Shell script integer value comparison, logic test, if statement, extract performance monitoring indicators

1、 Conditional test operation

[  Conditional expression   ]

        (1) File test
        [ The operator   File or directory ]
-d:         Test whether it's a directory (Directory)
-e:         Test whether the directory or file exists (Exist)
-f:         Whether the test is a file ((File)
-r:         Test whether the current user has permission to read (Read),
-w:         Test whether the current user has permission to write (Write)
-x:         Test if executable (Excute ) jurisdiction .
 

Test directory /media Whether there is ,$? The return value is 0, Indicates that this directory exists

[ -d  /media/  ]

echo $?

ls -ld /media/

If $? Return value not 0, Indicates that this directory does not exist

[ -d /media/cdrom ]

echo $?

ls -ld  /media/cdrom

Test whether the directory exists , The output result is more intuitive “yes” Indicates that the directory exists

[ -d /media/ ] && echo "yes"

nothing “yes” The output indicates that the directory does not exist

[ -d /media/cdrom ] && echo "yes"

         （2）、 Integer comparison
According to the given two integer values , Judge the relationship between the first number and the second number
        [  Integers 1  The operator   Integers 2  ]
-eq:         The first number equals ((Equal) The second number .
-ne:         The first number is not equal to (Not Equal) The second number .
-gt:         The first number is greater than (Greater Than) The second number .
 -It:         The first number is less than (Lesser Than) The second number .
-le:         The first number is less than or equal to (Lesser or Equal) The second number .
-ge:         The first number is greater than or equal to (Greater or Equal) The second number .

 

  Judge the number of currently logged in users , When more than 5 Hour output “too many”

who |wc -l

unum=`who | wc -l`

[ $unum -gt 5 ] && echo "too many"

  Determine the currently available free memory （buffers/cache） size , When it is lower than 2048MB Output specific values

Freecc=$(free -m | grep "Mem:" |awk '{print  $6}')

[ $Freecc -lt 1024 ] && echo ${Freecc}MB

        (3)  String comparison
String comparisons are often used to check user input 、 Whether the system environment meets the conditions
[  character string 1  The operator   character string 2  ]
 =:         The first string is the same as the second string .
!=:         The first string is not the same as the second string . among ‘!“ The sign means reverse .
-z:         Check if the string is empty (Zero Variables that are not defined or assigned null values will be treated as empty strings .

User input “yes” or “no” To confirm a task

read -p "shi fou fu gai xian you wen jian (yes/no)?"  ACK        yes
[ $ACK =  "yes" ] && echo "fu gai"


read -p "shi fou fu gai xian you wen jian (yes/no)?"  ACK        no
[ $ACK = "no" ]  && echo "bu fu gai"

        （4） Logic test
Logical testing refers to judging the dependency between two or more conditions
        command 1 The operator   command 2
&&：        and
   ||：        perhaps
 ！：        Logical not , Take the opposite
 

Judge the present linux Whether the kernel version of the system is greater than 2.4.

View kernel

uname -r

Judge

A=$(uname -r | awk -F.  '{print  $1}')
B=$(uname -r | awk -F.  '{print  $2}')
[ $A -ge 3  ]  &&  [  $B -gt 4 ] && echo "fu he yaoqiu"

2、IF Structure of statement

         （1） Single branch if
  Only in ‘ Conditions established “ The corresponding code will be executed , Otherwise, nothing will be done .

Grammar format ：
if Conditional test operation
then
    Command sequence
fi

Determine the mount point directory , If it doesn't exist, it will be created automatically

vim cdrom.sh

 #!/bin/bash
MOUNT_DIR="/media/cdrom/"
if [ ! -d $MOUNT_DIR ]
then
        mkdir -p $MOUNT_DIR
fi


chmod +x cdrom.sh
./cdrom.sh

Judge whether the current user is root, If not, report an error and execute “exit  1” Exit script , No more code to execute

vim /opt/root.sh
#!/bin/bash
if [ "$USER" !="root" ]
then
        echo " error ： Not root user , Insufficient authority ！"
        exit 1
fi
fdisk -l /dev/sda


su lisi
exit 


chmod +x /opt/root.sh

 Switch user execution  su - user name    /opt/root.sh

 Switch again root User authentication 

        （2） Double branch if sentence
    Ask for ‘ Conditions established “‘ Conditions not established ” In both cases, different operations are performed .
Grammar format ：
if Conditional test operation
then
    Command sequence 1
else
  Command sequence 2
fi
Determine whether the target host is alive or not , Show test results

