View, Redirect, and Append Files with cat

Red Hat Linux has a utility which can help you keep short lists, gather those lists together and, at the same time, show you a little of the power behind your system.

The utility is called cat, short for "concatenate," which means that it strings files together.

The command cat will also display the contents of an entire file on the screen (for example, type cat filename.txt). Using cat can be handy if the file is fairly short. But if a file is fairly long, it will quickly scroll past you on the screen, since cat displays the whole file.

But cat can also perform a quick demonstration of two important terms: standard input and standard output.

Standard input and standard output direct input and output (often referred to as I/O) to the user. If a program reads from standard input, it will by default be reading from the keyboard. If a program writes to standard output, by default it will be writing to the screen.

Start cat to see what this means. At the shell prompt, type:


The cursor moves to a blank line. Now, in that blank line, type:

stop by sneaker store

and press the [Enter] key. Your screen will look like:

[newuser@localhost newuser]$ cat
stop by sneaker store
stop by sneaker store

Figure 10-7. cat Demonstrates Standard Input and Standard Output

To quit cat, move the cursor to a blank line by pressing [Enter]. Then press [Ctrl]-[D].

The cat command has just demonstrated the definition of standard input and standard output; you typed words (standard input) and they appeared on the screen (standard output).

Using Redirection

Redirection means causing the shell to change what it considers to be standard input or where the standard output is going.

You used cat before to demonstrate the idea behind standard input and standard output. Now use cat to see how standard output can be redirected.

To redirect standard output, use the > symbol. Placing > after the cat command (or after any utility or application that writes to standard output) will direct its output to the filename following the symbol.

Try it. In a shell prompt type:

[newuser@localhost newuser]$ cat > sneakers.txt
buy some sneakers
then go to the coffee shop
then buy some coffee

Figure 10-8. Redirecting Output to a File

Now press [Enter] to go to an empty line, and use the [Ctrl]-[D] keys to quit cat.

Notice the difference (see Figure 10-8)? For one thing, there are no double entries. That is because the standard output from cat was redirected. That redirection was to a brand new file you made called sneakers.txt.

You can find the file in the directory you were in when you started cat(type ls if you want to see it listed).

You can even use cat to read the file, by typing:

cat sneakers.txt

at the prompt.

CautionDo Not Overwrite Files

Be careful when you redirect the output to a file, because you can easily overwrite an existing file! Make sure the name of the file you are creating does not match the name of a pre-existing file, unless you want to replace it.

Let's use output redirection for another file and call it home.txt. Type the following:

[newuser@localhost newuser]$ cat > home.txt
bring the coffee home
take off shoes
put on sneakers
make some coffee

Now, on an empty line, use the [Ctrl]-[D] keys again to quit cat.

We can check the file again by typing:

cat home.txt

at the prompt.

Use cat again to join home.txt with sneakers.txt and redirect the output of both files to a brand new file called saturday (you will find an example in Figure 10-9). Type the following:

[newuser@localhost newuser]$ cat sneakers.txt home.txt > saturday

Figure 10-9. Joining Files and Redirecting Output

Now it is time to check your work. Type:

[newuser@localhost newuser]$ cat saturday

and you should see something like this:

[newuser @localhost newuser]$ cat saturday
buy some sneakers
then go to the coffee shop
then buy some coffee
bring the coffee home
take off shoes
put on sneakers
make some coffee
[newuser @localhost newuser]$

You can see that cat has added home.txt where sneakers.txt left off.

TipCombining Files With cat

Creating and combining short files with cat can be a convenient alternative to using a text editor like Pico.

Appending Standard Output

You can use output redirection to add new information to the end of an existing file. Similar to when you used the > symbol, you tell your shell to send the information somewhere other than standard output.

However, when you use >>, you are adding information, rather than replacing it.

The best explanation is a demonstration, so take two files which have already been created — sneakers.txt and home.txt — and join them by using the append output symbol. You want to add the information in home.txt to the information already in sneakers.txt, so type:

cat home.txt >> sneakers.txt

Now check the file by typing:

cat sneakers.txt

And there it is, with the contents of home.txt at the end.

The command you typed told the system to "append the output from the file home.txt to the file sneakers.txt."

By appending the output, you save yourself a step or two (and a bit of disk clutter) by using existing files, rather than creating a new file.

Compare the results of the files sneakers.txt and saturday now, and you will see that they are identical. To make your comparison, type:

cat sneakers.txt; cat saturday

The contents of both files will be displayed — first sneakers.txt, then saturday (as shown in Figure 10-10).

CautionDo Not Replace When You Append

Remember that when you append output, have to include two greater-than symbols (>>). Otherwise, you will end up replacing the very file to which you want to append information!

Figure 10-10. Stringing Commands and Comparing Files

Redirecting Standard Input

Not only can you redirect standard output, you can perform the same type of redirection with standard input.

When you use the redirect standard input symbol <, you are telling the shell that you want a file to be read as input for a command.

Use a file you have already created to demonstrate this idea. Just type:

cat < sneakers.txt

Because you used the less-than symbol (<) to separate the cat command from the file, the output of sneakers.txt was read by cat.