Shell basics-day4-if and case condition judgment

1. The structure of the branch statement in the shell      

if 条件测试1; then            
    命令序列
elif  条件测试2; then
    命令序列      
elif 条件测试3; then              
    命令序列      
else 命令序列      
fi

2. Use if to install apache

#!/usr/bin/bash
#install apache
#v1.0 
gateway=192.168.122.1

ping -c1 www.baidu.com &>/dev/null
if [ $? -eq 0 ];then
    yum -y install httpd
    systemctl start httpd
    systemctl enable httpd
    firewall-cmd --premanent --add-service=http
    firewall-cmd --premanent --add-service=https
    firewall-cmd --reload
    sed -ri '/^SELINUX=/cSELINUX=disabled' /etc/selinux/config
    setenforce 0
elif ping -c1 $gateway &>/dev/null
    echo "check dns..."
else
    echo "check ip address"
fi

3. Case syntax structure

case 变量 in 
模式1)
    命令序列1
    ;;
模式2)
    命令序列2
    ;;
*)
    ;;
esac

       Note that the last one is not required;;, it should be noted that *) is similar to default in high-level languages. It should be noted that case can only be used for string pattern matching, and cannot be compared with the size of numbers.

4. Use case to delete users

Use if to delete, version one:

#!/usr/bin/bash
#delete user
#v1.0

read -p "Please input a username: " user

id $user &>/dev/null
if [ $? -ne 0 ]; then
    echo "no such user $user"
    exit 1
fi

read -p "Are you sure[y/n]: " action
if [ "$action" = "y" -o "$action" = "Y" -o "$action" = "yes" -o "$action" = "YES"]; then
    userdel -r $user
    echo "$user is deleted"
else
    echo "good!"
fi

 Use if to delete, version two:

#!/usr/bin/bash
#delete user
#v1.0

read -p "Please input a username: " user

case "$user" in
y|Y|yes|YES)
    userdel -r $user
    echo "$user is deleted!"
    ;;
esac

5. In the shell script, true and: have the same effect.

6. Landing on the springboard

      Supplementary knowledge: Mainly related to shell login, etc/profile; etc/bashrc (public); each user has his own profile and bashrc.

      If you are logging in with a password, you can write a script and place it under the login-related folder, and that's it.

      The method of using the secret key to log in has been introduced in the previous blog.