Shell script -- condition judgment

Logic control — conditional

One 、 Realization of condition judgment

• if
• case

Two 、if The simple structure of

if  Conditions  ; then
	 Actions performed 
	 Actions performed 
fi

1、 The way conditions are written

• shell command
  • Judge the execution status code of the command
• [ expression ]
  • Number expression
  • Character expression
  • File directory expression

1)、 Number expression

• [ Numbers -eq Numbers ] be equal to
• [ Numbers -ne Numbers ] It's not equal to
• [ Numbers -gt Numbers ] Greater than
• [ Numbers -ge Numbers ] Greater than or equal to
• [ Numbers -lt Numbers ] Less than
• [ Numbers -le Numbers ] 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 " The disc has been 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 " The CD has been successfully mounted $mount_name"
fi

• Automatically create users

#!/bin/bash/
#

read -p " Please enter a 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 users UID And group ID Is it consistent

#!/bin/bash/
#

read -p " Please enter a 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 usage exceeds 60%！！！"
else
        echo " The current directory usage is ：${usage}%,  The root directory usage rate does not exceed 60%"
fi

2)、 Character expression

• == The two characters are equal
• ！= Unequal
• [ -z character ] Judge whether the character is empty
• stay shell The space in the script also represents a certain character , If you want to if Add spaces in sentence condition judgment , Need to put “ conditional ” quotation marks

（1） Character expression script example

#!/bin/bash
#

read -p " Please input a password ：" pwd1
read -p " Confirm the password ：" pwd2
if [ "$pwd1" == "$pwd2" ]; then
        echo " Password changed successfully ！"
else
        echo " Password inconsistency ！"
fi    

3) File directory expression

• [ -e File directory name ] Determine if the file directory exists
• [ -f File name ] Judge text file
• [ -d File name ] Determine whether it is a directory
• [ -b File name ] Determine 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 support the use of ! Take the opposite [ ! -e /opt/file01 ]

2、 Single branch if

if  Conditions ; then
     Actions performed 
     Actions performed 
else
	 Actions performed 
	 Actions performed 
fi

3、 Multiple branches if

if  Conditions 1; then
	 Actions performed 
	 Actions performed  
elif  Conditions 2; then 
	 Actions performed 
	 Actions performed 
elif  Conditions 3; then 
	 Actions performed 
	 Actions performed 
else
	 Actions performed 
	 Actions performed 
fi 

1)、 Many conditions

• Note that as long as one of the conditions meets the conditions, it will exit

• and also

• • [ Conditions 1 -a Conditions 2 ]
  • [ Conditions 1 ] && [ Conditions 2 ]

• or perhaps

• • [ Conditions 1 -o Conditions 2 ]
  • [ Conditions 1 ] || [ Conditions 2 ]

（1） Example of multi condition script

• Judge user input

  Linux, linux Show redhat

  windows, Windows, Show Microsoft

  mac, MAC, Show apple

  Show others

#!/bin/bash
#
echo "---- Select the following system ----"
echo "Linux、linux"
echo "windows、Windows"
echo "mac、MAC"
echo "----------------------"
read -p " Please enter 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、 nesting if

• Applicable to multi-level judgment

if   Conditions ; then
      if   Conditions ; then
              Actions performed 
              Actions performed 
      else
              Actions performed 
              Actions performed 
      fi
else
         Actions performed 
         Actions performed 
fiw

1) Nested script example

• Delete blank lines and comment lines of 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 conditional

• 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）、 Positional variable

• $1 $2 $3 $4…$9 ${10}

  • $1 The first parameter of the command ,$2 The second parameter
  • $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  Successful launch "
 20                 else
 21                         echo "Nginx  Boot failure "
 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 failure "
 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  Boot failure ！！"
 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  Boot failure ！！"
 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 cancel the boot auto start "
103                 else
104                         echo " Failed to cancel the boot auto start ！！！"
105                 fi
106         else
107                 echo "Nginx  At present, the machine does not start automatically "
108         fi
109         ;;
110         *)
111         echo " help ：$0 <status|start|stop|reload|restart>"
112         exit 101
113         ;;
114 esac