the following are legal variable definitions

#!/bin/bash

projectName="pay"
project_name="query"
_project_name="notice"

2.2. Use variables

   use a defined variable, as long as the dollar sign ($) is added in front of the variable name, such as:

#!/bin/bash

projectName="pay"
echo $projectName
echo ${projectName}

Both methods are OK, but the second method is more used when string splicing variables, and will not report errors

2.3 read only variable

   variables with readonly are read-only variables and cannot be changed

#!/bin/bash

readonly systemName="Linux"
systemName="ubuntu"

You will be prompted: systemName: read only variable

2.4. Delete variable

  use the unset command to delete variables

#!/bin/bash

tempStr="abcdef"
unset tempStr
echo $tempStr

   then the variable result will be empty

3, Shell string

3.1 single quotation mark

• Any character in the single quotation mark will be output as it is, and the variable in the single quotation mark string is invalid
• A single single quotation mark cannot appear in a single quotation mark string (nor after using the escape character for single quotation marks), but it can appear in pairs and be used as string splicing

#!/bin/bash

str='this is a simple string'
echo $str

3.2 double quotation marks

• There can be variables in double quotation marks
• Escape characters can appear in double quotation marks

#!/bin/bash

operating_system="linux"
str="Hello, operating system is \"$system_name\"! \n"
echo -e $str

3.3 splicing string (double quotation marks)

   there can be variables in double quotation marks, and escape characters can also appear in double quotation marks

#!/bin/bash

username="alian"
greeting="Hello $username"
echo $greeting

Operation results:

Hello alian

3.4 splicing string (single quotation mark)

   any character in single quotation marks will be output as is, and the variable in single quotation marks string is invalid

#!/bin/bash

username='alian'
greeting='Hello $username'
echo $greeting

Operation results:

Hello $username

3.five. Get string length

#!/bin/bash

tempstr="abcd"
echo "Length of string: ${#tempstr}"

Operation results:

Length of string: 4

3.6. Extract substring

The subscript starts at 0

#!/bin/bash

string="Alian is learning java"
echo ${string:1:5}
string="runoob is a great site"
echo `expr index "$string" io`

Operation results:

lian

3.7. Find substring

   find the position of the character i or o (the position starts from 1, and the letter that appears first is calculated)

#!/bin/bash

string="Alian is learning java"
echo `expr index "$string" ia`

Operation results:

3

4, Shell array

4.1 array definition and value

   use parentheses to represent the array, and the array elements are separated by "space symbol", as follows:

   array name = (value 1, value 2... Value n)

#!/bin/bash

fruits=("Apple" "Banana" "Orange" "kiwifruit")
#Value by subscript, starting from 0
echo ${fruits[2]}
# Use the @ symbol or * symbol to get all the elements in the array, for example:
echo ${fruits[@]}
echo ${fruits[*]}

Operation results:

Orange
Apple Banana Orange kiwifruit
Apple Banana Orange kiwifruit

4.2. Get the length of the array

   the method of obtaining the length of an array is similar to that of a string

    ${# array name [*]} or ${# array name [@]}

#!/bin/bash

fruits=("Apple" "Banana" "Orange" "kiwifruit")
#Two ways to get the length of an array
echo "Method 1: array length: ${#fruits[*]}"
echo "Method 2: array length: ${#fruits[@]}"

Operation results:

Method 1: array length: 4
 Mode 2: array length: 4

5, Shell comments

5.1. Single line notes

    lines beginning with # are comments and will be ignored by the interpreter.

#!/bin/bash

#alian is summarizing the basics of Shell programming
#If you have any questions, you can contact me through the blog
#Blog address: https://blog.csdn.net/Alian_1223

echo "I'll be waiting for you"

Operation results:

I'll be waiting for you

5.2 multi line notes

   of course, you can set multi line comments by adding a # number to each line. Here we will talk about other ways

#!/bin/bash
echo "eof Note content in format"
:<<EOF
eof Format Note content
eof Format Note content
eof Note content format
EOF

echo "Exclamation mark format comment content"
:<<!
Exclamation mark format comment content
 Exclamation mark format comment content
 Exclamation mark format comment content
!

echo "Comment content in single quotation mark format"
: '
Single quotation mark format comment content
 Single quotation mark format comment content
 Single quotation mark format comment content
'

Operation results:

eof Note content in format
 Exclamation mark format comment content
 Comment content in single quotation mark format

Note that there is a space between colon and single quotation mark in the third way

