Mac OS X Leopard - ARCAism

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CHAPTER 18 INTRODUCING DARWIN AND THE SHELL
Pipes
Piping one command into another is a great way to make even the simplest Darwin tools do
powerful things. We saw this previously when we piped the ps command into the less command.
The pipe symbol is the | (which is the tall line that lives above the \ on a normal US Mac keyboard).
In practice, this takes the first command and sends the output into the second.
Commands like less rely almost entirely on the ability to pipe one command into it, and other
commands become much more useful with this ability. For example, the ability to take commands
that produce large amounts of output and pipe that content into a filter (like the grep
command) can save lots of time and headaches.
Redirects
A redirect allows you to alter what happens to the output of a command, or alternately direct the
content of a file into a command. The symbols for redirection are < and >. A very simple use of a
redirect is to create a text file using echo, like this:
Leopard:~ scott$ echo "Hello my name is Scott" > name.txt
Leopard:~ scott$ cat name.txt
Hello my name is Scott
The echo command normally would just print out whatever you feed into it back to your terminal,
but here we redirected the output to name.txt (which may or may not have existed).
CAUTION If you are redirecting data into an existing file, the entire contents of that file will
be replaced with the new data. So be very careful with this command.
If you wanted to redirect additional data into an existing file (rather than replace the content,
which the > always does), >> can be used to append the new data to the old:
Leopard:~ scott$ echo " Hello Scott" >> name.txt
Leopard:~ scott$ cat name.txt
Hello my name is Scott
Hello Scott
Leopard:~ scott$ echo "Ooops" > name.txt
Leopard:~ scott$ cat name.txt
Ooops
Background Tasks
Any Darwin command can be issued to run in the background with the & symbol tacked onto the
end of the command. This is particularly useful when you want to start a command that may
take a long time to finish, or when running a task that you want to keep running indefinitely. For
example, if we wanted to use the find command to find something with the name motd somewhere
on our system, knowing that this may take some time, we may want to run it in the
background. Here’s an annotated example of this:
Leopard:~ scott$ find / -name "motd" > found 2> found_err &
[1] 358
Here we start our find command in the background. We are redirecting our output to a file
named found. Also, the 2> found_err will redirect any error messages to a file named found_err
(otherwise, even though the command is running in the background, error messages would still
spam our terminal).
Leopard:~ scott$ jobs
[1]+ Running find / -name "motd" >found 2>found_err &