Standard Error

The third and final data stream is standard error, abbreviated as stderr. This stream is specifically for error messages and warnings, keeping them separate from normal output.

When a command fails, the error message goes to standard error, not standard output:

ls nonexistent_folder

The error message you see comes through stderr. This separation matters because you can redirect errors independently from regular output.

To redirect standard error, use 2> instead of >:

ls nonexistent_folder 2> errors.txt

The 2 refers to stderr's file descriptor number (stdout is 1, stderr is 2). This command captures the error message in errors.txt while keeping your terminal clean.

You can redirect both streams to different files:

ls existing_folder nonexistent_folder > output.txt 2> errors.txt

Successful output goes to output.txt, while any errors go to errors.txt. To send both streams to the same file, use 2>&1:

ls existing_folder nonexistent_folder > all_output.txt 2>&1

This redirects stderr to wherever stdout is going, combining everything into one file.