Case driven: a detailed guide from getting started to mastering shell programming

Reading tips ： It takes about 15 minute , This paper combines many practical cases , From getting started to mastering shell Programming , It takes a long time to learn , It is suggested to collect and learn slowly .

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One 、 summary

Shell It's a command line interpreter , Receive application user commands , To call the kernel of the operating system . It is also a programming language . As a command language , It interprets and executes commands entered by users interactively or automatically interprets and executes a series of preset commands . It is easy to write 、 Very flexible .

Shell Parser

Linux Provided Shell There are several parsers ：

cat /etc/shells 

stay centOS The default parser in is bash

echo $SHELL

Two 、 Introductory cases

shell Script to #!/bin/bash start （ Specify the parser ）

Case study ： Create a Shell Script , Output hello shell

First step ： To write shell Script

First create a helloShell.sh Script files , Then type in the following

#!/bin/bashecho "hellom shell"

The second step ： function shell Script

• The first one is ： use bash or sh+ Relative or absolute path of script （ Don't give script permission ）

sh helloShell.sh bash helloShell.shsh /root/Test/helloShell.sh bash /root/Test/helloShell.sh

• The second kind ： Execute the script using the absolute or relative path of the input script （ Must have executable permissions ）（ Without permission, the following error will occur ）
  

First of all, give helloworld.sh The script +x jurisdiction

chmod 777 helloShell.sh 

And then rerun

./helloShell.sh 

Case study ： stay /root/Test Create a zhangsan.txt The file of , Then add... To the file “ I love shell”.

The specific implementation is as follows ：

[[email protected] Test]# > demo1.sh[[email protected] Test]# vi demo1.sh  stay demo1.sh Enter the following in #!/bin/bashcd /root/Testtouch zhangsan.txtecho "I love shell" >>zhangsan.txt

Test run successful ：

3、 ... and 、Sell The variables in the

System variables

Common system variables $HOME、$PWD、$SHELL、$USER etc.

 View the value of the system variable 

Custom variable

Basic grammar

1. Defining variables ： Variable = value
2. Revoke variables ：unset Variable
3. Declare static variables ：readonly Variable , Be careful ： You can't unset

Other instructions

1. Variable names can be alphabetized 、 Numbers and underscores , But it can't start with a number , Environment variable name is recommended to be capitalized .

2. No spaces on both sides of equal sign

3. stay bash in , Variable default type is string type , No direct numerical operation

4. Value of variable if there is a space , Double or single quotes are required

Case study ：
1、 Defining variables S
2、 Revoke variables S
3、 Declare static variables A=2

Add ： Promote the variable to the global environment variable , For other purposes Shell Program usage .
grammar ：export Variable name

Case study ： Use shell Script output variables B

Special variables

$n

$n（ Function description ：n Is the number ,$0 Represents the script name ,$1-$9 Represents the first to ninth parameters , More than ten parameters , More than ten parameters need to be enclosed in braces , Such as${10}）

Case study ： Output the script file name 、 Input parameters 1 And input parameters 2 Value

#!/bin/bashecho "$0 $1 $2 "

$#

$# （ Function description ： Get the number of all input parameters , Commonly used in cycles ）

Case study ： Get the number of input parameters

#!/bin/bashecho $#

$*、[email protected]

$* （ Function description ： This variable represents all the parameters on the command line ,$* Treat all parameters as a whole ）

[email protected] （ Function description ： This variable also represents all the parameters on the command line , however [email protected] Treat each parameter differently ）

Case study ： Print all parameters entered

#!/bin/bashecho $*echo [email protected]

$？

$？ （ Function description ： Return status of last executed command . If the value of this variable is 0, Prove that the last command was executed correctly ; If the value of this variable is not 0（ Which number is it , It's up to the order itself ）, The last command was executed incorrectly .）

Case study ： Judge helloShell.sh Whether the script is executed correctly

Four 、 Operators and conditional judgments

Operator

Basic grammar

1. “$(( Arithmetic expression ))” or “$[ Arithmetic expression ]”
2. expr + , - , *, /, % .== Be careful ：expr Space between operators ==

Case study ： Use expr
1、 Calculation 3+2 Value

expr 2 + 3

2、 Calculation 2+3x4

expr `expr 2 + 3` \* 4

Case study ： Use $ Symbol
Calculation 2+3 ride 4

S=$[ (2+3) *4 ]

conditional

Basic grammar

[ condition ]（ Be careful condition Space before and after ） If the condition is not empty, it is true,[ lisi ] return true,[]  return false.

