Article Directory
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Table of Contents
Article directory
Preface
1.if statement syntax
double branch structure
multi-branch structure
case analysis
Summarize
Foreword
In the command connection symbols & amp; & amp; and || have their own judgment conditions. This is the condition judgment learned in the shell basics. In script writing, the shell script has its own condition judgment for process control.
一.if statement syntax
Grammar 1:
# Syntax 1:
if <conditional expression>
then
instruction
fi
Syntax 2:
#Syntax 2:
if <conditional expression>;then
instruction
fi
Double branch structure
grammar
if <conditional expression>
then
Instruction sequence 1
else
Instruction sequence 2
fi
Multi-branch structure
if conditional expression 1
then
Instruction sequence 1
elif conditional expression 2
then
Instruction sequence 2
else
Instruction sequencen
fi
Case Analysis
Case 1: Write the script choice1.sh, use a single branch structure to input 2 integers and output the maximum value
[root@server ~]# vim choice1.sh
#!/bin/bash
read -p "Please enter the first integer: " x
read -p "Please enter the second integer: " y
max=$x
if (($max<y))
then
max=$y
fi
echo "Maximum value: $max"
Case 2: Interview question, write the script choice2.sh to determine the remaining capacity of the system memory. If it is less than 100M, send a message to alert you.
#!/bin/bash
free_mem=$( free -m | grep Mem | tr -s
" " | cut -d " " -f4 )
if (($free_mem<100))
then
echo "Warning, remaining memory: $free_mem, less than
Warning Line 100MB"
else
echo "Remaining memory: $free_mem, enough space."
fi
# free -m: Indicates displaying memory, that is, virtual memory information, -m displays numbers in MB units
# grep Mem: means filtering the lines containing Mem
# tr -s " ": compress spaces into one
# cut -d " " -f4: Use space as the separator and take the 4th part
Case 3: Write the script choice3.sh to determine the executor of the current script. If it is not the root account, it will prompt and exit.
# 4 ways to check the current account
[root@server ~]# whoami
root
[root@server ~]# id -u
[root@server ~]# echo $USER
root
[root@server ~]# echo $UID
[root@server ~]# vim choice3.sh
#!/bin/bash
if [ "$USER" != "root" ]
then
echo "please switch user root"
fi
[root@server ~]# bash choice3.sh # Execute under root account
[root@server ~]# pwd
/root
[root@server ~]# mv choice3.sh / # To allow ordinary accounts to access the script, it needs to be moved to /
[root@server ~]# chmod + x /choice3.sh # Grant execution permissions
[root@server ~]# ll /choice3.sh
-rwxr-xr-x 1 root root 81 June 18 10:10 /choice3.sh
[root@server ~]# tail -1 /etc/passwd # View common account names
fox:x:1000:1000::/home/fox:/bin/bash
[root@server ~]# su fox #Switch account
[fox@server root]$ cd /
[fox@server /]$ bash choice3.sh
please switch user root
Case 4: Write the script choice4.sh to realize leap year judgment
[root@server ~]# vim choice4.sh
#!/bin/bash
read -p "Please enter a 4-digit integer year: " year
if [ -n "$year" ] # Non-empty test
then
if ((year%4==0)) & amp; & amp; ((year 0!=0))
|| ((year@0==0))
then
echo "leap year"
else
echo "Ping Nian"
fi
else
echo "Please enter a valid 4-digit integer year"
fi
Case 5: Write the script choice5.sh to determine whether sshd is running
# Analysis
#Judge based on the number of processes
[root@server ~]# ps -ef | grep sshd | grep -v grep | wc -l
#Judge based on port number
[root@server ~]# netstat -lntup | grep 22 | wc -l
[root@server ~]# vim choice5.sh
#!/bin/bash
num=$(ps -ef | grep sshd | grep -v grep |
wc -l)
if (($num>0))
then
echo "sshd is running"
else
echo "sshd is not running"
fi
Case 6: Interview question, write script choice6.sh to determine whether the host is alive
[root@server ~]# vim choice6.sh
#!/bin/bash
read -p "Please enter the IP address of the test host:" ip
ping -c 2 -w 3 $ip &> /dev/null
# -c 2 means sending 2 data packets, -w 3 means waiting for 3 seconds to end, note
Note: You cannot wait for 1 second to end. It is possible that the second data packet has not been pinged.
When it is over, an incorrect status code will be returned.
if [ $? -eq 0 ]
then
echo "Host $ip is already running"
else
echo "Host $ip is not running"
fi
# Modify the above example, use loop to test whether multiple hosts are alive, and set the display color
color
[root@server ~]# vim choice6.sh
#!/bin/bash
for ip in 192.168.48.{125..135}
do
ping -c 2 -w 3 $ip &> /dev/null
if [ $? -eq 0 ]
then
echo -e "\e[1;31m!!Host $ip
Run!!\e[0m" # Highlight, red
else
echo "Host $ip is not running"
fi
done
Case 7: Write the script choice7.sh, enter the hundred-point score, and determine the grade
[root@server ~]# vim choice7.sh
#!/bin/bash
read -p "Please enter the percentage score: " score
if [ -z $score ]
then
echo "Not entered, please re-enter"
elif [ $score -lt 0 -o $score -gt 100 ]
then
echo "Incorrect score input, please enter a number between 0-100"
elif[$score-ge90]
then
echo "Excellent results"
elif[$score-ge80]
then
echo "Good grades"
elif [ $score -ge 60 ]
then
echo "passing grade"
else
echo "make-up exam"
fi
Case 8: Write a script to determine whether the user input is numbers, letters, or other characters.
[root@server ~]# vim choice8.sh
#!/bin/bash
read -p "Please enter numbers, letters, symbols: " str
if echo $str | grep [a-zA-Z] > /dev/null
then
echo "letter"
elif echo $str | grep [0-9] > /dev/null
then
echo "number"
else
echo "symbol"
fi
Case 9: Write the script choice9.sh to determine the current host CPU manufacturer
[root@server ~]# vim choice9.sh
#!/bin/bash
vendor=$(grep "vendor_id" /proc/cpuinfo |
uniq | cut -d " " -f2)
if [ $vendor == GenuineIntel ]
then
echo "Intel"
elif [ $vendor == AuthenticAMD ]
then
echo "AMD"
else
echo "other"
fi
- The case statement will compare the value of the variable with the value in ) brackets. If it is equal to a certain value, the corresponding statement will be executed. When the “;;” symbol is encountered, the case statement is jumped out and the statement following the esac statement is executed. If there is no match with any value, execute the set of statements following *)
Case 10: Write the script choice10.sh to rewrite the hundred-point score judgment grade in Example 7 above.
[root@server ~]# vim choice10.sh
#!/bin/bash
read -p "Please enter the percentage score: " score
case $score in
9[0-9]|100)
echo "Excellent results"
;;
8[0-9])
echo "Good grades"
;;
6[0-9]|7[0-9])
echo "passing grade"
;;
*)
echo "make-up exam"
esac
Summary
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