Shell---loop structure

The loop structure under the Shell is Class C, also has a for loop, while loop, in addition, the shell also has a until loop. For Loop the basic syntax for a for loop

For variable in {list}
do
    statement1
    statement2
... Done

The syntax meaning of the For loop is the same as the C language, that is, the number of times to execute the loop body is determined by the list. The list here is a listing, which can be a series of numbers or strings, separated by spaces between elements

Example: Print out the 1~10 in turn.

1, listing list:

#! /bin/bash for

 I, 1 2 3 4 5 6 7 8 9 do
 echo ' number is
     $i ' done
~            

2. The list can be expressed as: {1..8}

#! /bin/bash for

 i ' {1..8} do
 echo ' number is
     $i '
 done

Run Result:
change the For loop growth step
In the For loop above, the variable can only grow at a time 1,shell allows the user to customize the growth step.
The basic syntax is:

For variable in {start ... End.. Step}
do
    statement1
    statement2
... Done

Start represents the starting value, end indicates the ending value, and step represents the growth value.

For example: output from the For loop 100 odd and:

#! /bin/bash

sum=0 for
i in {1..100..2} does let
    "sum+=i";
Done
echo ' result is: $sum '

for loop with no list
Let's take a look at the positional arguments in several shells:

$: Equivalent to main function parameter Argv[0]
$, $ ...: called positional parameter, the equivalent of main function parameter argv[1],argv[2] ....
$#: Equivalent to the main function of the argc-1, where the # does not represent annotation
$@: The argument list "$", "$", can be used in the for loop in the back
$?: Exit code for the previous command
$$: Process number of the current shell

Example: Output per command line for each parameter

For Val in $@
does
    echo "$val"
done

A loop with no list is implemented as:

For Val
doing
    echo "$val"
done

The above two implementations are equivalent. Class C-style for loops

For ((Expression1;expression2;expression3))
do
    statement1;
    Statement2;
    ...
Done

For example: Print 1~10 in this way

For ((i=1;i<10;i++))
do
    echo ' number is $i '
done

For loop processing array
Basic syntax:

For variable in ${array[*]}
do
    statement1;
    Statement2;
    ...
Done

Example: Print array contents

Array= (1 2 3 4 5 6 7 8)     #这句表示定义一个数组 for
i in ${array[*]}
doing
    echo $i
done

While Loop While loop basic usage
Basic syntax:

While expression
do
    statement1
    statement2
... Done

The syntax of the while loop is still the same as the C language while loop, as long as the condition is satisfied.

Example: Using a while loop to output 1~9 squared

I=1 while
["$i"-lt] does
let
    "Square=i*i"
    echo "$i * $i = $square" Let
    "i+=1"
done

Output results:
until cycle basic usage of until cycle
Basic syntax:

Until expression
do
    statement1
    statement2
... Done

The until cycle is similar to the Do...while loop in C language, which repeats the loop body until a condition is set to exit the loop body.

Example: Use the Until loop to perform the example of the while loop, which prints the square of the 1~9.

I=1
until ["$i"-GT] #这就需要判断i大于10时退出循环 do-let
    "square=i*i"
    echo "$i * $i = $square" Let
    " I+=1 "Done

using break and continue control loops use break to control loops
The function of a break is to jump out of a loop immediately.

Example: the multiplication table within the output 5

#! /bin/bash for ((i=1;i<9;i++)) does for ((
    j=1;j<=i;j++)) does let
        "result= $i * $j"
        printf "$i * $j = $result   "
    done
    printf "\ n"
#当i变量的值为5时退出循环
    If [$i-eq 5]
    then
        break
    fi
Done

The output results are:
using continue control loop
The function of continue is to skip the statement after the sentence in the current loop body and start again from the loop body.

Example: An even number within the output 10

#打印10以内的偶数 for
val in {1..10}
do
    if [["$val 2"-eq 1]]
    then
        continue
    fi
    printf "$val  "
done
printf "\ n"

The results of the operation are: