Replace Linux Shell Parameters

The $ symbol in Bash is used to replace the parameter name with the value represented by the parameter. For $, braces are optional, that is, $ A and $ {A} represent the same parameter.

$ {} There are several expressions with colons: $ {parameter:-word}

If parameter is null or not set, replace the entire parameter with the expression value of word.

$ {Parameter: = word}

If parameter is null or not set, the entire parameter replaces the expression value with word, and the parameter value is set to word.

$ {Parameter :? Word}

If parameter is null or not set, an error message is printed. Otherwise, the entire parameter replacement expression value is $ parameter.

$ {Parameter: + word}

If parameter is not null or is not set, replace the entire parameter with the expression value of word.

$ {Parameter: Offset }$ {parameter: offset: length}

The substring of the parameter value.

We can understand the following examples:

$ {}! There are several expressions: $ {! Prefix *} $ {! Prefix @}

Print out the parameter names with prefix

$ {! Name [@] }$ {! Name [*]}

This is for the name array and prints out the subscript of the name array.

We can understand the following examples:

$ {} Regular Expression: $ {parameter # Word }$ {parameter # Word}

Scan the word from the beginning and filter out the characters matching the regular expression of the word.

# For the shortest match, # For the longest match

$ {Parameter % word }$ {parameter % word}

Scan the word from the end and filter out the characters matching the regular expression of the word.

% Is the shortest match, and % is the longest match

We can understand the following examples:

$ {Parameter/pattern/string }$ {parameter // pattern/string}

Replace the pattern string corresponding to the parameter value with the string

/Indicates to replace only once

// Replace all

We can understand the following examples: