Linux advanced Variables

Advanced Variables
Basic Form
[1]. variable extension format $ {variable name}
[2]. Command extension format $ (command)
[3]. Arithmetic extended format $ (arithmetic ))
1. variable extension
Basic Format: $ {variable name}
Variable extension meaning: A simple condition judgment that assigns different values of variables to different conditions. It has a great relationship with the existence of variables.
1.1 test the basic usage of Variables
Syntax: $ {variable to be tested-default value}
Note: If the variable to be tested does not exist, the condition is met and the default value is returned for processing.
Resolution: If the variable to be tested does not exist, the default value is returned. Otherwise, the value of the variable is returned.
Instance:
Unset
Echo $ {a-'default'} # output is 'default'
1.2 If the test variable does not exist or its value is null, a default value is returned.
Syntax: $ {variable to be tested:-default value}
Instance:
A =
Echo $ {a:-'default'} # The output is 'default'
1.3 The test variable does not exist or its value is null. assign a default value to the variable.
Syntax: $ {variable to be tested: = default value}
Instance:
A =
Echo $ {a: = 'default '}
Echo $ a #2 rows of output 'default'
1.4 if the test variable does not exist or its value is null, a prompt is displayed.
Syntax: $ {variable to be tested :? Prompt message}
Instance:
A =
Echo r = {:? Prompt information} # output "prompt information"
1.5 if the test variable exists and is not null, the true value is returned.
 
Syntax: $ {variable to be tested: + true value}
Instance:
A = hello
Echo $ {a: + world} # output as "world"
Summary:
Symbol function
: Determines whether the variable is null. If it is null, it is true.
+ Forward: determines whether the variable exists and is not empty. If it exists and is not empty, it is true.
-Reverse: determines whether the variable does not exist. If the variable does not exist, it is null.
= Value assignment,
? Prompt information
2. Take string slices to obtain the string length.
2.1 take string slices
Syntax:
[1]. $ {Variable: Start position index}
Purpose: intercept all characters starting from the specified position and ending with the string. The index of the Start character is 0.
 
Instance:

Str = "hello world"
Echo $ {str: 6} # The output result is "world"

[2]. $ {Variable: Start position: length}
Purpose: intercept a string of the specified length starting from the specified start position.
 
Instance:
Str = "hello world"
Echo $ {str: 6: 2} # output is "wo"
2.2 calculate the string length
Syntax: $ {# variable name}
Instance:
Str = "hello world"
Echo $ {# str} # The output result is 11.
3. Comparison Style
Objective: To extract a part of a variable value (string)
Practice: delete or replace some strings that match the style
3.1 compare and delete the shortest matched items starting from the beginning of the string
Syntax: $ {variable to be tested # style}
Note: The minimum matching style string is deleted from the left side of the variable value to be tested.
Instance:

Filename = "/usr/sbin/ntpdate"
Echo $ {filename #/*/} # display result sbin/ntpdate
3.2 comparison between the longest matched person deleted from the beginning of the string
Syntax: $ {variable to be tested # style}
Instance:
Filename = "/usr/sbin/ntpdate"
Echo $ {filename ###/*/} # Show the result ntpdate
3.3 comparison between the shortest matched items after the string
Syntax: $ {variable to be tested % style}
Instance:
Filename = "/usr/sbin/ntpdate"
Echo $ {filename %/*} # output result/usr/sbin
3.4 Comparison Between the longest matched items after the string
Syntax: $ {variable to be tested % style}
Instance:
Filename = "hello/usr/sbin/ntpdate"
Echo $ {filename %/*} # output result hello

4. Replace or delete some strings
 
4.1 replace only the first string found
Syntax: $ {variable to be tested/style/replacement}
Instance:
Str = "hello"
Echo $ {str/hello/world} # The result is world hello.
4.2 replace all style strings found
Syntax: $ {variable to be tested // style/replace}
Instance:
Str = "hello"
Echo $ {str/hello/world} # The result is world.
4.3 Delete the first specified string.
Syntax: $ {variable to be tested/string to be deleted /}
Instance:
Str = "hello"
Echo $ {str/hello/} # The result is hello.
4.4 delete all specified strings.
Syntax: $ {variable to be tested // variable to be deleted /}
Instance:
Str = "hello world hello"
Echo $ {str // hello/} # The output result is world.
5. Command Extension
Syntax: $ (command)
Instance: time = $ (date) equivalent time = 'date'
6. Arithmetic Extension
Syntax: $ (arithmetic ))
Instance:
Echo "a = $ (5*5)" # The result is a = 25.

 
Author: "huangyandong"