OSXFAQ Mac OS X UNIX Tip-of-the-Day 

This week we'll look at some handy redirection tricks. They're of most use in shell scripts, but can be used directly on the command line too.

First off, here's how a script might send an error message to standard error instead of standard out. Normally, when your script writes any type of message using a command such as 'echo', output is sent to standard out.

Why would we want to write to standard error instead? Traditionally, Unix commands such as 'ls' and 'mount' write output to standard out and error messages to standard error. This convention ensures that if we were to pipe the command's output to another command (or redirect it to a file) error messages would still displayed on the terminal screen and would not get mixed up with the 'normal' output. We want our scripts to honour this convention.

As an example, let's write the error message "Can't open the file." to standard error.

The key is this after the echo command:

which merges standard out (stream 1) into standard error (stream 2), hence causing 'echo' to write to standard error.

We run it and see the error message:

To prove the message went to standard error and not standard out, we redirect standard out to the file 'out' and standard error to the file 'err', displaying the contents of each file: