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Using Redirection

2.7 Using Redirection

Now that we have a handle on what standard input and standard output are, it's time to expand a little.

Redirection means causing the shell to change what it considers 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 an xterm window type:

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

Figure 31: Redirecting the output to a file

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

Notice the difference (see Figure 31)? 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 your login directory (may we suggest using 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.

  • Tip: 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.

    [billy@localhost billy] 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] and [D] keys again to quit cat.

    We can check the file again by typing:

    cat home.txt
    

    at the prompt.

    Let's use cat again to join home.txt with sneakers.txt and redirect the output of both files to a brand new file we'll call saturday (you'll find an example in Figure 32).

    [billy@localhost billy] cat sneakers.txt home.txt > saturday
    

    That's it.

    Figure 32: Joining files and redirecting the output

    Now it's time to check our handiwork. Type:

    [billy@localhost billy] cat saturday
    

    and you should see something like this:

    [billy @localhost billy] 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!
    [billy @localhost billy]
    

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

  • Tip: Creating and combining short files with cat can be a convenient alternative to using a text editor like pico.

  • Summary: By using the output redirection symbol (>) you can send the output to a file instead of the terminal. The cat utility can be used along with output redirection to join files together into a single, unified file with one filename.


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