Linux Advanced variables

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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sbin/ntpdate

3.2 Delete the longest match by starting with the preceding string

Syntax: ${variables to be tested # #样式}

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