Linux commands: Let

The Let usage of Linux

1. Shell String Connection operation

[Email protected] sysroot]# var=1; var= $var +1;echo $var
Up

2, in the shell can use let to indicate that the following is an arithmetic expression, let expression inside the variable can not add $

[Email protected] sysroot]# var=3; Let "var=$var+1"; Echo $var
4
[Email protected] sysroot]# var=4; Let "Var=var+1"; Echo $var
5

[Email protected] sysroot]# var=5; Let "var+=1"; Echo $var
6 #Let must be a complete arithmetic expression that has both sides of the equals sign

3, the Let can be replaced by (()),Let″j=i*6+2″ equivalent to ((j=i*6+2)), like many of the cycle of the use of the same

[Email protected] sysroot]# var=7;  ((var++)); Echo $var

8

[Email protected] sysroot]# var=8; var=$[$var +1]; Echo $var
9

[Email protected] sysroot]# var=9; var= ' expr $var +1'; echo $var # plus no spaces next to string connections
9+1 # ' is not a single quote, is a command substitution, the key to the left of "1"

[Email protected] sysroot]# var=9; var=' expr $var + 1'; echo $var # plus no spaces next to arithmetic operations

10

[Email protected] sysroot]# var=9; Var=$ (expr $var + 1); echo $var # in the shell $ () and ' equivalent
10

[Email protected] sysroot]# var=9;  Var=$ (( $var + 1)); Echo $var
10

[Email protected] sysroot]# var=9; Echo $ (( $var + 1))
10

# (()), expr can be evaluated only to the right of the equals sign, by $ ((...)), $ (expr ...), ' expr ... ' to view the returned results

4. Processing floating-point BC

[Email protected] sysroot]# var=20; Var= ' echo ' scale=1; $var *3.5 "|  BC '; Echo $var
70.0

5. The method that supports floating-point arithmetic is awk

[Email protected] sysroot]# var=30; Var= ' echo ' $var 3 | awk ' {printf ("%g", $1+$2)} "; Echo $var
33

[[email protected] sysroot]# c=$ (awk ' begin{print 11.111*5-0.4} '); Echo $c
55.155

1: In the shell $ () is equivalent to '. Executes the command statement contained in the middle, returning the execution result.
2: From Efficiency let== (()) > expr > BC. Let and (()) run are built-in commands that use the same algorithm.
The operations of 3:let and expr are integer operations and do not include floating-point budgeting.
4:expr and BC are external programs, and the volume of expr is almost equal to 1/3 of BC, and less time is spent executing a load of memory at once.
5: From the computing power, BC ranked first.

---end---

Linux commands: Let