Getting started with shell Basics

Why study Shell Programming

1、Linux When the operation and maintenance engineer is managing the server cluster , You need to write Shell Program for Server Management .
2、 about javaEE and python For the programmer , The need for work , Your boss will ask you to write some Shell Script for program or server maintenance , For example, write a script to back up the database regularly .
3、 For big data programmers , You need to write Shell Program to manage clusters .

Shell What is it?

Shell Is a command interpreter , It provides a way for users to Linux The interface system program that the kernel sends a request to run the program , Users can use Shell To start up 、 Hang up 、 Stop even writing some programs .

Shell How the script is executed

• Script format requirements
  1、 Script to #!/bin/bash
  2、 Scripts need to have executable rights
• Common execution methods of scripts
  The way 1： Enter the absolute or relative path of the script
  explain ： First, give the script +x jurisdiction , Then execute the script
  such as ：./hello.sh Or use absolute path /root/shcode/hello.sh
  The way 2：sh+ Script
  explain ： Not scripted +x jurisdiction , Then execute the script
  such as ：sh hello.sh You can also use absolute paths

shell Variable

• Define variable rules
  Variable name without dollar sign
  Only English letters can be used for naming , Numbers and underscores , The first character cannot start with a number .
  No spaces in between , You can use underscores （_）.
  You can't use punctuation .
  Out of commission bash Keywords in （ You can use help Command to view reserved keywords ）
  1、 Defining variables ： Variable name = value
  2、 Revoke variables ：unset Variable
  3、 Declare static variables ：readonly Variable , Be careful ： You can't unset
  4、A= `date `` The quotation marks , Run the command inside , And return the result to the variable A
  5、A=$(date) Equivalent to backquotes

Set the environment variable

• Basic grammar
  1、export Variable name = A variable's value （ Function description ： take shell Variable output is environment variable / Global variables ）
  2、source The configuration file （ Function description ： Let the modified configuration options take effect immediately ）
  3、echo $ Variable name （ Function description ： Query the value of the environment variable ）

Be careful ： At output TOMCAT_HOME Before environment variable , It needs to take effect
source /etc/profile

Position parameter variable

When we execute a shell Script time , If you want to get the parameter information of the command line , You can use the position parameter variable .
such as ：./myshell.sh 100 200, This is an execution shell The command line , Can be in myshell Get the parameter information in the script .

• Basic grammar
  1、$ n：n Is the number ,$0 For the order itself ,$1- $ 9 Represents the first to ninth parameters , More than ten parameters need to be enclosed in braces , Such as ：$ {10}.
  2、$ *： This variable represents all the parameters on the command line ,$* Treat all parameters as a whole .
  3、 [email protected]： This variable also represents all the parameters on the command line , however [email protected] Treat each parameter differently .
  4、 $#： This variable represents the number of all parameters on the command line .

Predefined variables

Namely shell Variables that have been defined by the designer in advance , Can be directly in shell Use in script

• Basic grammar
  1、 $$： The process number of the current process （PID）
  2、 $!： The process number of the last process running in the background （PID）
  3、 $?： 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 .

Operator

• Basic grammar
  1、“$(( Operator ))” or “ $[ Operator ]” perhaps expr m + n
  2、 Be careful expr Space between operators
  3、expr m - n
  4、expr \ *,/,% ride , except , Remainder

Judge the condition

• Basic grammar

if [ condition ]  
then 
... 
elif [ condition ] 
then 
... 
fi（ Be careful condition Space before and after ）

# Not empty return true, You can use $? verification （0 by true,>1 by false）

• Application example
  [ test ] return true
  [] return false
  [ condition ] && echo OK || echo notok Conditions met , Execute the following statement
• Judgment statement
  1、= String comparison
  2、 A comparison of two integers
  -lt Less than
  -le Less than or equal to
  -gt Greater than
  -ge Greater than or equal to
  -ne It's not equal to
  3、 Judge according to the file authority
  -r Have read permission
  -w Have the right to write
  -x Have the authority to execute
  4、 Judge according to the document type
  -f The file exists and is a regular file
  -e File exists
  -d The file exists and is a directory
• Case study
  
