The let command performs integer arithmetic. bash provides a way to substitute integer values (for use as command arguments or in variables); base conversion is also possible.
Table 530. Syntax for arithmetic expressions
Expression 
Meaning 

(( 
Use the value of the enclosed arithmetic expression. 
bash uses arithmetic operators from the C programming language; the following list is in decreasing order of precedence. Use parentheses to override precedence.
Table 531. Arithmetic operators
Operator 
Meaning 


Unary minus 

Logical negation; binary inversion (one's complement) 

Multiplication; division; modulus (remainder) 

Addition; subtraction 

Bitwise left shift; bitwise right shift 

Less than or equal to; greater than or equal to 

Less than; greater than 

Equality; inequality (both evaluated left to right) 

Bitwise AND 

Bitwise exclusive OR 

Bitwise OR 

Logical AND 

Logical OR 

Assign value 

Reassign after addition/subtraction 

Reassign after multiplication/division/remainder 

Reassign after bitwise AND/XOR/OR 

Reassign after bitwise shift left/right 
See the let builtin command for more information and examples.
let "count=0" "i = i + 1" Assign i and count let "num % 2"; echo $? Test for an even number
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