Shell script learning notes

Reprint please indicate the source ：http://blog.csdn.net/zhaoyanjun6/article/details/122923763
This article from the 【 Zhao Yanjun's blog 】

What is? Shell

Shell It's a use. C A program written in a language , It is used by users Linux The bridge .Shell It's a command language , Another programming language .

Shell Script

Shell Script （shell script）, It's for shell Write the script .

Shell Environmental Science

Shell Programming with JavaScript、php Programming is the same , Just have a text editor that can write code and a script interpreter that can interpret execution .

Shell grammar

Open the text editor ( have access to vi/vim Command to create a file ), Create a new file test.sh, extension sh（sh representative shell）, The extension does not affect script execution , It's good to see the name and know the meaning , If you use php Write shell Script , Use the extension php Okay .

Grant Execution Authority

chmod +x ./test.sh  # Give the script permission to execute 
./test.sh  # Execute the script 

Log output echo

echo "Hello World !"

Variable

When defining variables , There can be no spaces between variable names and equal signs , This may not be the same as any programming language you are familiar with . meanwhile , The naming of variable names should follow the following rules ：

• Only English letters can be used for naming , Numbers and underscores , The first character cannot begin 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 ）.

Example ：

your_name="runoob.com"

Variable references

your_name="qinjx"
echo $your_name
echo ${
    your_name}

Curly braces outside variable names are optional , Add it or not , Curly braces are used to help the interpreter identify the boundaries of variables , For example, the following situation ：

your_name="qinjx"
echo $your_name
echo ${
    your_name}hello

A read-only variable

Use readonly Commands can define variables as read-only variables , The value of a read-only variable cannot be changed .

The following example attempts to change a read-only variable , The result is wrong ：

myUrl="https://www.google.com"
readonly myUrl
myUrl="https://www.runoob.com"

Run script , give the result as follows ：

/bin/sh: NAME: This variable is read only.

character string

Strings can be in single quotes , You can also use double quotes , You can also use no quotes .

• 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 （ You can't escape a single quotation mark ）, But in pairs , Use as string concatenation .

The advantages of double quotes ：

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

if else

if Sentence syntax format ：

if condition
then
    command1 
    command2
    ...
    commandN 
fi

give an example ：

a=10
b=10
if [ $a == $b ]
then
   echo "a  be equal to  b"
elif [ $a -gt $b ]; then
   echo "a  Greater than  b"
elif [ $a -lt $b ]; then
   echo "a  Less than  b"
else
  echo " There are no conditions that meet "
fi

for loop

for The general format of the loop is ：

for var in item1 item2 ... itemN
do
    command1
    command2
    ...
    commandN
done

Write in a line ：

for var in item1 item2 ... itemN; do command1; command2… done;

give an example ：

for i in {
    1..5} ; do
    echo " Traverse  $i"
done

Output ：

 Traverse  1
 Traverse  2
 Traverse  3
 Traverse  4
 Traverse  5

give an example ：

for str in Hello new world
do
  echo " Traverse  $str"
done

Output ：

 Traverse  Hello
 Traverse  new
 Traverse  world

while sentence

while Loop is used to execute a series of commands continuously , Also used to read data from the input file . Its grammatical form is ：

while condition
do
    command
done

give an example ：

int=1
while(( $int<=5 ))
do
    echo $int
    let "int++"
done

Run script , Output ：

1
2
3
4
5

The above example uses Bash let command , It is used to execute one or more expressions , There is no need to add $ To represent a variable .

give an example ：

num=18
while [ $num -le 100 ]  #-le  Less than or equal to 
do
    let num++
    echo "while $num"
done

case … esac

case … esac Select multiple statements for , And in other languages switch … case Statements like , It's a multi branch selection structure , Every case The branch begins with a right parenthesis , Use two semicolons ;; Express break, The end of execution , Jump out of the whole case … esac sentence ,esac（ Namely case In turn, ） As the closing tag .

case … esac The syntax is as follows ：

case  value  in
 Pattern 1)
    command1
    command2
    ...
    commandN
    ;;
 Pattern 2)
    command1
    command2
    ...
    commandN
    ;;
esac

case Work as shown above , Value must be followed by word in, The first mock exam must be closed with right brackets. . Value can be variable or constant , Match found that after the first mock exam meets a certain pattern, , During this period, all commands are executed until ;;.

