Shell $ (), '', $ {}, $ (), and (())

Tag: $ {} ''$ () Shell

1. $ (), ''For command replacement

2. $ {} Is used to replace variables (the following content is from shell 13)

(1). Definition of Variables

        $ A=B        $ echo $AB

When executing this command, the AB is regarded as a variable rather than the connection between the variable $ A and the letter B.

        $ A=B        $ echo ${A}B

After the command is executed, the following output is: bb

(2). Special functions are helpful for string processing.

First, define the variable file =/dir1/dir2/dir3/my.file.txt. The following command gets different values.

• Extract Based on characters in the string: # And %

  • $ {File # */}: remove the first "/" and Its left string->/dir1/dir2/dir3/my.file.txt

  • $ {File ### */}: remove the last "/" and Its left string-> my.file.txt

  • $ {File %/*}: remove the last "/" and Its right string->/dir1/dir2/dir3

  • $ {File %/*}: remove the first "/" and Its right string-> Null Value

• Extract specified characters in length at a specified position

  • $ {File: 0: 5}: extract the leftmost five characters->/dir1

  • $ {File: 5: 5}: extract 5 consecutive 5 characters from the right of 5th characters->/dir2

• String replacement

  • $ {File/DIR/path}: Replace the first dir with Path->/path1/dir2/dir3/my.file.txt.

  • $ {File // DIR/path}: replace all dir with Path->/path1/path2/path3/my.file.txt

• Calculate the length of the variable value: $ {# Var}

  • $ {# File}-> 27 is strlen (file)

(3) array-related

• 
  

  • A = "a B C Def": the variable value is a string.

  • A = (a B C DEF): defines $ A as an array.

• 
  

  • $ {A [@]} or $ {A [*]}: obtain all arrays.

  • $ {A [0]}: first element

  • $ {# A [@]} or $ {# A [*]}: Number of array elements

3. $ () is used for calculation.

    $ a=5;b=7    $ echo $((a+b))    12

4. () You can redefine the variable value.

A = 5; (A ++) You can redefine $ A as 6

This article is from the "Linux" blog, please be sure to keep this source http://4983206.blog.51cto.com/4973206/1436474