Linux bash in operation

1. First, the use of ((expression))

   root@root:~# a=3
   root@root:~# ((b=a+3))
   root@root:~# echo $b
   6

2. The second use let

   root@root:~# let "c=$a+4"
   root@root:~# echo $c
   7

3. Use expr expression, note the space and backtick

   root@root:~# d=`expr 4 + 3`
   root@root:~# echo $d
   7

4. On $ [expression], which is bash the recommended standard treatment

   root@root:~# a=4
   root@root:~# b=$[$a-9]
   root@root:~# echo $b
   -5

5. In the for-loop style C, may be used as follows

   root@root:~# for((a=1,b=10;a<=10;a++,b--))
   do
   ((c=a*b))
   echo "$c"
   done
   10
18
24
28
30
30
28
24
18
10

6. Floating-point operations, with bc (bash calculator) operation

   基本格式: variable=`echo "option;expression" | bc`
   root@root:~# a=`echo "scale=4;3.25/3" | bc`
   root@root:~# echo $a
   1.0833
   root@root:~# a=6.6
   root@root:~# b=3.5
   root@root:~# c=`echo "scale=5;$a/$b"|bc`
   root@root:~# echo $c
   1.88571

7. If a floating-point expression is more than one line, you can put multiple lines computing

   基本格式:
   variable=`bc << EOF
   options
   statements
   expressions
   EOF
   `
   演示:
   root@root:~# a=1.2
   root@root:~# b=2.4
   root@root:~# c=3.5
   root@root:~# d=2
   root@root:~# e=`bc << EOF
   scale=4
   ab=($a+$b)
   cd=($c-$d)
   ab+cd
   EOF
   `
   root@root:~# echo $e
   5.1