Manipulating Files with cat

Red Hat Linux 8.0: The Official Red Hat Linux Getting Started Guide
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# Manipulating Files with `cat`

Red Hat 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` 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 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, see the Section called Pipes and Pagers.

## Using Redirection

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 the output of `cat` to a file, type the following at a shell prompt (pressing the [Enter] key takes you to the next blank line):

    cat > sneakers.txt

Figure 11-5. Redirecting Output to a File

Press [Enter] to go to an empty line and use the [Ctrl]-[D] keys to quit `cat`.

Do you notice anything different in 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:

    cat sneakers.txt

| 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:

    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:

    cat sneakers.txt home.txt > saturday.txt

Figure 11-6. Joining Files and Redirecting Output

You can see that `cat` has added `home.txt` where `sneakers.txt` ended.

## 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 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. 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.

    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!

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).

Figure 11-7. 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`.

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