Value will detect every matching pattern . Once the patterns match , After the corresponding command of matching mode is executed, other modes will not be continued . If there is no matching pattern , Use the asterisk * Capture the value , Then execute the following command .

give an example ：

echo ' Input  1  To  4  Number between :'
echo ' The number you enter is :'
read aNum
case $aNum in
    1)  echo ' You chose  1'
    ;;
    2)  echo ' You chose  2'
    ;;
    3)  echo ' You chose  3'
    ;;
    4)  echo ' You chose  4'
    ;;
    *)  echo ' You didn't enter  1  To  4  Number between '
    ;;
esac

string matching

site="runoob"

case "$site" in
   "runoob") echo " Novice tutorial "
   ;;
   "google") echo "Google  Search for "
   ;;
   "taobao") echo " TaoBao "
   ;;
esac

The output is ：

Novice tutorial

Array

Example 1：

name=(zhao zhang li)
echo " Elements ：${name[1]}"
echo " All the elements ：${name[*]}"
echo " The length of the array ：${#name[*]}"
echo " The length of the array ：${#name[@]}"

Example 2：

# Copy to an element individually 
name[1]=wang

function

shell The definition format of the function in is as follows ：

[ function ] funname [()]

{
    

    action;

    [return int;]

}

explain ：

• 1、 You can take function fun() Definition , It can also be direct fun() Definition , Without any parameters .
• 2、 Parameter return , Can display plus ：return return , If not , Results will be run with the last command , As return value . return Heel value n(0-255)

give an example

demoFun(){
    
    echo " This is my first  shell  function !"
}
echo "----- The function starts executing -----"
demoFun
echo "----- The function is finished -----"

Output results ：

----- The function starts executing -----
 This is my first  shell  function !
----- The function is finished -----

Here's a definition with return Function of statement ：

function sum() {
    
    a=10
    b=20
    return $((a+b))
}

echo ' Start function '
sum
 # Receive function return value 
echo "result: $?" 

Output results ：

 Start function 
result: 30

The return value of the function passes through $? To obtain a .

Be careful ： All functions must be defined before use . This means that you have to put the function at the beginning of the script , until shell When the interpreter first discovered it , Can be used . Call a function using only its function name .

Function arguments

stay Shell in , You can pass parameters to a function when you call it . Inside the body of the function , adopt $n To get the value of the parameter , for example ,$1 Represents the first parameter ,$2 Represents the second parameter …

Examples of functions with arguments ：

function sum() {
    
    # $1 Represents the first parameter ,$2 Represents the second parameter .
    a=$1
    b=$2
    return $((a+b))
}

echo ' Start function '
sum 2 3
 # Receive function return value 
echo "result: $?"

Output ：

 Start function 
result: 5

give an example ：

funWithParam(){
    
    echo " The first parameter is zero  $1 !"
    echo " The second parameter is  $2 !"
    echo " The tenth parameter is  $10 !"
    echo " The tenth parameter is  ${10} !"
    echo " The eleventh parameter is  ${11} !"
    echo " The total number of parameters is  $#  individual !"
    echo " Output all parameters as a string  $* !"
}
funWithParam 1 2 3 4 5 6 7 8 9 34 73

Output results ：

 The first parameter is zero  1 !
 The second parameter is  2 !
 The tenth parameter is  10 !
 The tenth parameter is  34 !
 The eleventh parameter is  73 !
 The total number of parameters is  11  individual !
 Output all parameters as a string  1 2 3 4 5 6 7 8 9 34 73 !

