How to Redirect stderr to Both stdout and stderr in Bash

Understanding the Stream Duplication Challenge

In Bash scripting, standard output (stdout, File Descriptor 1) and standard error (stderr, File Descriptor 2) operate as separate streams. A common requirement when logging or monitoring execution is to merge stderr into stdout so you have a single unified output stream, while simultaneously keeping stderr intact so error listeners or log aggregators receive errors independently.

Specifically, the goal is to achieve:

• stdout stream: Contains all original stdout lines interleaved with original stderr lines.
• stderr stream: Contains only original stderr lines.

The Solution: Bash Process Substitution with tee

If you are using Bash, the cleanest and most efficient approach utilizes process substitution (>(...)) combined with tee: