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:
and press the [Enter] key. Your screen will look like:
[newuser@localhost newuser]$ cat
stop by sneaker store
stop by sneaker store |
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).
Redirection means causing the shell to change what it considers
to be standard input or where the standard output is going.
We used cat before to demonstrate the idea behind
standard input and standard output. Now, let's use
cat to see how standard output can be redirected.
To redirect standard output, we'll 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.
Let's 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 |
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's 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:
at the prompt.
 | Don't 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're creating doesn't 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
relax! |
Now, on an empty line, use the
[Ctrl]-[D] keys
again to quit cat.
We can check the file again by typing:
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'll
find an example in Figure 10-9). Type the
following:
[newuser@localhost newuser]$ cat sneakers.txt home.txt > saturday |
Now it's 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
relax!
[newuser @localhost newuser]$ |
You can see that cat has added
home.txt where sneakers.txt
left off.
 | Combining Files With cat |
|---|
| | Creating and combining short files with cat can
be a convenient alternative to using a text editor like
Pico.
|
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're 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:
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'll see that they're
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).
| Don't Replace When You Append |
|---|
| | Remember that when you append output, you've got to include two
greater-than symbols (>>). Otherwise, you'll end up replacing
the very file to which you want to append information!
|
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're telling the
shell that you want a file to be read as input for a command.
Use a file you've already created to demonstrate this idea. Just
type:
Because you used the less-than symbol (<) to separate the
cat command from the file, the output of
sneakers.txt was read by cat.
