WHAT ARE WILDCARD CHARACTERS?(chapter 6)

WHAT ARE WILDCARD CHARACTERS?

 

Wildcards are characters that represent all possible other characters. As you might remember from Chapter 4, the question mark character (?) can represent every other potential character, and the asterisk (*) can represent any potential combination of characters. Why are wildcards useful? Basically, they let your commands affect more than one file at a time. 

Let’s say you wanted to use the DEL command to erase a file called test.txt:

DEL test.txt

That command (assuming you had the proper permissions) would delete the test.txt file and do nothing else. But if you add wildcards into the mix, the command can affect multiple files. For instance, add a single question mark to the command:

DEL test?.txt

This command will not delete the test.txt file. It will, however, delete every file in the directory that has “test” in its name, “txt”, as its extension, and additional single character before the extension. So test1.txt, test2.txt, and test3.txt, would all be deleted by this single command. Much more efficient than typing DEL over and over again!

However, any of the files that had two characters before the extension – like test10.txt and test11.txt – will not be deleted. This is because the question mark character only serves as a wildcard for a single character. It’s useful if you want your command to affect a very narrow subset of files – if you only want to copy a few files, or to delete a few particular files.

To cast a wider net with your command, you need the asterisk (*) wildcard character. The question mark wildcard character represents a single character, but the asterisk wildcard represents any number of characters. For example, consider what would happen if you used an asterisk instead of a question mark with the DEL command:

DEL test*.txt

This command will delete every single file in the current directory that begins with “test” and has an extension of “txt”. No matter how many characters come after “test” and before “txt”, the DEL command will delete them all. 

You can make the DEL command even more powerful by removing the filename and leaving only the asterisk:

DEL *.txt

This command will delete every file in the current directory that has an extension of “txt.” We’ve already mentioned how an asterisk wildcard character can represent any number of characters – here will delete any file, regardless of its name, that has the “txt” extension. 

So think about what would happen if you typed this command:

DEL *.*

The use of the asterisk wildcard characters here means that the DEL command will delete any file with any name and any extension. To put it more simply, DEL will delete every single file in the current directory! Needless to say, you should exercise extreme caution while using wildcards with the DEL command.

The wildcard characters work with file manipulation commands other than DEL. You can use the wildcards to make copying and moving files from the Command Prompt far quicker and more efficient.

 

REDIRECTION

 

To understand redirection at the Command Prompt, you first to need to understand the ideas of input and output. Input is any information you enter into the computer – a mouse click, or the commands you type at the Command Prompt. Output is information that the computer returns to you – the menu that appears when you right-click the mouse, or the text Command Prompt displays when you enter a command.

Using redirection, you can redirect the output from one command to another command. The most common use of redirection is with the MORE command. If you type the DIR command in a directory that contains a great deal of files, the output will probably scroll off the top of the screen. You could use the DIR /P command to view the output one screen at a time. You have another option with the MORE command and a redirection pipe:

DIR | MORE

The pipe character (|) redirects the output from the DIR command to MORE. The MORE command takes the output and displays it one screen at a time, letting you scroll down line by line. You can also use the pipe character and MORE with the TYPE command. The TYPE command displays the contents of a plain-text file (generally a file with the “txt” extension, though some system INI files are plain-text) on the screen. See this example:

TYPE test.txt | MORE

If test.txt is too long to display on the screen, the MORE command will parse it out one screen of text at a time.

You can also use output redirection create files with the output from the commands. For example, what if you wanted to keep a listing of all the files in a particular directory? You could type the DIR command, of course, but the output would disappear as soon as you closed the Command Prompt window, or even if you typed enough other commands. However, you can avoid the problem if you use DIR with the greater than (>) sign:

DIR > output.txt

When you hit the Enter key, it might seem initially that nothing has happened. However, a new file named "output.txt" has been created in your current directory, and it contains the output from the DIR command. You can display the file with this command:

TYPE output.txt

You can then view the contents of the file.

It is important to take care while using the greater than sign to redirect output. In the previous example, if there's already a file named "output.txt" in the directory, the command will overwrite it, and any data in the original file will be lost. So make sure that you pick a new filename for your output.

However, by using a double greater than sign (>>) you can append the output to an existing file. Rather than overwriting an existing file, the double greater than sign will simply add the output to the end of the existing file. Here's an example:

DIR >> output.txt

You can then read output.txt at your leisure, whether with the TYPE command or with the Notepad application. 

Redirecting output to text files might not seem very useful with the DIR command, but it really comes in handy when working with Command Prompt's networking commands. (We'll discuss networking more in Chapter 8.) Many of the networking commands produce long and complicated outputs, and you might want to put the output in a text file to peruse at your leisure.