Shell -- circular statements (for, while, until)

One 、for loop

In practice , There are often situations where a task needs to be performed many times , And each time it is executed, it is only processed
Different objects , The other commands are the same . for example , Create a system account according to the name list in the address book , Check the survival status of each host according to the server list
When repeating tasks in the face of various lists , Easy to use if Statement has been difficult to meet the requirements , Writing all the code in sequence is extremely cumbersome 、 The difficulties

1、for Loop structure

• List loop
• Loop without list
• class c Style for loop

Format

for  Variable name  in {}
do
   echo $i
done
 give an example 
for i in {1..5}
do 
   echo $i
done
# perform 
1
2
3
4
5
 perhaps 
for i in `seq 5`  # You can introduce variables 
do 
   echo $i
done
# perform 
1
2
3
4
5

Take a chestnut ： use for loop , Output 10 An odd number within

for i in $(seq 1 2 10)
do 
echo $i
done
# perform 
1
3
5
7
9
#for i in $(seq 1 2 10)
1-10 The odd number , middle 2 Step length 

2、 class c Style for loop

Format

for ((expr1;expr2;expr3))
do
   command
done

• expr1: Define variables and assign initial values

• expr2: Decide whether to cycle or not

• expr3: Decide how the loop variable changes , Decide when the loop will exit

for example

for ((i=1;i<=5;i++))
do
  echo $i
done
 On the first output 1, Then start entering the loop , Each cycle will add 1, To 5 end  
 Execution results ：
1
2
3
4
5

notes ：i++ ：i=1+1 Let's start with the assignment i=1 after Again +1

​ ++i : 1+1=i Operation before assignment 1+1 after Again =1

Take a chestnut

1. Output 1-10 Odd sum of

sum=1
for ((i=1;1<=10;1+=2))
do 
  let sum=$sum+$i
done
echo "$sum"
# perform 
25

2. Random roll call

touch name       # First create a list file 
vim name          # Write name 
#!/bin/bash
w=$[RANDOM%5+1]    # Define random number variables , The random number is 1-5 Within the scope of （RANDOM% Random numbers are from 0 At first, therefore +1）

for i in $(grep -w $w ~/for/name)   
 do
     echo "$i"
done
# Then execute it 

3. Batch add users

#!/bin/bash
ULIST=$(cat /root/users.txt) 

for UNAME in $ULIST
do
useradd $UNAME
echo "123456" | passwd --stdin $UNAME &>/dev/null 
done

4. Volume delete user's script

vim udelfor.sh	

#!/bin/bash
ULIST=$(cat /root/users.txt) 
for UNAME in $ULIST
do
userdel -r $UNAME &>/dev/null 

done

Two 、while sentence

while Loops are generally used for conditional judgment loops , If the judgment condition is true , Then enter the cycle , When the condition is false, jump out of the loop

Format

while  expression 
do
  command
  done

for example

i=1
n=0
while [ $i - le 5 ]
do
   echo $i
   let i++    #/ Note that if this doesn't change $i Value , It's going to be a dead cycle 
   let n++
done
echo " The final value ：$i; loop ：$n Time "

Take a chestnut

• Output 1-100 Can't be 3 Divisible numbers

#!/bin/bash
i=1
while [ $i -le 100 ]    #i The value is less than 100
 do
 if [[ $i%3 -ne 0 ]]    # And i except 3 The value of the remainder is equal to 0
 then echo "$i"         # Can be output and displayed 
 fi
 let i++
done

1、while Dead cycle

Format

while [ 1 -eq 1 ]  // Write an expression that is always true ,1 be equal to 1 This condition is always true , So this script will continue to loop 
do
    command
done

Take a chestnut

• Guess the number game （ Guess wrong and keep guessing ）

#!/bin/bash
one=$[RANDOM%10+1]
i=0
while true 
do
read -p " Please enter 1-10 A number in ：" o
let i++
 if [ $i -gt 3 ];then
   if [ $o -eq $one ];then
    echo " The game failed , Next time try "
     break
        fi
else 
    if [ $o -eq $one ];then
       echo " congratulations , You guessed right in three times "
     exit
fi
fi  
  if [ $o -gt $one ];then
    echo " Guess the , Once again "
elif [ $o -lt $one ];then
    echo " Guess a little , Once again "   
 
  fi

done
  echo " How many times do you enter $i"

3、 ... and 、until loop

Follow while contrary , If the condition is false, enter the loop , If the condition is true, exit the loop

Grammatical structure

until  expression 
do
    command
done

• Calculation 1-50 And 1275 There are two ways to write the first

#!/bin/bash
i=0;s=0
until [ $i -eq 51 ] 
do
let s+=i    #s+=i, Equivalent to s=s+i, Use plus assignment 
let i++    
done
echo $s
# The second kind 
#!/bin/bash
i=1
sum=0
until [ $i -eq 51 ]   # Note here that if it is 50 If the condition is true, the loop ends and it calculates 1-49 And ,until [ $i -gt 50 ] It's OK 
  do
  sum=$[$i+$sum]
  let i++
  done
echo "$sum"

1、 Dead cycle structure

until false
do
    command
done

until [ 1 -ne 1 ]
do
    command
done

Take a chestnut

• Sign in zhangsan user Use root Send a message zhangsan

#!/bin/bash
username=$1
# Determine the format of information 
if [ $# -lt 1 ];then
  echo "Usage:`basename $0` <username> [<message>]" 
  exit 1
fi
# Judge whether the user exists 
if grep "^$username:" /etc/passwd >/dev/null ;then : 
else
   echo " The user doesn't exist "
   exit 1
fi

# Is the user online , If not online, every 5 Contact once a second 
until who|grep "$username" >/dev/null
do
	echo " The user doesn't exist "
        sleep 5
done

mes=$* 
echo $mes | write $username

notes ： Switch the user during the test

Four 、 Loop control statement

for Loops are generally executed together with conditional judgment statements and process control statements , Then you need to skip the loop and abort the loop , The commands to control the loop are as follows 3 individual

1、continue

continue It means to continue , but The following code in the loop body will not be executed 了 , Start Restart the next cycle

for example ： Print 1-5 The number of ,3 No printing

#!/bin/bash
for ((i=1;i<=5;i++))
do
        if [ $i -eq 3 ];then
        continue
        else
        echo $i
        fi
done

The result is 1245,3 No output , Because jump out of the back echo Statement executes the next loop

2、break

break Indicates interruption , Stop this cycle immediately , Execute the extracorporeal code

for example ：1-10 The number of ,7 The back ones don't print

#!/bin/bash
for ((i=1;i<=10;i++))
do
        if [ $i -eq 8 ];then
        break
        else
        echo $i
        fi
done

3、exit

exit Express Jump out of the program , It can be followed by a status return code, such as exit 1 wait

for example ：1-5 The number of , To 3 Then exit the loop with a return code exit 100

for i in {1..5}
do
if [ $i -eq 3 ];then
        exit  100 
else
        echo $i
fi

done
echo hi

Jump out of the program directly, so the final... Will not be executed echo hi, And the return code is 100 adopt $? see