Shell basics-day5-loop syntax

1, for loop     

for 变量名 [ in 取值列表 ]
do
    循环体
done

By default, the for loop separates the variable values ​​by spaces. Note that the number of loops for the for loop is fixed.

for val in a b c
do
    echo $val
done

        Note that the above script will output abc

The following example realizes that for creating multiple users, you need to create a file. The format of each line in the file is: user name password

#!/usr/bin/bash
#判断脚本是否有参数
if [ $# -eq 0 ]; then
    echo "usage: `basename $0` file"
    exit 1
fi

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

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

2. The while loop

while 条件测试
do
    循环体
done

     Because for uses a space as a delimiter, while while can directly use a carriage return as a delimiter, when the file needs to be processed, the priority is while instead of for.

#!/usr/bin/bash
#while create user

#会从文件user.txt里面读入一行给user
#当读到文件尾时，read line不会成功
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 $user already exists"
    else
        useradd $user
        echo "$pass" |passwd --stdin $user &>/dev/null
        if [ $? -eq 0 ];then
            echo "$user is created"
        fi
    fi 
done < user.txt