What are STDOUT and STDERR?

In Unix-like operating systems, every process or command generates three standard streams for input and output:

STDOUT (Standard Output):

This is where a program sends its normal output (results, responses, or information that it needs to display).

STDERR (Standard Error):

This is where a program sends its error messages (warnings, errors, or issues it encounters).

Both streams are represented as file descriptors in the background: - STDOUT is file descriptor 1 - STDERR is file descriptor 2

By default, both streams are displayed on the terminal.

Example 1: STDOUT (Normal Output)

Let's look at a simple example of a command that generates normal output (STDOUT):

echo "Hello, World!"

Output (STDOUT): 

Hello, World!

In this case, the command echo outputs the text Hello, World! to the terminal. This is STDOUT because it’s the normal, expected output of the command.

Example 2: STDERR (Error Output)

Now, let's look at an example of a command that generates an error (STDERR):

ls non_existent_directory

Output (STDERR): 

ls: cannot access 'non_existent_directory': No such file or directory

Here, the ls command is trying to list a directory that doesn’t exist. The error message cannot access 'non_existent_directory': No such file or directory is printed to STDERR because it’s an error, not normal output.

Redirecting STDOUT and STDERR

You can redirect these outputs to different places, like a file or /dev/null (to discard the output). Here's how you can redirect each:

1. Redirect STDOUT (Normal Output):

If you only want to redirect the normal output (STDOUT) to a file:

echo "Hello, World!" > output.txt

Result: The output.txt file will contain: 

Hello, World!

But, if there was an error generated in this case (e.g., trying to write to a file you don't have permission to), the error would still be shown on the terminal because we haven’t redirected STDERR.

2. Redirect STDERR (Error Output):

To redirect the error output (STDERR) to a file:

ls non_existent_directory 2> error.txt

Result: The error.txt file will contain: 

ls: cannot access 'non_existent_directory': No such file or directory

But the normal output (if any) will still be printed on the terminal.

3. Redirect Both STDOUT and STDERR:

You can redirect both STDOUT and STDERR to the same place (e.g., a file or /dev/null):

To redirect both to a file:

ls non_existent_directory > output.txt 2>&1

This will redirect both normal output and error messages to output.txt.

To discard both STDOUT and STDERR (suppress everything):

ls non_existent_directory > /dev/null 2>&1

This will suppress all output and errors, so nothing is printed to the terminal.

Visualizing STDOUT and STDERR:

Here’s a visualization of what happens when you run a command in a terminal:

Normal Output (STDOUT):

$ echo "Hello, World!"
Hello, World!  <-- This is STDOUT

Error Output (STDERR):

$ ls non_existent_directory
ls: cannot access 'non_existent_directory': No such file or directory  <-- This is STDERR

Summary:

  • STDOUT is the normal output stream that contains the expected result of a command.
  • STDERR is the error output stream that contains error messages when things go wrong.
  • By redirecting these streams, you can control where the output goes (file, terminal, etc.) and decide which type of output you want to suppress or capture.