigned to the variable aa and bb
[root@localhost ~]# declare -i cc=$aa+$bb
Type cc # declare variables of type integer, and its value is aa and bb
[root@localhost ~]# echo $cc
33
# This time finally summed up
Example 2: array variable type
Note that the array index starts from 0, and when calling array values, required {$ array [index]} way to read.
[root@localhost ~]# name[0]="li "
[root@localhost ~]# name[1]="ming"
[root@localhost ~]# name[2]=" ang"
[root@localhost ~]# echo ${name}
at the
# Output array of content, if just write the name of the array, then only the output of the first index variable
[root@localhost ~]# echo ${name[*]}
li ming ang
Example 3: Environment Variables
In fact, we can also use the command to declare the variable is declared as an environment variable, and the role of export orders is the same:
[root@localhost ~]# declare -x test=123
# Declare the variable test environment variable
Example 4: read-only attribute
Note that once the read-only attribute is set to a variable, then this variable can neither change the value of the variable, nor can you delete variables can not even use the "+ r" option to cancel the read-only attribute. Command is as follows:
[root@localhost ~]# declare -r test
# Gives read-only attribute to test
[root@localhost ~]# test=456
-bash: test: readonly variable
#Test variable value can not be changed
[root@localhost ~]# declare +r test
-bash: declare: test: readonly variable
# You can not cancel the read-only attribute
[root@localhost ~]# unset test
-bash: unset: test: cannot unset: readonly variable
# Can not delete variables.
Example 5: Query variable attributes and variable attributes canceled
Query uses a variable property "-p" option, eliminating the use of variable attributes "+" option. Command is as follows:
[root@localhost ~]# declare -p cc
declare -i cc="33"
#cc variable of type int
[root@localhost ~]# declare -p name
declare -a name='([0]=" li " [1]="li " [2]="ming")'
#name variable is an array type
[root@localhost ~]# declare -p test
declare -rx test="123"
#test environment variables and variables are read-only variables
[root@localhost ~]# declare +x test
# Cancel test variable environment variable properties
[root@localhost ~]# declare -p test
declare -r test="123"
# Note that the read-only attribute variables can not be canceled.
Examples 6: shell operation
Shell arithmetic calculation performed:
(1) $ (($ num1 + $ num2))
(2) declare -i is shaping declare variables
（3）dd=$(expr $aa + $bb)
Value #dd is aa and bb and. Note that "+" about the number must be a space on both sides
(4) let ee = $ aa + $ bb
（5）gg=$[ .
+ $ B]
Shell operator:
13-- + unary minus, n monocular
12!, ~ Non-logical, or bitwise complement
11 *, /,%, multiply, divide, modulo
10 + - Add, Subtract
9 << >> Bitwise left, Bitwise Right Shift
8 <=,> =, <,> less than or equal to, greater than, or equal to, less than, greater than
7 ==,! = Equal, not equal
/ * & Bitwise AND /
5 ^ bitwise XOR
4 | Bitwise or
3 && Logical AND
2 || logical OR
1 =, + =, - =, * =, / =,% =, & =, ^ =, | =, << =, >> = assignment, calculation and Fu
Substantially: Commonly used four operations, &&, ||,
XOR operation: a binary operation, & 0 is 0, | 1 of 1
Testing and contents of the variable displacement:
Test whether a variable has a value:
Happening:
(1) no variable declarations, echo output is empty. set -u echo $ name is not being given, recognition programs need to use the shell's own test
(2) variable declaration name = "" is empty
x = $ {y- new value new value} x = x x = $ y is empty
If you do not declare y x = new
If y = "" x = Empty
If y = 123 x = 123