3. stderr (Standard Error)

Let's explore what happens when a command produces an error. Try to list the contents of a directory that doesn't exist and redirect the output to a file named peanuts.txt.

ls /fake/directory > peanuts.txt

Instead of a clean prompt, you will see an error message on your screen:

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

You might be wondering why this message wasn't sent to the file. This is because another I/O stream is at play: standard error, or stderr.

What is Standard Error in Linux?

In Linux, stderr is a default output stream used by programs to send error messages and diagnostics. It is completely separate from the standard output (stdout) stream, which is used for normal program output. By default, both stdout and stderr send their output to your terminal screen, which is why you see the error message directly.

To control stderr, you need a different redirection method.

Understanding File Descriptors

To manage I/O streams like stdin, stdout, and stderr, the system uses file descriptors. A file descriptor is a non-negative number that the kernel uses to identify an open file or stream. The default file descriptors are:

• 0: stdin (standard input)
• 1: stdout (standard output)
• 2: stderr (standard error)

The number 2 is the dedicated stderr file descriptor, and we can use it to control where error messages go.

Redirecting stderr to a File

To redirect stderr to a file, you use the file descriptor 2 followed by the > operator. This command will send any error messages into the specified stderr file.

ls /fake/directory 2> peanuts.txt

Now, your terminal will be quiet, and the error message will be inside peanuts.txt.

Combining stdout and stderr

What if you want to capture both normal output and error messages in the same file? You can achieve this by redirecting both streams.

ls /fake/directory /etc/passwd > peanuts.txt 2>&1

Let's break this down:

1. > peanuts.txt redirects stdout (file descriptor 1) to the peanuts.txt file.
2. 2>&1 redirects stderr (file descriptor 2) to the same location that stdout (file descriptor 1) is currently pointing to.

The order is important. 2>&1 sends stderr to the current destination of stdout. In this case, stdout is pointing to a file, so stderr is also sent to that file.

A more modern and shorter way to redirect both stdout and stderr is by using &>.

ls /fake/directory /etc/passwd &> peanuts.txt

Discarding Error Messages

Sometimes, you might want to run a command and completely ignore any potential error messages. To do this, you can redirect stderr to a special file called /dev/null, which discards any data written to it.

ls /fake/directory 2> /dev/null

This command will execute, and any error output from stderr will be sent to /dev/null and discarded, leaving your screen clean.