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. If the file is fairly long, it will quickly
scroll past you on the screen. To prevent this, use the cat
filename.txt | less command.
Using the pipe (|) and the less
command displays the file one page at a time. You can then use the left
and right arrow keys to move back and forth through the pages. For more on
pipes, see the Section called Pipes and Pagers.
Redirection means causing the shell to change what it considers to
be standard input or where the standard output should be going.
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.
For example, using cat by itself simply
outputs whatever you input to the screen as if it were repeating the
line you just typed. The following example shows
cat repeating every line that is entered:
[sam@halloween sam]$cat
buy some sneakers
buy some sneakers
then go to the coffee shop
then go to the coffee shop
then buy some coffee
then buy some coffee |
To redirect cat output to a file, type
the following at a shell prompt (pressing the [Enter]
key takes you to the next blank line):
[sam@halloween sam]$cat > sneakers.txt
buy some sneakers
then go to the coffee shop
then buy some coffee |
Press [Enter] to go to an empty line and use the
[Ctrl]-[D] keys to
quit cat.
Notice the difference (see Figure 11-5)?
There are no repeated 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).
As you learned earlier, you can then use cat to
read the file. At the prompt, type:
 | Caution |
|---|
| | 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.
|
Use output redirection again for another file and call it
home.txt. For this example, type the following:
[sam@halloween sam]$cat > home.txt
bring the coffee home
take off shoes
put on sneakers
make some coffee
relax! |
Now, on an empty line, use the [Ctrl]-[D] key combination again to quit
cat.
Next, use cat to join
home.txt with sneakers.txt
and redirect the output of both files to a brand new file called
saturday.txt (you will find an example in Figure 11-6). Type the following:
[sam@halloween sam]$cat sneakers.txt home.txt > saturday.txt |
You can see that cat has added
home.txt where sneakers.txt
ended.
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. 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 using the command cat
sneakers.txt. The final output shows the contents of
home.txt at the end of the file.
[sam@halloween sam]$cat sneakers.txt
buy some sneakers
then go to the coffee shop
then buy some coffee
bring the coffee home
take off shoes
put on sneakers
make some coffee
relax!
[sam@halloween sam]$ |
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 time (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.txt now, and you will see that they are
identical. To make your comparison, type:
cat sneakers.txt; cat saturday.txt |
The contents of both files will be displayed — first
sneakers.txt, then saturday.txt
(as shown in Figure 11-7).
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:
Because you used the less-than symbol (<) to separate the
cat command from the file, the output of
sneakers.txt was read by cat.