vim ping.sh

#!/bin/bash
ping -c 3 -i 0.2 -W 3 $1 &> /dev/null
if [ $? -eq 0 ]
then
echo "Host $1 is up."
else
echo "Host $1 is down."
fi


ifconfig ens33 192.168.1.10     （ Temporary modification IP Address ）

chmod +x ping.sh

 ./ping.sh 192.168.1.10        （ping Their own IP Address ）

Show is up. It means success

  Check vsftpd Whether the service is running , If running, list the listening address ,PID Number ; Otherwise output the prompt “ Warning ,vsftpd Service not available ”.（ Insert linux System installation CD , Install it. vsftpd software package ）
 

vim vsftpd.sh

#!/bin/bash
systemctl status vsftpd &> /dev/null
if [ $? -eq 0 ]
then
echo " Monitor address :$(netstat -anpt | grep vsftpd | awk '{print $4}')"
echo " process PID Number :$(pgrep -x vsftpd)"
else
echo " Warning :vsftpd Service not available !"
fi


chmod +x vsftpd.sh

 ./vsftpd.sh

Display when not started ： Warning vsftpd If the service is unavailable, it will succeed
Display at startup ： Listening port ：：：：21, process PID Number ：61072 The successful

（3） Multiple branches if sentence
    because if The statement can be established according to the test results 、 Do not perform operations separately , So it can be nested , Make multiple judgments .
Grammar format ：
if Conditional test operation 1
then
Command sequence 1
elif Conditional test operation 2
then
  Command sequence 2
else
  Command sequence 3
fi

Excellent students are distinguished according to the test scores entered , qualified , Unqualified third gear

vim gradediv.sh


#!/bin/bash
read -p " Please enter your score (0-100):" GRADE
if [ $GRADE -ge 85 ] &&  [ $GRADE -le 100  ]
then
        echo "$GRADE  branch , good !"
elif [ $GRADE -ge 70 ] &&  [ $GRADE -le 84  ]
then
        echo "$GRADE  branch , qualified !"
else
        echo "$GRADE  branch , unqualified !"
fi


chmod +x gradediv.sh

./gradediv.sh

3、 Extract performance monitoring indicators （ Disk usage 、CPU Use 、 Memory usage ）

        （1）  Use df The command extracts the disk occupancy of the root partition , Assign a value to a variable DUG.
                 Use mpstat Command extraction CPU Usage rate , Assign a value to a variable CUG.
                 Use free The command extracts the memory usage , Assign a value to a variable MUG.

vim /root/sysmon.sh

#!/bin/bash
#  Extract performance monitoring indicators （ Disk usage 、CPU Use 、 Memory usage ）
DUG=$(df -h | grep "/$" | awk '{print $5}' | awk -F% '{print $1}')
CUG=$(expr 100 - $(mpstat | tail -1 | awk '{print $12}' | awk -F. '{print $1}'))
MUG=$(expr $(free | grep "Mem:" | awk '{print $3}') \* 100 / $(free | grep "Mem:" | awk '{print $2}'))
#  Set the alarm log file 、 Alarm mailbox 
ALOG="/tmp/alert.txt"
AMAIL="root"
#  Judge whether the alarm is recorded 
if [ $DUG -gt 90 ]
then
    echo " Disk occupancy ：$DUG %" >> $ALOG
fi
if [ $CUG -gt 80 ]
then
    echo "CPU Usage rate ：$CUG %" >> $ALOG
fi
if [ $MUG -gt 90 ]
then
    echo " Memory usage ：$MUG %" >> $ALOG
fi
#  Judge whether to send alarm email , Finally, delete the alarm log file 
if [ -f $ALOG ]
then
    cat $ALOG | mail -s "Host Alert" $AMAIL
    rm -rf $ALOG
fi


chmod  +x  /root/sysmon.sh
/root/sysmon.sh

Disk usage 80%、CPU Usage rate 90%、 Memory usage 90% Will send to /tmp/alert.txt Send a warning email ,（ You can reduce the value as 2、3、4 Executing script tests ）

        （2） View the result of executing the command ,

mail

exit （ sign out ）

        （3） Set up crontab Planning tasks
 

systemctl   status  crond    （ green running  Indicates running ）


crontab –e
*/30 * * * * /root/sysmon.sh    （ Call every half an hour sysmon.sh Script program ）