6, Shell operator

• Arithmetic operator
• Relational operator
• Boolean operator
• Logical operator
• String operator
• File test operator

6.1 arithmetic operators

#!/bin/bash
a=100
b=20
# addition
val=`expr $a + $b`
echo "a + b : $val"
#subtraction
val=`expr $a - $b`
echo "a - b : $val"
#Multiplication (pay attention to escape symbols)
val=`expr $a \* $b`
echo "a * b : $val"
#division
val=`expr $b / $a`
echo "b / a : $val"
#Seeking module
val=`expr $b % $a`
echo "b % a : $val"
#Equality judgment
if [ $a == $b ]
then
   echo "a be equal to b"
fi
#Unequal judgment
if [ $a != $b ]
then
   echo "a Not equal to b"
fi

Operation results:

a + b : 120
a - b : 80
a * b : 2000
b / a : 0
b % a : 20
a Not equal to b

It should be noted that the conditional expression should be placed between square brackets with spaces. For example: [$a==$b] is wrong and must be written as [$a==$b]

6.2. Relational operators

#!/bin/bash

a=100
b=20

if [ $a -eq $b ]
then
   echo "$a -eq $b : a be equal to b"
else
   echo "$a -eq $b: a Not equal to b"
fi

if [ $a -ne $b ]
then
   echo "$a -ne $b: a Not equal to b"
else
   echo "$a -ne $b : a be equal to b"
fi

if [ $a -gt $b ]
then
   echo "$a -gt $b: a greater than b"
else
   echo "$a -gt $b: a Not greater than b"
fi

if [ $a -lt $b ]
then
   echo "$a -lt $b: a less than b"
else
   echo "$a -lt $b: a Not less than b"
fi

if [ $a -ge $b ]
then
   echo "$a -ge $b: a Greater than or equal to b"
else
   echo "$a -ge $b: a less than b"
fi

if [ $a -le $b ]
then
   echo "$a -le $b: a Less than or equal to b"
else
   echo "$a -le $b: a greater than b"
fi

Operation results:

100 -eq 20: a Not equal to b
100 -ne 20: a Not equal to b
100 -gt 20: a greater than b
100 -lt 20: a Not less than b
100 -ge 20: a Greater than or equal to b
100 -le 20: a greater than b

It should be noted that there are spaces in if, [, variable, relational operator, variable,] and do not omit them

6.3 Boolean operators

#!/bin/bash
  
a=100
b=20

if [ $a != $b ]
then
   echo "$a != $b : a Not equal to b"
else
   echo "$a == $b: a be equal to b"
fi

#And operation
if [ $a -lt $b -o $a -lt 200 ]
then
   echo "$a -lt $b -o $a -lt 200:  return true"
else
   echo "$a -lt $b -o $a -lt 200:  return false"
fi

#And operation
if [ $a -gt $b -a $a -gt 200 ]
then
   echo "$a -gt $b -a $a -gt 200:  return true"
else
   echo "$a -gt $b -a $a -gt 200:  return false"
fi

Operation results:

100 != 20 : a Not equal to b
100 -lt 20 -o 100 -lt 200:  return true
100 -gt 20 -a 100 -gt 200:  return false

6.4 logical operators

 & & is logical and, |𞓜 is logical or

#!/bin/bash

a=100
b=20

if [[ $a -gt fifty && $b -gt 50 ]]
then
   echo "return true"
else
   echo "return false"
fi

if [[ $a -gt 50 || $b -gt 50 ]]
then
   echo "return true"
else
   echo "return false"
fi

Operation results:

return false
 return true

6.5. String operator

#!/bin/bash

a="abcdefg"
b="hijklmn"

if [ $a = $b ]
then
  echo "$a = $b Test for equality, return true"
else
  echo "$a = $b Test for equality, return false"
fi

if [ $a != $b ]
then
  echo "$a != $b Test whether it is not equal, return true"
else
  echo "$a != $b Test whether it is not equal, return false"
fi

if [ -z $a ]
then
  echo "-z $a Test whether it is 0 and return true"
else
  echo "-z $a Test whether it is 0 and return false"
fi

if [ -n $a ]
then
  echo "-n $a Test whether it is not returned for 0 true"
else
  echo "-n $a Test whether it is not 0, return false"
fi

if [ $a ]
then
  echo "$a Test whether it is empty, return true"
else
  echo "$a Test whether it is empty, return false"
fi

