Shell script-the difference between for and while

The difference between for and while

The difference between row reading

#!/bin/bash
df -hl |awk 'int($5) >30' > df.txt
result=$(df -hl |awk 'int($5) >30')
echo "-------------- for遍历变量 ---------------------"
for i in $result
do
        echo $i
done

echo "--------------- while 遍历遍量 ------------------------------"
echo $result |while read line
do
        echo $line
done

echo "-------------- for 遍历文件 --------------------"
line=$(cat ./df.txt)
for i in line
do
        echo $line
done

echo "-------------- while 遍历文件 -------------------"
while read line
do
        echo $line
done <df.txt

The result of executing the above script is:

1. while loop: read the file by line, the default separator is space or Tab;

2. for loop: Read the file with spaces, that is, when you encounter a space, the loop body will be executed, so if you need to read in a line, you must convert the space into other characters.

while loop through files
for loop to traverse variables

Use different loops for different scenarios

Create user script from file

假设文件是ip.txt
maomao 123
zhuzhu 456

niuniu 789

If there are blank lines in the file, you need to use forCustom separator

#!/bin/bash
#v1.0 by stanZ 2020-12-15
if [ $# -eq 0 ];then
        echo "usage：`basename $0` file"
        exit 1
fi

if [ ! -f $1 ];then
        echo "error file"
        exit 2
fi

#希望for处理文件按回车分隔，而不是空格或者tab空格
#重新定义分隔符
#IFS内部字段分隔符
#IFS=$'\n'
IFS='
'

for line in `cat $1`
do
        user=`echo "$line" |awk '{print $1}'`
        passwd=`echo "$line" |awk '{print $2}'`
        id $user &>/dev/null
        if [ $? -eq 0 ];then
                echo "user $user already exists"
        else
                useradd $user
                echo "$passwd" |passwd --stdin $user &>/dev/null
                if [ $? -eq 0 ];then
                        echo "$user is created"
                fi
        fi
done

Use while

#!/bin/bash
#while create user
#v1.0 by stanZ 2020-12-17
if [ $# -eq 0 ];then
        echo "usage：`basename $0` file"
        exit 10
fi

if [ ! -f $1 ];then
        echo "error file"
        exit 5
fi

while read line
do
        user=`echo $line |awk '{print $1}'`
        pass=`echo $line |awk '{print $2}'`
        id $user &>/dev/null
        if [ $? -eq 0 ];then    
                echo "$user already exists"
        else
                useradd $user
                echo "$pass" |passwd --stdin $user &>/dev/null
                if [ $? -eq 0 ];then
                        echo "$user is created."
                fi
        fi
        done < $1

In summary, while processing files is more convenient than for

Scripts that count various states in the system

Count the number of various states of shells in /etc/passwd

#!/bin/bash
# 首先使用for
declare -A shells
shell=`cat /etc/passwd |awk -F: '{print $NF}'`
for i in $shell
do
        let shells[$i]++
done

for i in ${!shells[*]}
do
        echo "$i：${shells[$i]}"
done

# 执行脚本
[root@maomao 1.11]# bash shells1.sh 
/sbin/nologin：25
/bin/sync：1
/bin/bash：12
/sbin/shutdown：1
/sbin/halt：1

#!/bin/bash
# 使用while
declare -A shells
while read line
do
        shell=`echo $line |awk -F: '{print $NF}'`
        let shells[$shell]++

done </etc/passwd

for i in ${!shells[*]}
do
        echo "$i：${shells[$i]}"
done

# 执行脚本
[root@maomao 1.11]# bash shells1.sh 
/sbin/nologin：25
/bin/sync：1
/bin/bash：12
/sbin/shutdown：1
/sbin/halt：1

Count the number of TCP connection states

#!/bin/bash
# count tcp status
# by stanZ 1.3
while :
do
unset status	# 取消一次变量 不然数值会重叠
declare -A status
        type=$(ss -an|grep :80 |awk '{print $2}')
        for i in $type
        do  
                let status[$i]++
        done

        for j in ${!status[@]}
        do  
                echo "$j:${status[$j]}"
        done

        sleep 1
        clear

done