1. Let integer arithmetic
let variable name = variable 1 operator variable 2
let++
Note that the let command can only perform integer-related operations, and the operation results can only save integers ( division is rounded, and the rest has an error if there is a decimal )
2. Expr integer operation
Arithmetic operations can only be integer operations ( division is rounded, the rest are reported with decimal errors ), and string operations can also be performed
expr expression1 operator expression2
The operator * must be preceded by '\' for escaping , and there must be a space between the operator and the two expressions (different from let)
expr as an external command must be enclosed in backticks
例：res=`expr $1 \* 4 +$3`
3. bc floating point arithmetic
Pipeline application: variable = `echo" OPTION; OPERATIONS "| bc`, where scale is one of the most important options of bc, used to specify the number of decimal places output
例：n=`echo "scale=3;13/2" | bc`
echo $n
Result: 6.500
4. String operation
1. Length of output string
Method 1: echo $ {# str1}: the length of the output variable $ str1
Method 2: expr length $ str1: the length of the output variable $ str1
2. The operation of taking substring
Method 1: expr substr $ string $ position $ length Note that the position number starts from 1
str = 'abcde123 "
expr substr $str 3 3
Result: cde
Method 2: echo $ {string: $ pos: $ length} Note that the position number starts from 0
str="abcde123"
echo ${str:2:3}
Result: cde
3. String concatenation operation
$str3="${str1}$str2"
echo $ str3
4. String replacement operation
str="you and you and zhangsan"
echo $ {str / you / YOU} # Replace only once
结果：YOU and you and zhangsan
echo $ {str // you / YOU} #Replace all
YOU and YOU and zhangsan
5. Integer comparison
6. String comparison
7. Logic operation
8. File testing