|Red Hat Enterprise Linux 3: Red Hat Enterprise Linux Step By Step Guide|
|Prev||Chapter 3. Shell Prompt Basics||Next|
Red Hat Enterprise Linux has a utility which can help you keep short lists, gather lists together, and even show you information about your system.
The utility is called cat, short for concatenate, which means to combine files.
The command cat also displays the contents of an entire file on the screen (for example, type cat filename.txt). If the file is fairly long, it quickly scrolls past you on the screen. To prevent this, use the cat filename.txt | less command.
Using the pipe (|) and the less command together displays the file one page at a time. You can then use the up and down arrow keys to move backward and forward through the pages. For more on using pipes to combine two separate functions, refer to Section 3.9 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) directs its output to the file name following the symbol.
For example, using cat by itself 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:
To redirect the output of cat to a file, type
the following at a shell prompt (pressing the
cat > sneakers.txt
Do you notice anything different in Figure 3-6? 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 also use cat to read the file you have created. At the prompt, type:
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 command
cat > home.txt, then
bring the coffee home take off shoes put on sneakers make some coffee relax!
Now, on an empty line, use the
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 (as seen in Figure 3-7). Type the following:
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 to a file, rather than replacing the contents of a file entirely.
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. To add the information in home.txt to the information already in sneakers.txt, 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:
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!
The command you typed appended 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, and you can see that they are identical. To make your comparison, type:
cat sneakers.txt; cat saturday.txt
The contents of both files are displayed — first sneakers.txt, then saturday.txt (as shown in Figure 3-8).
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. 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.