Shell script loop statement
1. for loop
1.1 Execution mechanism: Traversal
Execution mechanism: Assign the elements in the list to the "variable name" in sequence; execute the loop body once after each assignment; until the elements in the list are exhausted, the loop ends. If [in WORDS...] is omitted, the positional parameter variable is used at this
time in “$@”
1.2 Basic format
The first
for 已知循环次数
for tlj（变量） 循环次数（取值列表）
do
需要循环的事件
done
the second
for (( 表达式1; 表达式2; 表达式3 )); do 命令; done
for ((expr1;expr2;expr3))
do
command
done
expr1：定义变量并赋初值
expr2：决定是否循环
expr3：决定循环变量如何改变，决定循环什么时候退出
1.3 Script example
Print a column of question marks
#!/bin/bash
for i in {
1..9}
do
echo -e " ? "
done
print a square
#!/bin/bash
for j in {
1..9}
do
for i in {
1..9}
do
echo -e " * \c"
#\c换行
done
echo
#换行
done
multiplication table
#!/bin/bash
for j in {
1..9}
do
for i in `seq $j`
do
echo -e "${i}x${j}=$[i*j] \t\c"
#\t tab键可以对齐
done
echo
done
Method 1 for finding the sum of 1 to 10
#/bin/bash
sum=0
for i in {
1..10}
do
sum=$[sum+i]
let i++
done
echo "$sum"
Method 2 for adding up to 10
#!/bin/bash
sum=0
for i in {
1..100}
do
let sum=$i+$sum
#sum=$[i+sum]
#两种方法都可以
done
echo "$sum"
Add users in batches
[root@localhost opt]# vim add.sh
#!/bin/bash
ulist=$(cat /opt/user.txt)
for uname in $ulist
do
useradd $uname
echo "123123" |passwd --stdin $uname &>/dev/null
done
2. while loop: stop when judged to be false
Compared with for, we need to know the number of loops. We only know the stop condition. If we don’t know the number of times, we need to use while until the condition is reached.
2.1 Basic format
Repeatedly test a certain condition, as long as the condition is true, it will be executed repeatedly, and it will stop when the command is judged to be false
格式：
while [ 条件测试操作 ]
do #do代表循环的开始
判断式/命令序列
done #done代表循环的结束
2.2 Self-made mini games
Guess the price game
#!/bin/bash
p=`echo $[RANDOM%1000+1]`
time=0
while true
do
let time++
read -p "请输入您猜测的价格（1-1000）：" h
if [ $h -eq $p ]
then
echo "恭喜您猜中了，您一共猜测了$time次"
exit
elif [ $h -gt $p ]
then
echo "您猜测的价格过高"
else
echo "您猜测的价格过低"
fi
done
Method three for adding up to 10
#!/bin/bash
i=0
sum=0
while [ $i -le 100 ]
do
sum=$[i+sum]
let i++
done
echo $sum
3. Until loop: Stop when it is judged to be true.
3.1 Basic format
Repeatedly test a certain condition, as long as the condition is not true, it will be executed repeatedly
格式：
until [ 条件测试操作 ]
do
判断式/命令序列
done
3.2 Script example
Method 4 for adding up to 10
#!/bin/bash
sum=0
i=0
until [ $i -gt 100 ]
do
sum=$[sum+i]
let i++
done
echo "{1..100}的和：$sum"
4. Nested loops
Using another loop inside a loop is called nested loop
Format:
#!/bin/bash
for ((i=1;i<5;i++))
do
echo 此${
i}为外部循环
for((j=1;j<4;j++))
do
echo -e "\t此${j}为内部循环"
done
done
5. The use of break, exit and continue in the loop statement
5.1Use of break
When the condition is met, break will jump out of the current loop body
#!/bin/bash
for ((i=1;i<5;i++))
do
echo 此${
i}为外部循环
for((j=1;j<4;j++))
do
if [ $j -eq 3 ] <------如果j的值为3
then
break <------跳出当前循环（内部）
fi
echo -e "\t此${j}为内部循环"
done
done
5.2Use of exit
When the conditions are met, exit will directly exit the current script
#!/bin/bash
for ((i=1;i<5;i++))
do
echo 此${
i}为外部循环
for((j=1;j<4;j++))
do
if [ $j -eq 3 ] <------如果j的值为3
then
exit <------结束当前脚本
fi
echo -e "\t此${j}为内部循环"
done
done
5.3Usage of continue
continue aborts a command in a loop, but does not completely abort the entire command
#!/bin/bash
for ((i=1;i<5;i++))
do
echo 此${
i}为外部循环
for((j=1;j<4;j++))
do
if [ $j -eq 2 ]
then
continue
fi
echo -e "\t此${j}为内部循环"
done
done
