Wildcards in Linux are special characters that can be used to represent other characters. They are often used in shell commands to execute commands that display results. Wildcards are also used in regular expressions to match patterns in the system.

There are three basic types of wildcards in Linux:

Asterisk (*): Matches any number of characters, including zero
Question mark (?): Matches exactly one character
Square brackets []: Matches multiple characters or a range

Here are some examples of how wildcards can be used in Linux:

ls a* (To list all files in the current directory that start with the letter "a")

ls *.txt (To list all files in the current directory that have the extension ".txt")

ls .txt .pdf (To list all files in the current directory that have the extension ".txt" or ".pdf",)

The following regular expression would match any line of text that contains the word "hello" followed by any number of spaces and then the word "world":

hello\s+world

The question mark (?) is a wildcard character in Linux

For example, the command ls* .txt will list all of the files in the current directory that end with .txt.

The command grep "hello?" myfile.txt will search the file myfile.txt for any line that contains the word "hello" followed by any single character.

ls f?le.txt will list all of the files in the current directory that start with the letter "f" and end with the word "file.txt".

grep "h[ae]llo" myfile.txt will search the file myfile.txt for any line that contains the word "hello" or "hallo".

`find . -name "*.txt"` will find all of the files in the current directory and all of its subdirectories that end with .txt.

Square brackets are used in Linux to match specific characters. They are also known as wildcards. Wildcards are special characters that can be used to represent a range of characters.
The following command would list all files that start with the letter "a" and end with the letter "t":

ls [a-t]*

The following command would list all files that start with the letter "a" and end with the letter "t" or "z":

ls [a-tz]*

You can also use square brackets to exclude characters. For example, the following command would list all files that start with the letter "a" but do not end with the letter "t":

ls [a-t]*[^t]

You can also use square brackets to match a range of numbers. For example, the following command would list all files that start with the number "1" and end with the number "9":

ls [1-9]*

Wildcards can be a powerful tool for working with files and text in Linux. By understanding how to use wildcards, you can save yourself a lot of time and effort.