Logic control - condition judgment
1, Realization of conditional judgment
- if
- case
2, Simple structure of if
if condition ; then Actions performed Actions performed fi
1. Writing of conditions
- shell command
- Determine the execution status code of the command
- [expression]
- Numeric expression
- Character expression
- File directory expression
1) , numeric expression
- [number -eq number] equals
- [number ne number] is not equal to
- [number gt] greater than
- [number -ge number] greater than or equal to
- [number - lt number] less than
- [number -le number] less than or equal to
(1) Example of numeric expression script
- Auto mount CD
#!/bin/bash
#
mount -a &> /dev/null
if df -Th | grep "iso9660" &> /etc/null ; then
mount_dir=$(df -Th | grep "iso9660" | awk '{print $7}')
echo "Disc mounted to $mount_dir"
else
sed -ri '$a \/dev/sr0 /mnt/ iso9660 defaults 0 0' /etc/fstab
mount -a &> /dev/null
mount_name=$(df -Th | grep "iso9660" | awk '{print $1}')
echo "Successfully mounted the disc $mount_name"
fi
- Automatically create users
#!/bin/bash/
#
read -p "Please enter user name:" user
read -p "Please input a password" passwd
grep "^$user:" /etc/passwd &> /etc/null
if [ $? -ne 0 ]; then
useradd $user
echo $passwd | passwd --stdin $user &> /etc/null
echo "$user User created successfully"
else
echo "$user Already exists"
fi
- Judge whether the user UID is consistent with the group ID
#!/bin/bash/
#
read -p "Please enter user name:" user
grep "^$user:" /etc/passwd &> /etc/null
id_num=$(id $user -u)
grp_num=$(id $user -g)
if [ $id_num -eq $grp_num ]; then
echo "Good,user ID And group ID agreement!"
else
echo "No,user ID And group ID atypism!"
fi
- Get an alarm if the disk utilization exceeds
#!/bin/bash
#
usage=$(df -Th | grep "/$" | awk '{print $6}' | awk -F% '{print $1}')
if [ $usage -ge 60 ]; then
echo "Warning! Warning! The current directory usage is: ${usage}% ,Directory disk utilization exceeds 60%!!!"
else
echo "The current directory usage is: ${usage}%, The root directory usage rate does not exceed 60%%"
fi
2) , character expression
- ==Two characters are equal
- != Unequal
- [- z character] judge whether the character is empty
- In the shell script, a space also represents a character. If you want to add a space to the conditional judgment of the if statement, you need to add double quotation marks to the "conditional judgment"
(1) Character expression script example
#!/bin/bash
#
read -p "Please input a password:" pwd1
read -p "Confirm password:" pwd2
if [ "$pwd1" == "$pwd2" ]; then
echo "Password modification succeeded!"
else
echo "The password is inconsistent!"
fi
3) File directory expression
- [- e file directory name] judge whether the file directory exists
- [- f file name] judge text file
- [- d file name] determines whether it is a directory
- [- b file name] determines whether it is a block device file
- [- l file name]
- [- s file name]
- [- r file name] [- w file name] [- x file name]
All monocular expressions are supported! Reverse [! - e /opt/file01]
2. Single branch if
if condition; then
Actions performed
Actions performed
else
Actions performed
Actions performed
fi
3. if multi branch
if Condition 1; then Actions performed Actions performed elif Condition 2; then Actions performed Actions performed elif Condition 3; then Actions performed Actions performed else Actions performed Actions performed fi
1) . multi conditional writing
- Note that multiple conditions. As long as one of them meets the conditions, it will exit
-
and also
-
- [condition 1 -a condition 2]
- [condition 1] & & [condition 2]
-
Or or
-
- [condition 1 -o condition 2]
- [condition 1] | [condition 2]
(1) Multi condition script example
-
Judge user input
Linux, linux displays redhat
windows, Windows, display Microsoft
mac, MAC, display apple
Show other
#!/bin/bash
#
echo "----Select the following systems----"
echo "Linux,linux"
echo "windows,Windows"
echo "mac,MAC"
echo "----------------------"
read -p "Please enter the system:" system
if [ $system == linux -o $system == Linux ]; then
echo
echo "The system is: RedHat System"
elif [ $system == windows -o $system == Windows ]; then
echo
echo "The system is: Microsoft System"
elif [ $system == mac -o $system == MAC ]; then
echo
echo "The system is: Apple System"
else
echo "Other operating systems"
4. Nested if
- Applicable to multi-layer judgment
if condition; then
if condition; then
Actions performed
Actions performed
else
Actions performed
Actions performed
fi
else
Actions performed
Actions performed
fiw
1) Nested script example
- Delete empty lines and comment lines from the file
#!/bin/bash # read -p "Please enter the source file to filter:" file_name if [ -e $file_name ]; then if grep "^$" $file_name &> /dev/null; then Hnumber=$(grep -n "^$" $file_name) echo "The line number of the blank line is:" echo $Hnumber sed -i -e '/^$/d' -e '/^#/d' $file_name echo "The file after deleting the blank line is:" echo cat $file_name fi else echo "Source file does not exist!" fi
5. case condition judgment
- It is applicable to judge that a variable has multiple fixed values
case variable in
Value 1)
Actions performed
Actions performed
;;
Value 2)
Actions performed
Actions performed
;;
Value 3)
Actions performed
Actions performed
;;
*)
Actions performed
Actions performed
;;
esac
1) . location variable
-
$1 $2 $3 $4...$9 ${10}
- First argument of $1 command, $2 second argument
- $0 command itself
- $#
- Number of parameters
-
case script example
#!/bin/bash
#
if [ $# -eq 0 ]; then
echo "Command input error, help information: $0 <linux|Linux|windows|Windows|mac|MAC>"
exit 78
fi
case $1 in
linux|Linux)
echo "RedHat"
;;
windows|Windows)
echo "Microsoft"
;;
mac|MAC)
echo "Apple"
;;
*)
echo "Others"
;;
esac
- Location variable script example
1 #!/bin/bash 2 # 3 4 nginx_file=/usr/local/nginx/sbin/nginx 5 nginx_pid=/usr/local/nginx/logs/nginx.pid 6 7 if [ -z $1 ]; then 9 exit 100 10 fi 11 12 case $1 in 13 start) 14 if [ -e $nginx_pid ]; then 15 echo "Nginx Running" 16 else 17 $nginx_file 18 if [ $? -eq 0 ]; then 19 echo "Nginx Successfully started" 20 else 21 echo "Nginx Start failed" 22 fi 23 fi 24 ;; 25 stop) 26 if [ -e $nginx_pid ]; then 27 $nginx_file -s stop 28 if [ $? -eq 0 ]; then 29 echo "Nginx Successfully stopped" 30 else 31 echo "Nginx Stop failed" 32 fi 33 else 34 echo "Nginx Not running" 35 fi 36 ;; 37 reload) 38 kill -1 $(cat $nginx_pid) 39 if [ $? -eq 0 ]; then 40 echo "Nginx Successfully Reloaded" 41 else 42 echo "Reload failed" 43 fi 44 ;; 45 restart) 46 if [ -e $nginx_pid ]; then 47 $nginx_file -s stop 48 if [ $? -eq 0 ]; then 49 echo "Nginx Restarting" 50 else 51 echo "Nginx Start failed!!" 52 fi 53 sleep 1 54 $nginx_file 55 if [ $? -eq 0 ]; then 56 echo "Nginx Restart successful" 57 else 58 echo "Nginx Restart failed" 59 fi 62 $nginx_file -s stop 63 if [ $? -eq 0 ]; then 64 echo "Nginx Restarting" 65 else 66 echo "Nginx Start failed!!" 67 fi 68 sleep 1 69 $nginx_file 70 if [ $? -eq 0 ]; then 71 echo "Nginx Restart successful" 72 else 73 echo "Nginx Restart failed" 74 fi 75 fi 76 ;; 77 status) 78 if [ -e $nginx_pid ]; then 79 echo "Nginx PID's $(cat $nginx_pid) Running" 80 else 81 echo "Nginx Has stopped" 82 fi 83 ;; 84 start_auto) 85 chmod a+x /etc/rc.d/rc.local 87 if [ $? -eq 0 ]; then 88 echo "Writing to boot configuration file......" 89 source /etc/rc.d/rc.local 90 if [ $? -eq 0 ]; then 91 echo "Nginx Successful startup and self startup!" 92 fi 93 else 94 echo "Failed to write boot auto configuration file!!!" 95 fi 96 ;; 97 Cancel_startup) 98 if grep "nginx" /etc/rc.d/rc.local &> /dev/null; then 99 echo "Canceling startup" 100 sed -ri '/nginx/d' /etc/rc.d/rc.local 101 if [ $? -eq 0 ]; then 102 echo "Successfully canceled the startup and self startup" 103 else 104 echo "Failed to cancel startup and self startup!!!" 105 fi 106 else 107 echo "Nginx At present, it does not start automatically" 108 fi 109 ;; 110 *) 111 echo "help: $0 <status|start|stop|reload|restart>" 112 exit 101 113 ;; 114 esac
nginx/d' /etc/rc.d/rc.local
101 if [ $? -eq 0 ]; then
102 echo "successful cancellation of startup and startup"
103 else
104 echo "failed to cancel boot auto start!!!"
105 fi
106 else
107 echo "Nginx does not start automatically at present"
108 fi
109 ;;
110 *)
111 echo "help: $0 < status|start|stop|reload|restart >"
112 exit 101
113 ;;
114 esac