Be careful ,$10NocanatakeThe\; firstTenindividualginsengCount,atakeThe\; firstTenindividualginsengCountNeed\; to\; bewant${10}. When n>=10 when , Need to use ${n} To get the parameters .

in addition , There are also a few special characters for processing parameters ：

• $* ： Display all the parameters passed to the script in a single string
• $@：And$* identical , But use quotation marks , And return each parameter in quotation marks .

External file references

Like any other language ,Shell You can also include external scripts . This can easily encapsulate some common code as a separate file .

Shell The file contains the following syntax format ：

. filename   #  Note the number (.) There is a space between the file name and the file name 

 or 

source filename

This method , Many tool classes can be encapsulated

First, define a tool class util.sh：

url="baidu.com"

function sum() {
    
    # $1 Represents the first parameter ,$2 Represents the second parameter .
    echo " All the parameters ：$* "
    a=$1
    b=$2
    return $((a+b))
}

test.sh

# External file references 
. util.sh

echo " Variable references :$url"

echo ' Start function '
#sum  Function in util.sh in 
sum 2 3
 # Receive function return value 
echo "result: $?"

Determines if the string is empty

str="abc"

if [ -z $str ]; then
  echo " The string is empty "
else
  echo " String is not empty "
fi

test command

Shell Medium test The command is used to check if a condition holds , It can do numerical 、 Character and file testing .

• The numerical test

-eq ： Equal is true
-ne ： True if not equal
-gt ： Greater than is true
-ge ： True if greater than or equal to
-lt ： Less than is true
-le ： True if less than or equal to

give an example ：

num1=100
num2=100
if test $[num1] -eq $[num2]
then
    echo ' Two numbers are equal ！'
else
    echo ' The two numbers are not equal ！'
fi

Output results ：

 Two numbers are equal ！

• Basic operation

In code [] Perform basic arithmetic operations , Such as ：

a=5
b=6

result=$[a+b] #  Note that there should be no spaces on either side of the equal sign 
echo "result  by ： $result"

The result is :

result  by ： 11

• String test

= Equal is true
!= Inequality is true
-z character string A zero length string is true
-n character string True if the length of the string is not zero

example

num1="ru1noob"
num2="runoob"
if test $num1 = $num2
then
    echo ' Two strings are equal !'
else
    echo ' Two strings are not equal !'
fi

Output results ：

Two strings are not equal !

• File test

give an example ：

if test -e ./bash
then
    echo ' file already exist !'
else
    echo ' file does not exist !'
fi

Output results ：

 file already exist !

in addition ,Shell It also provides ( -a )、 or ( -o )、 Not ( ! ) Three logical operators are used to connect test conditions , Its priority is ： ! The highest , -a second , -o The minimum . for example ：

if test -e ./notFile -o -e ./bash
then
    echo ' At least one file exists !'
else
    echo ' Neither file exists '
fi

Output results ：

 At least one file exists !

http curl

curl The command uses... By default get Mode sending http request .

curl www.baidu.com

Practice one ： Traversal folder

# Define Directory , Traverse the current directory 
fileDir="./*"

# Traverse the directory 
for file in $fileDir ; do
    echo " Traverse ：$file"
done

Output ：

 Traverse ：./main
 Traverse ：./test
 Traverse ：./test.sh
 Traverse ：./util.sh

Output files in directory 、 Folder

# Define Directory 
fileDir="./*"

# Traverse the directory 
for file in $fileDir; do

  if test -f "$file"; then
    echo " Traverse ：$file  It's a document "
  elif test -d "$file"; then
    echo " Traverse ：$file  Is a directory "
  fi
done

Actual combat II ： Deep traversal of folders

# Folder Directory 
fileDir="../assets"

function allFile() {
  # Receiving parameters 
  dir=$1

  # Traverse the directory 
  for file in ${dir}/*; do

    if test -f "$file"; then
      echo "$file  It's a document "
    elif test -d "$file"; then
      echo "$file  Is a directory "
      allFile "$file"
    fi
    
  done
}

echo " Start traversing the directory "
allFile $fileDir

Output results ：