  

case sentence

• 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

for loop

• Basic grammar

# grammar 1
for  Variable  in  value 1  value 2  value 3...
do
 Program 
done
# grammar 2
for(( Initial value ; Cycle control conditions ; Variable change ))
do
 Program 
done

while loop

• Basic grammar

while [  Conditional judgment  ]# Space inside parentheses 
do
 Program ...
done

read 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 ）, If it is not entered in the specified time , I don't wait .
  Parameters ： Specifies the variable name of the read value
  
  
  

function

shell Programming is like other programming languages , There are system functions , You can also customize functions . In system function , Let's introduce two .
System function

• basename Basic grammar
  basename [pathname] [suffix]
  basename [string] [suffix]
  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 .
  
• dirname Basic grammar
  function ： Return to full path last / The front part of , Commonly used in the return path part
  dirname File absolute path ： Remove filename from given filename with absolute path （ Non catalogue part ）, Then return to the rest of the path （ Part of the catalog ）
  
  Custom function
• Basic grammar

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

Call to write the function name directly ：funname [ value ]

• Case study ： Calculate the sum of the two input parameters （ Get dynamic ）,getSum
  
  

Shell String

The string is shell The most commonly used and useful data types in programming , Strings can be single - booted , You can also use double quotes , You can also use no quotes .

• Single quotation marks ：
  Any character in a single quotation mark will be output as is , Variables in a single quoted string are invalid ;
  A single quotation mark cannot appear in a single quotation mark string , But in pairs , Used as a string concatenation .

• Double quotes ：
  You can have variables in double quotes
  Escape characters can appear in double quotes

# Declaration string 
str1="hello world 1"
str2="hello world 2"

# String splicing -- Double quotes 
name='helloworld'
name1="hello, "$name" !"
name2="hello, ${name} !"

# String splicing -- Single quotation marks 
passwd=’123456‘
passwd1=’hello, ‘$passwd’ !‘
passwd2='hello, ${passwd} !'
echo $passwd2 # hello,${passwd} !

# Length of string 
email="[email protected]"
echo ${
    #email}
echo ${
    email:1:4}

Shell Array

bash Supports one dimensional array （ Multidimensional arrays are not supported ）, And there's no limit to the size of the array
The subscripts of array elements are 0 Numbered starting . To get the elements in an array, you need to use subscripts , Subscripts can be integers or arithmetic expressions , Its value should be greater than or equal to 0 .

# Define an array   Parentheses are used to represent arrays , For array elements “ Space ” Symbol split 
 Array name =( value 1  value 2 .... value n)
favs=(" football " " Basketball " " Table Tennis " " The bowling ball ")

# Read array  ${ Array name [ Subscript ]}
fav=${
    favs[1]}

# Use  @  The symbol can get all the elements in the array 
echo ${
    favs[@]}

# Gets the length of the array 
length1=${
    #favs[@]}
length2=${
    #favs[*]}

Shell Notes

With # The first line is the comment , Will be ignored by the interpreter .
Add one... To each line # Number to set multiline comment

#-----------------------
# This is a comment 
#author：
#site：
#-----------------------
####  Server configuration -start ####
#
#
#
#
#
####  Server configuration -end ####

#  Special multiline comments 

:<<EOF
 The comment ...
 The comment ...
 The comment ...
EOF

:<<!
 The comment ...
 The comment ...
 The comment ...
!

Shell Parameter passing

perform Shell Script time , Pass parameters to script , The format of obtaining parameters in the script is ：$n.n Represents a number

Processing parameters	Parameter description
$#	The number of parameters passed to the script
$*	Display all the parameters passed to the script in a single string .
$$	The current process the script runs ID Number
$!	Of the last process running in the background ID Number
$?	Display the exit status of the last command .0 No mistakes , Any other value indicates an error .
$0	File name of execution

#！/bin/bash

echo "Shell  Pass parameter instance !"
echo " File name of execution ：$0"
echo " The first parameter is zero :$1"
echo " The second parameter is :$2"
echo " The third parameter is zero :$3"

# ./hello.sh 11 22 33 44