Shell in parentheses summary: {}, (), (()), [], [[]]

Parentheses Summary() Single parenthesis

• Command group, the command in parentheses will open a new child shell execution

  • Variables in parentheses cannot be used by left script

  • Semicolon separated between commands

  • There can be no space between the command and the parentheses

• Command substitution, equivalent to ' anti-quote '

  • $ (XXX) will be replaced with command execution results

• Initializing an array

  • Arr= (a B c d)

FileNum=$ (ls | wc-l) 

${filenum}

FileNum=' ls | wc-l ' 

${filenum}

?

Arr= (a b c d)

${arr[0]}  //a

${arr[@]}  //4

(()) Double parenthesis

• Integer calculation, floating point not supported

  • ((TMP=A+B))

  • c=$ ((a+b))

• Comparison of arithmetic operations

((c=2+1))    

    3

D=$ ((1+2)) 

    3

?

a= =true 

if ($a= =   Then

    "A==1"

Fi

/a= =true  

if ((a= =  Then

    "A==1"

Fi

[] Single bracket

• Judgment command: The General argument is a comparison expression

  • Returns an exit status code based on the results of the comparison

  • The available comparison operators are:

    • String: = =,! =

    • Integers:-eq,-gt,-lt,-ne,-ge,-le

    • Logic:-A (| |),-O (&&)

= = AA

Then

    "AA = = AA"

Else

    "AA! = AA"

Fi

1 < 2

Then

    "1 < 2"

Fi

= = BB

Then

    "1-lt 2-o aa = = BB"

Fi

[[]] double brackets

• Keywords in bash program language, payment in double brackets does not occur parameter extension or command substitution

• Support for string pattern matching and regular matching

• Support logic judges: &&, | |, <, >

Hello and Hell? You can add parentheses, or do not add

Then

    "Hello = = Hell?"

Fi

Then

    "2 > 1 && Hello = = Hell?"

Fi

{} curly braces

• Use wildcards to extend curly brace content

  • Whitespace is not allowed (unless whitespace is referenced or escaped)

• Code block:

  • Will not open a new shell, variables can be used by the rest of the script

  • Every command must have a semicolon.

  • There are spaces on both sides of parentheses

' Touch {1..4}.txt '  4.txt

' Touch {ex1, ex2}.txt '  touch ex1.txt ex2.txt

Shell in parentheses summary: {}, (), (()), [], [[]]