Running Linux, 5th Edition

Saving Your Output

System administrators (and other human beings too) see a lot of critical messages fly by on the computer screen. It's often important to save these messages so that you can scrutinize them later, or (all too often) send them to a friend who can figure out what went wrong. So, in this section, we'll explain a little bit about redirection, a powerful feature provided by Unix shells. If you come from Windows, you have probably seen a similar, but more limited, type of redirection in the command-line interpreter there.

If you put a greater-than sign (>) and a filename after any command, the output of the command will be sent to that file. For instance, to capture the output of ls, you can enter:

$ ls /usr/bin > ~/Binaries

A listing of /usr/bin will be stored in your home directory in a file named Binaries. If Binaries had already existed, the > would wipe out what was there and replace it with the output of the ls command. Overwriting a current file is a common user error. If your shell is csh or tcsh, you can prevent overwriting with the command:

$ set noclobber

In bash, you can achieve the same effect by entering:

$ noclobber=1           It doesn't have to be 1; any value will have the same effect.

Another (and perhaps more useful) way to prevent overwriting is to append new output. For instance, having saved a listing of /usr/bin, suppose we now want to add the contents of /bin to that file. We can append it to the end of the Binaries file by specifying two greater-than signs: ...