3. stderr (Standard Error)

Lesson Content

Let’s try something a little different now, let’s try to list the contents of a directory that doesn’t exist on your system and redirect the output to the peanuts.txt file again.

$ ls /fake/directory > peanuts.txt 

What you should see is:

ls: cannot access /fake/directory: No such file or directory

Now you’re probably thinking, shouldn’t that message have been sent to the file? There is actually another I/O stream in play here called standard error (stderr). By default, stderr sends its output to the screen as well, it’s a completely different stream than stdout. So you’ll need to redirect its output a different way.

Unfortunately the redirector is not as nice as using < or > but it’s pretty close. We will have to use file descriptors. A file descriptor is a non-negative number that is used to access a file or stream. We will go in depth about this later, but for now know that the file descriptor for stdin, stdout and stderr is 0, 1, and 2 respectively.

So now if we want to redirect our stderr to the file we can do this:

$ ls /fake/directory 2> peanuts.txt

You should see just the stderr messages in peanuts.txt.

Now what if I wanted to see both stderr and stdout in the peanuts.txt file? It’s possible to do this with file descriptors as well:

$ ls /fake/directory > peanuts.txt 2>&1

This sends the results of ls /fake/directory to the peanuts.txt file and then it redirects stderr to the stdout via 2>&1. The order of operations here matters, 2>&1 sends stderr to whatever stdout is pointing to. In this case stdout is pointing to a file, so 2>&1 also sends stderr to a file. So if you open up that peanuts.txt file you should see both stderr and stdout. In our case, the above command only outputs stderr.

There is a shorter way to redirect both stdout and stderr to a file:

$ ls /fake/directory &> peanuts.txt

Now what if I don’t want any of that cruft and want to get rid of stderr messages completely? Well you can also redirect output to a special file call /dev/null and it will discard any input.

$ ls /fake/directory 2> /dev/null

Exercise

What is the following command doing?

$ ls /fake/directory >> /dev/null 2>&1

Quiz Question