Introduction to Shell programming
Symbols of references:
Single quotation mark: WYSIWYG
Double quotation marks: parse variables (treat the input as a whole)
Backquotes: parsing commands
No quotation marks: similar to double quotation marks
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Why learn Shell programming
Shell will be widely used in Linux system, and we also need to realize business automation in our work,
For example: automatic backup, monitoring and automatic installation services.Shell programming is a programming language that Linux operation and maintenance personnel must know. The simplest programming language.
Programming is also a necessary skill for operation and maintenance personnel:
Shell and Python are the skills that operation and maintenance personnel must possess -
What is Shell
Shell is a command interpreter (translator). The commands and scripts of the command line will be interpreted through the shell, passed to the operating system, processed and output to the user
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What is a Shell script
Programs and commands are executed in files. This file and script are called Shell script files
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Shell variable
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Global variable (environment variable)
It takes effect in the whole system, usually in uppercase letters. There are some such variables in the system by default to meet the needs of the system and program.
Example of system default environment variables:
[root@centos ~]#echo $PS1 [\[\e[34;1m\]\u@\[\e[0m\]\[\e[32;1m\]\H\[\e[0m\]\[\e[31;1m\] \w\[\e[0m\]]\$ [root@centos ~]#echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin [root@centos ~]#echo $HOME /root [root@centos ~]#echo $SHELL /bin/bash [root@centos ~]#echo $PWD /root
Custom environment variables:
Method 1:
[root@centos ~]#export JACK="I am Jack" [root@centos ~]#echo $JACK I am Jack
Method 2:
[root@centos ~]#ANDY="I am Andy" [root@centos ~]#echo $ANDY I am Andy [root@centos ~]#echo ${ANDY} #You can use {} to distinguish multiple variables I am AndyCenOS environment variable priority profile
/etc/profile ~/.bash_profile ~/.bashrc /etc/bashrc #It is recommended to put the environment variable into / etc/bashrc in the future, with the highest priority
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Local variable (ordinary variable)
Local variables are the most commonly used variables in Shell Scripting
Representation:
Variable name = value
a=1
Naming rules:
Letters, underscores, numbers, cannot start with numbers
= Assignment operator value Variable content (Quotation marks, single quotation marks, and double quotation marks)
Note: there must be no spaces on either side of the assignment symbol
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expression
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[] bracket expression
There must be spaces at both ends of the contents in brackets
The string should be enclosed in double quotation marks
[root@centos ~]#[ "a" = "b" ] && echo 1 || echo 0 0 [root@centos ~]#[ "a" = "a" ] && echo 1 || echo 0 1 If the expression holds, print 1, otherwise print 0
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Integer expression
1) There must be spaces at both ends of the integer expression character
2) There must be spaces at both ends of the brackets.
3) The contents of variables and comparisons do not need quotation marks.> -gt greater than greater than >= -ge greater equal Greater equal to < -lt less than less than <= -le less equal Little equal to = -eq equal be equal to ≠ -ne not equal Not equal to
[root@centos ~]#a=1 [root@centos ~]#[ $a -eq 2 ] && echo 1 || echo 0 0 [root@centos ~]#[ $a -eq 1 ] && echo 1 || echo 0 1 If the expression holds, print 1, otherwise print 0
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String expression
-n When the variable content length [not] is 0, it is true(The expression is correct)not zero -z True when the variable content length is 0(The expression is correct) zero a == a Are the strings the same```
Note: double quotation marks shall be added to the string
[root@centos ~]#jack="I am Jack" [root@centos ~]#echo $jack I am Jack [root@centos ~]#[ -z "$jack" ] && echo 1 || echo 0 0 [root@centos ~]#[ -n "$jack" ] && echo 1 || echo 0 1 by jack Assign a value, and then check whether the variable length is 0. If yes, print 0, otherwise print 1
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Shell judgment sentence
if judgment sentence grammar
if [ expression ] then command fi
[root@centos ~]#cat test.sh #!/bin/bash a=10 if [ $a -gt 1 ] then echo 1 fi [root@centos ~]#bash test.sh 1 #If the variable a is greater than 1, print 1If... Otherwise example:
[root@centos ~]#cat test.sh #!/bin/bash a=10 if [ $a -gt 1 ] then echo 1 else echo 0 fi [root@centos ~]#bash test.sh 1 #If the variable a is greater than 1, print 1, otherwise print 0Fun try:
[root@centos ~]#cat test.sh #!/bin/bash boy="potential share" if [ "$boy" == "potential share" ] then echo "Talk to you first, friend" else echo "In vain" fi [root@centos ~]#bash test.sh Talk to you first, friend[root@centos ~]#cat test.sh if [ $(date +%w) -eq 6 ] then echo "Let's travel together" else echo "Let's have a good class" fi [root@centos ~]#bash test.sh Let's have a good classRealize multiple conditions
Method 1:
[root@centos ~]#cat test.sh if [ $(date +%w) -eq 6 ] || [ $(date +%w) -eq 0 ] then echo "Let's travel together" else echo "Let's have a good class" fi [root@centos ~]#bash test.sh Let's have a good classMethod 2:
read Command to read user input interactively -p "Reminder:" -t "How long to wait for input"
[root@centos ~/shsh]#cat test.sh #!/bin/bash read -p "Please enter the instruction:" ps if [ "$ps" == "123" ] then echo "Correct instruction,Start the ship" elif [ "$ps" == "456" ] then echo "Special instruction,Go to earth" elif [ "$ps" == "789" ] then echo "Super command, go home" else echo "Instruction error,Start self destruction" fi [root@centos ~/shsh]#bash test.sh Please enter the instruction: 123 Correct instruction,Start the ship [root@centos ~/shsh]#bash test.sh Please enter the instruction: 456 Special instruction,Go to earth [root@centos ~/shsh]#bash test.sh Please enter instruction: 789 Super command, go home [root@centos ~/shsh]#bash test.sh Please enter the instruction: 000 Instruction error,Start self destruction -
Exercise: Shell programming to compare the size of two integers
Method 1:
[root@centos ~]#bash daxiao.sh Please enter two numbers: 11 22 11<22 [root@centos ~]#bash daxiao.sh Please enter two numbers: 22 11 22>11 [root@centos ~]#bash daxiao.sh Please enter two numbers: 11 11=11 [root@centos ~]#cat daxiao.sh #!/bin/bash read -p "Please enter two numbers:" a b if [ $a -gt $b ] then echo "$a>$b" elif [ $a -eq $b ] then echo "$a=$b" else echo "$a<$b" fiMethod 2:
Special location variables: $1 The first parameter of the script file is assigned to $1 $2 The second parameter of the script file is assigned to $2 [root@centos ~]#cat b.sh echo $1 $2 [root@centos ~]#bash b.sh b1 b2 b1 b2 #b1 is the output of $1, and b2 is the output of $2 #Assign b1 to $1 and b2 to $2 ()
[root@centos ~]#cat daxiao.sh #!/bin/bash a=$1 b=$2 if [ $a -gt $b ] then echo "$a>$b" elif [ $a -eq $b ] then echo "$a=$b" else echo "$a<$b" fi [root@centos ~]#bash daxiao.sh 11 22 11<22 [root@centos ~]#bash daxiao.sh 22 11 22>11 [root@centos ~]#bash daxiao.sh 22 22 22=22 After the script, enter the number directly and compare the values -
for loop
Syntax:
for n in Value list do Execute command doneExample:
[root@centos ~]#cat for.sh #!/bin/bash for n in 1 2 3 4 5 do echo "$n" done [root@centos ~]#bash for.sh 1 2 3 4 5 [root@centos ~]#bash -x for.sh # -x view execution process + for n in 1 2 3 4 5 + echo 1 1 + for n in 1 2 3 4 5 + echo 2 2 + for n in 1 2 3 4 5 + echo 3 3 + for n in 1 2 3 4 5 + echo 4 4 + for n in 1 2 3 4 5 + echo 5 5bash -x view execution
Sequence for loop
[root@centos ~]#cat for.sh #!/bin/bash for n in {1..5} do echo "$n" done [root@centos ~]#bash -x for.sh + for n in '{1..5}' + echo 1 1 + for n in '{1..5}' + echo 2 2 + for n in '{1..5}' + echo 3 3 + for n in '{1..5}' + echo 4 4 + for n in '{1..5}' + echo 5 5 [root@centos ~]#bash for.sh 1 2 3 4 5Advanced: print 254 IP addresses 192.168.0.1-192.168.0.254
[root@centos ~]#cat forip.sh #!/bin/bash for n in {1..254} do echo "192.168.0.$n" done[root@centos ~]#cat forip.sh #!/bin/bash for n in 192.168.0.{1..254} do echo $n done -
Script writing habits:
1. Take sh end.
2. The first line at the beginning of the script #/ Who explains the contents of the bin/bash script.Since bash is the default under Linux, this line can also be omitted.
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After class practice
1. Compare integer sizes.
2. Judge the date, travel every Saturday and Sunday, and have classes at other times
3. Take out the Ip address of the current system and judge whether it is 192.168.0.111. If the prompt is correct, if not, give a prompt
[root@centos ~/shsh]#cat ip.sh #!/bin/bash ipadd=$(ifconfig eth0 | sed -nr 's#^.*inet (.*)net.*$#\1#gp') if [ $ipadd == "192.168.0.111" ] then echo "system IP 192.168.0.111 Verification passed" else echo "system IP Verification failed, current IP Address: $ipadd" fi4. Print 192.168.0.100 – 192.168.0.120 IP addresses. When the IP address is the system IP address, give a prompt that an IP is the system IP
#!/bin/bash for n in 192.168.0.{100..120} do echo "$n" if [ $n == "192.168.0.111" ] then echo "******$n Match successful, this IP For system IP address******" else echo "Match failed" fi done