Operation results:

abcdefg = hijklmn Test for equality, return false
abcdefg != hijklmn Test whether it is not equal, return true
-z abcdefg Test whether it is 0 and return false
-n abcdefg Test whether it is not returned for 0 true
abcdefg Test whether it is empty, return true

6.6 document test operator

   there are too many operators here, so I won't demonstrate them one by one. Just pick some commonly used ones

#!/bin/bash

file="/home/shell/logic.sh"
echo "file[$file]The properties are as follows:"
if [ -r $file ]
then
   echo "File readable"
else
   echo "File unreadable"
fi

if [ -w $file ]
then
   echo "File writable"
else
   echo "The file is not writable"
fi

if [ -x $file ]
then
   echo "File executable"
else
   echo "The file is not executable"
fi

if [ -f $file ]
then
   echo "The file is ordinary"
else
   echo "The document is a special document"
fi

if [ -d $file ]
then
   echo "The file is a directory"
else
   echo "File is not a directory"
fi

if [ -s $file ]
then
   echo "File is not empty"
else
   echo "File is empty"
fi

if [ -e $file ]
then
   echo "File exists"
else
   echo "file does not exist"
fi

Operation results:

file[/home/shell/logic.sh]The properties are as follows:
File readable
 File writable
 File executable
 The file is ordinary
 File is not a directory
 File is not empty
 File exists

7, Shell echo command

   Shell echo command is used for string output. Let's talk about several common commands

7.1. Display string

#!/bin/bash

#Double quotes are not required
echo study hard and make progress every day
#With double quotation marks
echo "study hard and make progress every day"
#Use escape characters to display double quotes
echo \"study hard and make progress every day\"

Operation results:

study hard and make progress every day
study hard and make progress every day
"study hard and make progress every day"

7.2 line feed and no line feed

#!/bin/bash

#-n can output without line break
echo -n "Hello ";echo "Shell"
#Output wrap
echo -e "I'll wrap: Echo output wrap \n"
#Output does not wrap
echo -e "I won't wrap: Echo output does not wrap \c"
#Output next line
echo "isn't it?"

Operation results:

Hello Shell
 I'll wrap: Echo output wrap 

I won't wrap: Echo output does not wrap isn't it?

7.3. Display results directed to file

#!/bin/bash

echo "Test echo output to file" > echoToFile.txt

If it is an additional file, it changes from > to > >

#!/bin/bash

echo "Test echo output to file" >> echoToFile.txt

7.4. Display command execution results

#!/bin/bash

#Displays the current time
echo `date`
#Display network information
echo "`ifconfig`"

Operation results:

2022 Wednesday, February 16, 2010:24:25 CST
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 172.26.163.146  netmask 255.255.240.0  broadcast 172.26.115.255
        inet6 fe80::216:3eff:fe1c:c5e5  prefixlen 64  scopeid 0x20<link>
        ether 00:16:3e:1c:c5:e5  txqueuelen 1000  (Ethernet)
        RX packets 764979  bytes 149477332 (142.5 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 951246  bytes 182426261 (173.9 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

8, Shell printf command

#!/bin/bash

printf "%-10s %-8s %6s %11s\n" Fruit unit price quantity total price
printf "%-10s %-8s %-6d %1.2f\n" Apple 5.00 10 50.00
printf "%-10s %-8s %-6d %1.2f\n" Banana 3.00 5 15.00
printf "%-10s %-8s %-6d %1.2f\n" Cherry 50.00 2 100.00

Operation results:

Fruits     Unit price quantity      Total price
 Apple     5.00     10     50.00
 Banana     3.00     5      15.00
 Cherry     50.00    2      100.00

• %S% c% d% F are all format substitutes,% s outputs a string,% d integer output,% c outputs a character,% f outputs a real number and outputs it in decimal form.
• %-8s refers to a width of 8 characters (- left aligned, no right aligned)
• %1.2f refers to formatting as decimal, where. 2 refers to retaining 2 decimal places.

9, Shell test command

   first, let's look at the meaning of the operators mentioned earlier, as follows:

9.1 digital test

#!/bin/bash
  
a=100
b=20
if test $[a] -eq $[b]
then
    echo "$a and $b equal!"
else
    echo "$a and $b Unequal!"
fi

if test $[a] -gt $[b]
then
    echo "$a greater than $b "
else
    echo "$a less than $b"
fi

Operation results:

100 And 20 are not equal!
100 Greater than 20

9.2 string test

#!/bin/bash

a="abcdefg"
b="hijklmn"
if test $a = $b
then
    echo "$a and $b Two strings are equal"
else
    echo "$a and $b The two strings are not equal!"
fi

if test -z $a
then
    echo "$a The string length is zero"
else
    echo "$a String length is not zero"
fi

Operation results

abcdefg and hijklmn The two strings are not equal!
abcdefg String length is not zero

9.3 document test

   in fact, the file test operator has been discussed in section 6.6 of this article, so I won't say more. See the above for details.

