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Circular statements in shell programming
2022-07-02 01:03:00 【Hadron's blog】
The function of the loop statement
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 , Clear according to the server Single check the survival status of each host. When facing repeated tasks in various lists , Easy to use if Statement has been difficult to meet the requirements , And write all generations in sequence The code is extremely cumbersome 、 The difficulties
One ,for loop
1、 Grammatical structure
（1） List loop
（2） Loop without list
（3） class C Style for loop
Traverse
for Variable name in {list}
do
command
done
for i in {a..c}
do
echo $i
done
for i in {1..5} //{} Variables are not recognized inside
do
echo $i
done
for i in `seq 5` //seq 5=1、2、3、4、5 Sequence
do
echo $i
done
a=10
for i in `seq $a` //seq You can refer to variables
do
echo $i
done
for Variable name in a b c
do
command
done
for i in a b c //i There is actually no call here , So it's equivalent to in The following parameters will be cycled several times
do
echo 123
done
for i in a b c // Here we call the i The variable , So it displays normally i Value （a\b\c）
do
echo $i
done
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2. skill
for i in {1..50..2} 1-50 The odd number
for i in {2..50..2} 1-50 An even number of
for i in {10..1} 1-10 Reverse order
for i in $(seq 10) 1-10 Positive order
for i in $(seq 10 -1 1) 1-10 Reverse order
for i in $(seq 1 2 10) 1-10 The odd number , The middle is the step size
for i in $(seq 0 2 10) 1-10 An even number of , The middle is the step size
Example ：
for i in $(seq 0 2 10);do echo $i ;done
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3, Example
1. Print 1-5 this 5 A digital
vim for.sh
#!/bin/bash
for i in {1..5}
do
echo $i
done
2. Print 5 Time hello world
Be careful ： Although we define a variable i, But I didn't use it , It just controls the number of cycles
vim for.sh
#!/bin/bash
for i in {1..5}
do
echo hello world
done
3. Output 0-50 Even number between
vim for1.sh
#!/bin/bash
for i in {0..50..2} //..2 The representative step size is 2, every other 2 individual
do
echo $i
done
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4. class C Style for loop
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
1. Print 1-5
iteration
vim for.sh
#!/bin/bash
for ((i=1;i<=5;i++))
do
echo $i
done
2. Print 1-10 The odd number
[[email protected] ~]# vim for3.sh
#!/bin/bash
for ((i=1;i<=10;i+=2)) //i=i+2
do
echo $i
done
3. Script batch add users =====
#!/bin/bash
ULIST=$(cat /root/users.txt)
for UNAME in $ULIST
do
useradd $UNAME
echo "123456" | passwd --stdin $UNAME &>/dev/null
done
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notes ：i++ : i=1+1 Assign first and then calculate i=1 after Again +1
++i : 1+1=i Operation before assignment 1+1 after Again =i
Two ,while loop
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
1. Grammatical structure ：
while expression
do
command
done
1. Print 1-5
vim while.sh
#!/bin/bash
i=1
while [ $i -le 5 ]
do
echo $i
let i++ // Note that if this doesn't change $i Value , It's going to be a dead cycle
# i=$[$i+1] // Two ways of writing
done
echo " Last i The value of is : $i"
2. Output 1-100 Can't be 3 Divisible numbers
vim 3.sh
#!/bin/bash
i=1
while [ $i -le 100 ]
do
if [[ $i%3 -ne 0 ]]
then echo "$i"
fi
let i++
done
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2.while Dead cycle
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
while true
do
command
done
while :
do
command
done
Guess the number , Guess wrong and keep guessing
num=10
while true
do
read -p " Please enter a number ：" shu
if [ $shu -eq $num ];then
echo " You guessed it "
break
elif [ $shu -gt $num ];then
echo " You guessed big "
elif [ $shu -lt $num ];then
echo " You guess it's small "
fi
done
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3、 ... and ,until loop
1. 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
2. Dead cycle structure
until false
do
command
done
until [ 1 -ne 1 ]
do
command
done
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 , But I won't execute the following code in the loop body , Start and restart the next cycle
example 1： Print 1-5 The number of ,3 No printing
vim for4.sh
#!/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
interrupt , Stop this cycle immediately , Execute the extracorporeal code
example 2：1-10 The number of ,7 The back ones don't print
vim for4.sh
#!/bin/bash
for ((i=1;i<=10;i++))
do
if [ $i -eq 8 ];then
break
else
echo $i
fi
done
3、exit
Jump out of the program , It can be followed by a status return code, such as exit 1 wait
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
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