Linux Dummies 9th

Using Redirection and Pipes
Chapter 17: Working without the GUI
Redirection and pipes facilitate the flow of information. A pipe is exactly what
it sounds like: It directs the output of one program to the input of another
program. A pipeline may consist of several utilities plumbed together by
pipes. At either end of this pipeline is, optionally, a redirection.
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Almost all Linux utilities that require input and output have been plumbed
with the following common interfaces: stdin (standard input), stdout
(standard output), and stderr (standard error). By having a common
method to feed input to a program or read data from the output of a program,
you can glue utilities together into sophisticated solutions.
Redirecting command output
We discuss redirecting command output here because it’s by far the most
common form of information detouring. One example of output redirection
involves telling a command to send its results to a file rather than to the
screen, as you’ve probably been used to seeing. Start in some familiar territory
by typing ls -l and then pressing Enter, to produce something like the
following:
rich@testbox:~$ ls -l
total 36
drwxr-xr-x 2 rich rich 4096 2009-05-20 18:56 Desktop
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Documents
drwxr-xr-x 4 rich rich 4096 2009-05-17 17:42 GNUstep
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Music
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Pictures
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Public
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Templates
drwxr-xr-x 2 rich rich 4096 2009-05-17 14:09 Videos
rich@testbox:~$
Want to send this information to a file instead? You can use the > redirection
operator to tell bash to send the data into a file rather than onto your screen.
Enter the following command to send the information to a file named listing:
ls -l > listing
Notice that nothing shows up on-screen, where normally you’d expect to see
the output from the ls command. That’s because the shell has rerouted the
output to a file named listing. To verify that the directory listing is there,
enter the following command:
cat listing