10, Shell process control

   process control can also be carried out in the Shell, such as condition judgment and circulation

10.1,if

   in fact, we have seen a lot of if judgments before. Its grammatical structure is roughly as follows:

if condition
then
    command
fi

Define a string. If the string is not empty, splice another string, and then output the result

#!/bin/bash

tempStr="Hello"
if test -n $tempStr
then
    tempStr="$tempStr, Shell"
fi
echo $tempStr

Operation results:

Hello, Shell

10.2,if else

The syntax structure of   if else is as follows:

if condition
then
    command1 
else
    command2
fi

Define two numbers and judge whether they are equal

#!/bin/bash

a=100
b=20
if test $a -eq $b
then
    echo "$a and $b equal!"
else
    echo "$a and $b Unequal!"
fi

Operation results:

100 And 20 are not equal!

10.3,if else-if else

The syntax structure of   if else if else is as follows:

if condition1
then
    command1
elif condition2 
then 
    command2
else
    commandN
fi

#!/bin/bash

a=100
b=20
if [ $a == $b ]
then
    echo "$a be equal to $b"
elif [ $a -gt $b ]
then
    echo "$a greater than $b"
else
    echo "$a less than $b"
fi

Operation results:

100 Greater than 20	

Note: else if is actually elif

10.4 for cycle

The syntax structure of   for loop is as follows:

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

  simple demonstration of a for loop

#!/bin/bash
  
for num in 1 2 3 4 5 6
do
    echo "For loop value is: $num"
done

Operation results:

For loop value is: 1
For loop value is: 2
For loop value is: 3
For loop value is: 4
For loop value is: 5
For loop value is: 6

10.5. while cycle

The syntax structure of   while loop is as follows:

while condition
do
    command
done

#!/bin/bash
  
maxcount=3
count=1
pwd="123456"
while [ $count -le $maxcount ]
do
  echo -n "The first ${count}Enter your password once:"
  read input
  if [ $pwd -eq $input ]
  then
        echo "The first ${count}The password entered is correct, and the login is successful"
        break
  fi

  echo "Wrong password ${count}second"

  if [ $count -gt $maxcount ]
  then
    echo "Maximum number of password errors:$maxcount"
    break
  fi
  count=`expr $count + 1`

done

Operation results:

Enter your password for the first time: 123123
 Wrong password, wrong input once
 Enter your password for the second time: 222222
 Wrong password, wrong input twice
 Enter your password for the third time: 123456
 The password entered for the third time is correct and the login is successful

10.6 infinite cycle

   infinite loop means that the condition is always true. The syntax structure of infinite loop is as follows:

while true
do
  command
done

or

while :
do
  command
done

or

for (( ; ; ))

10.7,case ... esac

The syntax structure of   case... esac loop is as follows:

case Mode value in
 Mode 1)
    command1
    ;;
Mode 2)
    command2
    ;;
esac

  suppose you go to the lottery by selecting numbers

#!/bin/bash

echo "Enter a number between 1 and 3"
echo -n "The number you entered is:"
read aNum
case $aNum in
"1")
    echo "Congratulations on winning 100 yuan!"
    ;;
"2")
    echo "Congratulations on winning 200 yuan!"
    ;;
"3")
    echo "Unfortunately, I didn't win the prize"
    ;;
esac

Operation results:

Enter a number between 1 and 3
 The number you entered is:2
 Congratulations on winning 200 yuan!

Enter a number between 1 and 3
 The number you entered is:3
 Unfortunately, I didn't win the prize

10.8,break

  we write an infinite loop to demonstrate that the user enters a number within the specified range, otherwise exit the program.

#!/bin/bash

while true
do
  echo -n "Please enter a number from 1 to 100:"
  read num
  if [[ $num -lt 1 || $num -gt 100 ]]
  then
     echo "Sorry, the number you entered ${num}Not a number between 1 and 100"
     break
  fi
  echo "The number you entered is: $num"
done

Operation results:

Please enter a number from 1 to 100: 1
 The number you entered is: 1
 Please enter the number from 1 to 99
 The number you entered is: 99
 Please enter a number from 1 to 100: 120
 Sorry, the number 120 you entered is not in the middle of 1 to 100

It should be noted that if there are multiple nested loops, the break keyword is to jump out of the current loop, not the whole loop.

