Original: https://blog.csdn.net/weixin_46659843/article/details/124139867
1. single branch if conditional statement
then followed by the program executed after the conditions are met. You can put it after [], and use; separate. You can also write on a new line, so you do not need ";" Yes.
For example:
if [ Conditional judgement ]
then
program
fi
1.1 example: determine whether the directory exists, and create it if it does not exist
#!/bin/bash
#date: 2022-4-13
#Description: example of if single branch statement to judge whether the directory exists
read -p "Please enter the directory to judge:" name
if [ ! -d $name ]
then
echo "$name Directory does not exist, creating..."
mkdir -p $name
echo "$name Directory creation completed."
fi
echo "$name Directory already exists, exiting..."
- Double branch if conditional statement
if [ Conditional judgement ]
then
Procedures to be implemented when conditions are satisfied.
else
Another procedure to be performed when the condition is not satisfied.
fi
2.1 example: monitor and automatically restart apache service scripts
In daily work, the services on the server are often down. If we do not monitor the server well, the service in the server will be down, but the administrator does not know. This is where we can write a script to listen to local services. If services are stopped or down, they can be restarted automatically. Take apache for example:
First, the port scan command and nmap port scan command are introduced,
Format: nmap -sT domain name or IP
Sub options:
-s scan -T scan all open TCP ports
The port displayed after the nmap scan must be alive.
To use the nmap command in the script, first install nmap using Yum -y install.
apache services are also installed using yum.
[root@xiaopeng ~]# cat autostart.sh
#!/bin/bash
port=$(nmap -sT 192.168.22.222 | grep tcp | grep http | awk '{print $2}')
if [ "$port" == "open" ]
then
echo "$(date) httpd is ok!" >> /tmp/autostart-acc.log
else
/etc/rc.d/init.d/httpd start &> /dev/nullecho "$(date) restart httpd!!" >> /tmp/autostart-err.log
fi
(first, use the nmap command to check whether apache is enabled and assign a value to port.
Then make conditional judgment. If the service is enabled, output the current time + httpd is ok to / tmp/autostart-acc.log.
If the value of the variable port is not open, perform the operation under else. First, start the apache service, output the post startup information to the bit bucket, and then click / TMP / autostart err Log. In this script, the nmap command uses
The IP lookup port does not refer to DNS, so an error that DNS does not exist will be reported, but the result will not be affected.)
- Multi branch if conditional statement
if [ Conditional judgment formula 1 ]
then
When conditional judgment formula 1 is true, execute procedure 1.
elif [ Conditional judgment formula 2 ]
then
When conditional judgment 2 is true, execute procedure 2.
......(More conditions can be added)
else
When all conditions are not satisfied, this procedure shall be executed finally.
fi
3.1 example: judge whether the user has entered a file or a directory
#!/bin/bash
#date:2022-4-13
#Description: judge file type
read -p "Please enter a file:" file
if [ -z $file ]
then
echo "Error! The file entered is empty."
elif [ ! -e $file ]
then
echo "Error! The file you entered does not exist."
elif [ -f $file ]
then
echo "$file Is a normal file"
elif [ -d $file ]
then
echo "$file Is a directory"
else
echo "$file Is a different type of file"
fi
4.case conditional statement
Multi branch case conditional statement
case $Variable name in
"Value 1 ")
If $If the variable is equal to the value 1, program 1 is executed
；；
"Value 2 ")
If $Variable equals value 2, program 2 is executed
；；
....ellipsis...
*)
If $Execute this procedure if the value of the variable is not the above value
；；
esac
4.1 example: create a startup script and let the service command manage apache
[root@xiaopeng htdocs]# vim /etc/init.d/apached
#!/bin/bash
# chkconfig: 2345 64 36
# description: A very fast and reliable SQL database engine
httpd=/usr/local/apache2/bin/apachectl
case $1 in
start)
$httpd start
;;
stop)
$httpd stop
;;
restart)
$0 stop
sleep 0.05
$0 start
;;
configtest)
$httpd -t
;;
*)
echo "usage:$0 start|stop|restart|configtest."
;;
esac
4.2 example: create a startup script and let the service command manage nginx
[root@xiaopeng conf]# vim /etc/init.d/nginx
#!/bin/bash
#Author: liu
#chkconfig: 2345 99 33
#description: nginx server control tools
ngxc="/usr/local/nginx/sbin/nginx"
ngxc_fpm="/usr/local/php/sbin/php-fpm"
case "$1" in
start)
$ngxc -t &> /dev/null
if [ $? -eq 0 ];then
$ngxc
$ngxc_fpm
echo "nginx service start success!"
else
$ngxc -t
fi
;;
stop)
$ngxc -s stop
killall php-fpm
echo "nginx service stop success!"
;;
restart)
$0 stop
$0 start
;;
reload)
$ngxc -t &> /dev/null
if [ $? -eq 0 ];then
$ngxc -s reload
pkill -HUP php-fpm
echo "reload nginx config success!"
else
$ngxc -t
fi
;;
*)
echo "please input stop|start|restart|reload."
exit 1
esac