8、 ... and 、shell Process control -for Loop statement

When the script executes a task , There will always be times when circular execution is needed , For example, we need the script to execute every five minutes ping The operation of , Except for planning tasks , We can also use scripts to do , So we use the loop statement .

1、for Loop Introduction

Many people put for A cycle is called a recommendation cycle , perhaps for i in . In fact, what the former says is for Characteristics of ,for The number of given conditions is proportional to the number of given cycles , You give 5 Conditions , He just circulates 5 Time , The latter says for The grammar of .
Cycle advantages , Save memory ; Structure is clearer ; Save time and cost

2、for grammar

2.1.for grammar _one

for var in value1 value2 value2 …
do
commands
done
Next , Instance output 1-9

[root@localhost ssh]# cat shell_for.sh
#!/bin/bash
#seq You can output numbers with specified rules , By default, you can add 1; If you need to output 1——9, Every time add 2 Words , Then you can `1 2 9`; If you need to output 9-1, Then you can `9 -1 1`;
for i in `seq 1 9`
        do
                echo $i
                # every other 1 Output once per second 
                sleep 1
done
[root@localhost ssh]# sh shell_for.sh
1
2
3
4
5
6
7
8
9
[root@localhost ssh]# cat shell_for_test.sh
#!/bin/bash
for var in jjl\'s is cool, jjl\'s is nice
 do
   echo "word: $var"
done
[root@localhost ssh]# sh shell_for_test.sh
word: jjl's word: is word: cool, word: jjl's
word: is
word: nice

2.2.for grammar _two

c Type for command

for (( Variable ; Conditions ; Self increasing and decreasing operation ))
do
Code block
done

Code demonstration

[root@localhost ssh]# cat shell_for_two.sh
#!/bin/bash
# i++  Express i=i+1
for ((i=1;i<10;i++))
        do
                echo $i
done
[root@localhost ssh]# sh shell_for_two.sh
1
2
3
4
5
6
7
8
9

Multiple conditions for loop

[root@localhost ssh]# cat shell_for_more.sh
#!/bin/bash
for ((a=0,b=9;a<10;a++,b--))
        do
                echo $a $b
done
[root@localhost ssh]# sh shell_for_more.sh
0 9
1 8
2 7
3 6
4 5
5 4
6 3
7 2
8 1
9 0

for Infinite loop Use ((;;)) Condition can realize infinite loop

[root@localhost ssh]# cat shell_for_del.sh
#!/bin/bash
for ((;;))
        do
                echo 'hahah'
done

3、 Loop control statement

3.1 sleep N The script executes to this step N second

Code demonstration ：

#!/bin/bash

for i in `seq 9 -1 1`
        do
                echo -n -e "\b$i"
                # every other 1 Output once per second 
                sleep 1
done
echo
[root@localhost ssh]# sh sleep.sh
 count down ：1

Monitor whether the host is alive

[root@localhost useful]# cat control_active.sh
#!/bin/bash
# Script to monitor the survival of the host 
for ((;;))
        do
                ping -c1 $1 &>/dev/null
                if [ $? -eq 0 ]
                        then
                                echo -e "`date +"%F %H:%M:%S"`: $1 is \033[32m UP \033[0m"
                else
                                echo -e "`date +"%F %H:%M:%S"`: $1 is \033[31m Down \033[0m"
                fi
                # Script rhythm control   More than one minute is recommended in production 
                sleep 5
done
[root@localhost useful]# sh control_active.sh 192.168.104.44
2021-12-24 16:28:17: 192.168.104.44 is  UP
2021-12-24 16:28:22: 192.168.104.44 is  UP

3.2 continue Skip one of the loops , Continue with next cycle

Default loop output 1-9, But use continue Skip output 5

[root@localhost ssh]# cat continue.sh
#!/bin/bash
#  Default loop output 1-9, But use continue Skip output 5
for ((i=1;i<10;i++))
 do
  if [ $i -eq 5 ]
   then
    continue
  else
   echo $i
  fi
done
[root@localhost ssh]# sh continue.sh
1
2
3
4
6
7
8
9

3.3 break Jump out of the loop and continue to execute subsequent code

[root@localhost ssh]# cat for_break.sh
#!/bin/bash
# Ask the user to enter a letter Q, When the input Q To exit the loop 
for ((;;))
  do
    read -p "char:" ch
    if [ $ch == "Q" ]
     then
      echo " Input correct "
      break
    else
      echo " The letter you typed is wrong , The letter you entered is ：$ch"
    fi
done
[root@localhost ssh]# sh for_break.sh
char:S
 The letter you typed is wrong , The letter you entered is ：S
char:E
 The letter you typed is wrong , The letter you entered is ：E
char:R
 The letter you typed is wrong , The letter you entered is ：R
char:W
 The letter you typed is wrong , The letter you entered is ：W
char:Q
 Input correct 

Nine 、shell Process control -while Loop statement

One 、 Use scenarios

Clearly know the number of cycles, use for, Use when you don't know the number of cycles while

Two 、while Cyclic grammar

while [ condition ]  # Be careful ： Condition is true while Will be recycled , The condition is false ,while Stop the cycle ,** If there is a variable in the condition, enclose the variable name in quotation marks **.
 do
  commands
done

3、 ... and 、 example

#!/bin/bash
#while Basic grammar learning 
# The condition can be any of the five kinds of transportation we learn 
# Mathematics compares transportation   String comparison   file type   Logical transportation   The assignment operation 

# Enter a number , If this number is greater than 0, Then output “ Greater than ”
read -p "NUM:" num1
while [ $num1 -gt 0 ]
 do
  echo " Greater than "
  sleep 3
done



# Enter a string , If the input string is “root“, Then jump out of the loop 
read -p "login:" account
while [ $account != 'root' ]
 do
  read -p "login:" account
done


# If home There is no jjl Folder , Then it will always output “not found /home/jjl”
while [ ! -d /home/jjl ]
 do
  echo "not found /home/jjl"
  sleep 3
done


# If money Less than 100000,car_num Less than 1,house Less than 1, Then continue the cycle 
read -p "money:" money
read -p "car:" car_num
read -p "house:" house
while [  $money -lt 100000 ] || [  $car_num -lt 1 ] || [  $house -lt 1 ]
  do
    echo "not"
    read -p "money:" money
    read -p "car:" car_num
    read -p "house:" house
done
echo "ok"

Ten 、shell Process control -until Loop statement

One 、until Introduce

and while Just the opposite ,until If the condition is false, start execution , If the condition is true, stop execution

Two 、until grammar

until [ condition ] # Be careful ： The condition is false until Just start the cycle , If the condition is true, stop the cycle
do
commands Code block
done