10.9,continue

We demonstrate the need for an even number of outputs between 1 and 10

#!/bin/bash

for num in 1 2 3 4 5 6 7 8 9 10
do
  if [[ `expr $num % 2` != 0 ]]
  then
     continue
  fi
  echo "Even number obtained: ${num}"
done

Operation results:

Even number obtained: 2
 Even number obtained: 4
 Even number obtained: 6
 Even number obtained: 8
 Even number obtained: 10

11, Shell pass parameters

11.1 basic usage

Let's start with a simple example

#!/bin/bash

echo "Shell Transfer parameters"
echo "Executed file name: $0"
echo "The first parameter passed in is: $1"
echo "The second parameter passed in is: $2"
echo "The third parameter passed in is: $3"

We execute:

sh /home/shell/params.sh apple orange banana

Operation results:

Shell Transfer parameters
 Executed file name:/home/shell/params.sh
 The first parameter passed in is: apple
 The second parameter passed in is: orange
 The third parameter passed in is: banana

11.2 description of other parameters

Let's see the effect separately

#!/bin/bash

echo "1,Number of parameters passed by executing script: $#"
echo "2,Execute all parameters passed by the script(A single string): $*"
echo "3,The current process in which the script is running ID(important): $$"
echo "4,Of the last process running in the background ID number: $!"
echo "5,Shell Current options used: $-"
echo "6,Exit status of command: $?"
echo "7,\$@use,Remember to use double quotation marks"
for var in "$@"
do
   echo $var
done

Operation results:

1,Number of parameters passed by executing script: 3
2,Execute all parameters passed by the script(A single string): apple orange banana
3,The current process in which the script is running ID(important): 1338260
4,Of the last process running in the background ID number:
5,Shell Current options used: hB
6,Exit status of command: 0
7,$@use,Remember to use double quotation marks
apple
orange
banana

12, Function

12.1. Simple use

In Shell, you can pass parameters to a function when you call it. Inside the function body, the value of the parameter is obtained in the form of $n. for example, $1 represents the first parameter, $2 represents the second parameter..., as shown in the following example:

#!/bin/bash

testFuction(){
    echo "Function call received"
    echo "The parameters passed in are: $*"
    echo "full name: $1"
    echo "Gender: $2"
    echo "Age: $3"
}

echo "-----Function call start-----"
testFuction Alian Male 18
echo "-----End of function call-----"

Operation results:

-----Function call start-----
Function call received
 The parameters passed in are: Alian Male 18
 full name: Alian
 Gender: Male
 Age: 18
-----End of function call-----

12.2. Get the return value of the function

The return values of     functions are generally 0 and 1, indicating success and failure

#!/bin/bash

add(){
  echo "Addition calculation"
  return `expr $1 + $2`
}
add 1 2
echo "Result of addition calculation: $?"

Operation results:

Addition calculation
 Result of addition calculation: 3

  looks like we passed $? It seems right to get the result 3. However, by default, the return value of the function is 0-255. If it exceeds 0, it will start from 0. For example, calculating add 200 58 will get 2.
   the return keyword in the Shell function is used to indicate the exit status of the function, not the return value of the function. If you must return other integers, define a global variable and change it in the function.

#!/bin/bash

res=0
dosomething(){
  echo "Addition calculation"
  echo "Other operations..."
  echo "Other operations..."
  res=`expr $1 + $2`
}
dosomething 200 58
echo "Result of addition calculation: $res"

Operation results:

Addition calculation
 Other operations...
Other operations...
Result of addition calculation: 258

12.3 function assignment description

  see the following code

#!/bin/bash

newAdd(){
  echo "New addition calculation"
  echo "augend: $1"
  echo "Addend: $2"
  return `expr $1 + $2`
}
res=`newAdd 1 2`
echo "Result of addition calculation: $res"

Operation results:

Result of addition calculation: new addition calculation
 Addend: 1
 Addend: 2

   we didn't get the expected result 3. In fact, res=`newAdd 1 2 'is to pass the standard output of function newAdd to res

epilogue

   the syntax of Shell is relatively simple. Compared with other languages, it is different from the same. It mainly needs more practice and practice.