Common judgment conditions

• Compare two integers
  

Case study ： Compare 2 Greater than 1

[ 2 -gt 1 ]

• Judge according to the file authority
  

Case study ：helloShell.sh Whether you have write permission

[ -w helloShell.sh ]

• Judge according to the document type

Case study ：/root/Test/helloShell.sh Does the file in the directory exist

[ -e /root/Test/helloShell.sh ]

5、 ... and 、 Process control

if

Basic grammar

if [  Conditional judgment  ];then    Program  fi if [  Conditional judgment  ]   then      Program  fi

matters needing attention ：

• [ Conditional judgment ], There must be a space between the bracket and the conditional judgment
• if Space after

Case study ： Enter a number , If it is 1, The output the number is 1, If it is 2, The output the number is 2, If other , Output nothing .

#!/bin/bashif [ $1 -eq "1" ]then         echo "the number is 1"elif [ $1 -eq "2" ]then         echo "the number is 2"fi

case

Basic grammar

case $ Variable name  in   " value 1"）      If the value of the variable is equal to the value 1, Then execute the procedure 1     ;;   " value 2"）      If the value of the variable is equal to the value 2, Then execute the procedure 2     ;;   … Omit other branches …   *）      If none of the values of the variables are above , Then execute this procedure      ;; esac

matters needing attention ：

• case Line ending must be a word “in”, Each pattern match must be in right parenthesis “）” end .
• Double a semicolon “;;” Indicates the end of the command sequence , amount to java Medium break.
• final “*）” Represents the default mode , amount to java Medium default.

Case study ： Enter a number , If it is 1, The output the number is 1, If it is 2, The output the number is 2, If other , Output other number.

#!/bin/bashcase $1 in "1")        echo "the number is 1";;"2")        echo "the number is 2";;*)        echo "other number";;esac

while

Basic grammar

while [  Conditional judgment  ]   do      Program   done

Case study ： from 1 Add to 100

#!/bin/bashs=0i=1while [ $i -le 100  ]do        s=$[$s+$i]        i=$[$i+1]doneecho $s

for

for ((  Initial value ; Cycle control conditions ; Variable change  ))   do      Code   done  for  Variable  in  value 1  value 2  value 3…   do      Program    done

Case study ：
1、 Print all input parameters

#!/bin/bashfor i in $*do        echo "value is : $i"done 

2、 from 1 Add to 100

#!/bin/bashs=0for((i=0;i<=100;i++))do        s=$[$i+$s]doneecho $s

6、 ... and 、 Read console input

Basic grammar

read( Options )( Parameters ) -  Options ： - `-p： Specify the prompt when reading the value ;`	`-t： Specifies the time to wait while reading the value （ second ）` Parameters 	 Variable ： Specifies the variable name of the read value 

Case study ： Tips 5 Seconds , Read the name entered by the console

#!/bin/bashread -t 5 -p "input your name" NAMEecho $NAME

7、 ... and 、 function

System function

basename Basic grammar

basename [string / pathname] [suffix]  Function description ：basename The command will delete all prefixes including the last （‘/’） character , Then display the string . Options ：suffix For the suffix , If suffix Designated ,basename Will pathname or string Medium suffix Get rid of .

Case study ： Intercept this /root/Test/helloShell.sh File name of the path

basename /root/Test/helloShell.sh 

dirname Basic grammar

dirname  File absolute path 	 Function description ： Remove filename from given filename with absolute path （ Non catalog part ）, Then return to the rest of the path （ Part of the catalog ）

Case study ： obtain helloShell.sh Path to file

dirname /root/Test/helloShell.sh 

Custom function

Basic grammar

[ function ] funname[()]{	Action;	[return int;]}funname

Additional explanation ：

• Must be before calling function place , Declare function first ,shell The script is run line by line . It doesn't compile first like any other language .
• Function return value , Only through $? System variable acquisition , Can display plus ：return return , If not , Results will be run with the last command , As return value .return Heel value n(0-255)

Case study ： Calculate the sum of the two input parameters

#!/bin/bashfunction sum(){        s=0        s=$[ $1 + $2 ]        echo $s}read -p "input first number: " N1read -p "input second number: " N2sum $N1,$N2;