4.9. Manipulating
Information from the Shell
From the shell's point of view, there are three kinds of
information: standard input, standard output, and standard error.
In simple terms, standard input is the
information that a user enters from the keyboard for the shell to
use, such as commands and filenames. Standard
output is essentially the information the shell prints on the
screen after evaluating a user's commands. Standard error is the information that indicates
that something has gone wrong. Collectively, these three types of
information are referred to as standard
I/O (input/output).
The ability to manipulate and use information from the shell is
one of the strengths of Red Hat Enterprise Linux. This section
discusses the simpler aspects of standard I/O manipulation.
4.9.1. Pipes and
Pagers
In Section 4.8.5
Viewing and creating files with cat, we saw that using cat to view a large file causes the contents to
scroll off the screen. Using a pipe, we
can control that behavior. A pipe is the ( | ) symbol. It is used
to connect the standard output of one command to the standard input
of another command. Essentially, it allows a user to string
commands together. Pagers are commands (such as less) that display text in the terminal window.
Using cat, the pipe (|), and less 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.
The above command opens the file named <filename> using the cat command, but does not allow it to scroll off the
screen.
Using the pipe with a pager is also useful when examining large
directories with ls. For example, view the
/etc/ directory with the ls command.
Notice that the contents scroll past too quickly to view. To get
a closer look at the output of the ls
command, pipe it through less.
Now you can view the contents of /etc/
one screen at a time. Remember that you can navigate forward and
backward through the screens and even search for specific text
using the [/] key.
You can combine redirection with wildcards.
The above displays all files and directories in /etc/ that start with the letter "a" one screen at
a time.
4.9.2.
Using Redirection
Redirection means changing where
standard input comes from or where the standard output goes.
To redirect standard output, use the > symbol. Placing >
after the cat command (or after any
utility that writes to standard output) redirects its output to the
file name following the symbol.
Remember that the cat command echoes
the text you enter on the screen. Those echoes are the standard
output of the command. To redirect this output to a file, type the
following at a shell prompt and press Enter: cat > foo.txt. Enter a few more lines of text,
and use the [Ctrl] -[D] key combination to quit cat.
The following screen is an example, redirecting three lines of
text into the file foo.txt.
The following screen is another example, redirecting three more
lines of text to create the file bar.txt.
The following screen demonstrates cat's
concatenate function, adding the contents of bar.txt to the end of foo.txt. Without redirection to the file example1.txt, cat displays
the concatenated contents of foo.txt and
bar.txt on the screen.
The following screen displays the contents of example1.txt, so that the user can see how the
files were added together.
 |
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.
|
4.9.3.
Appending Standard Output
The symbol >> appends standard
output. This means it adds the standard output to the end of a
file, rather than over-writing the file.
The following screen shows the command to append bar.txt to foo.txt. The
contents of bar.txt are now permanently
added to the end of foo.txt. The
cat command is called a second time to
display the contents of foo.txt.
To compare example1.txt and the
modified foo.txt, use the diff command. diff compares
text files line-by-line and reports the differences to standard
output. The following screen shows the comparison between
example1.txt and foo.txt. Because there are no differences,
diff returns no information.
4.9.4.
Redirecting Standard Input
You can also 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.
The following screen shows foo.txt
being redirected as input for cat:
