Linux Advanced variables

Advanced variables
Basic form
[1]. variable extension format ${variable name}
[2]. command extension Format $ (command)
[3]. Arithmetic extension format $ (arithmetic)
1. Variable expansion
Basic form: ${variable name}
Variable extension meaning: a simple conditional judgment that assigns different values to different conditions. It has a large relationship with the existence or otherwise of a variable.
1.1 Basic usage of test variables for existence
Syntax: ${variables to be tested-default value}
Note: If the variable to be measured does not exist, then it conforms to the judging condition, and returns the default value as the disposition.
Parse: Returns the default value if the variable being tested does not exist, otherwise returns the value of the variable
Instance:
Unset A
echo ${a-' default ' #输出为 ' default '
1.2 A default value is passed back when the test variable does not exist or its value is empty
Syntax: ${variables to be tested:-Default value}
Instance:
A=
echo ${a:-' default ' #输出为 ' default '
1.3 The test variable does not exist or its value is empty to assign a default value to the variable
Syntax: ${variables to be tested: = default value}
Instance:
A=
echo ${a:= ' default '}
echo $a #输出为2行 ' default '
1.4 Test variable does not exist or its value is empty, output a hint message
Syntax: ${variables to be tested:? Prompt message}
Instance:
A=
echo r={a: Prompt message} #输出 "Tips"
1.5 The test variable exists and is non-null returns the truth value

Syntax: ${variables to be tested: + truth}
Instance:
A=hello
Echo ${a:+world} #输出为 "World"
Summarize:
Symbolic effect
: Determines whether the variable is empty and true if it is empty
+ Forward, determine if the variable exists and is not empty, true if it exists and is not empty
-Reverse, determine if the variable does not exist, or null if it does not exist
= assigned value,
? Prompt information
2. Take a string slice to get the string length
2.1 Taking a string slice
Grammar:
[1].${variable: The starting position index}
Function: intercepts all characters starting at the specified position to the end of the string, with the starting character indexed to 0

Instance:

str= "Hello World"
Echo ${str:6} #输出结果为 "World"

[2].${variable: Start position: Length}
Function: Intercepts a string of the specified length starting at the specified starting position.

Instance:
str= "Hello World"
Echo ${str:6:2} #输出为 "Wo"
2.2 Calculating string Lengths
Syntax: ${#变量名称}
Instance:
str= "Hello World"
echo ${#str} #输出结果为11
3. Contrast style
Purpose: To intercept a part of a variable value (string)
Procedure: Delete or replace a partial string that conforms to the style
3.1 Starting from the front of the string delete the shortest match
Syntax: ${variables to be measured # style}
Description: Deletes the shortest matching style string from the left of the value of the variable to be measured
Instance:

Filename= "/usr/sbin/ntpdate"
Echo ${filename#} #显示结果ntpdate
3.3 Starting from behind the string delete the shortest match
Syntax: ${the variable to be measured% style}
Instance:
Filename= "/usr/sbin/ntpdate"
Echo ${filename%/*} #输出结果/usr/sbin
3.4 Delete the longest match by starting from behind the string
Syntax: ${the variable to be measured in percent style}
Instance:
Filename= "Hello/usr/sbin/ntpdate"
Echo ${filename%%/*} #输出结果hello

4. Replace or delete a partial string

4.1 Replace only the first string found
Syntax: ${variables to be tested/style/replace}
Instance:
str= "Hello hello"
Echo ${str/hello/world} #结果为world Hello
4.2 Replace all found style strings
Syntax: ${variables to be tested//style/replace}
Instance:
str= "Hello hello"
Echo ${str/hello/world} #结果为world World
4.3 Delete the first specified string
Syntax: ${the variable to be tested/the string to delete/}
Instance:
str= "Hello hello"
Echo ${str/hello/} #结果为hello
4.4 Delete all the specified strings
Syntax: ${variables to be tested//variables to be deleted/}
Instance:
str= "Hello World Hello"
Echo ${str//hello/} #输出结果为world
5. Command Extensions
Syntax: $ (command)
Example: time=$ (date) equivalent time= ' Date '
6. Arithmetic expansion
Syntax: $ ((arithmetic))
Instance:
echo "a=$ ((5*5))" #结果为a =25

Linux Advanced variables