Redirection & Pipelines | stdin, stdout & tee

I composed single-line data pipelines using redirection, pipes, and tee to filter, transform, and log output in one pass. Understanding the three standard streams let me capture stderr separately and split a pipeline to both a file and the screen.

Objective & Context

The Unix philosophy is small tools composed via streams. This lab masters file descriptors 0/1/2, pipe composition, and tee for branching, the glue behind every filter and scripting lab.

Environment & Prerequisites

A Bash shell with coreutils.
A command that writes to both stdout and stderr.
A log destination.

flowchart LR C[command] -->|stdout| F[filter] C -->|stderr 2>| E[error log] F --> T[tee] T --> Scr[screen] T --> Log[file]

Step-by-Step Execution

1. Separate stdout and stderr

make 2> build-errors.log 1> build-out.log

2. Filter through a pipe

journalctl -u nginx | grep -i error | tail -20

3. Split output with tee

dmesg | tee dmesg.log | grep -i fail

[  3.21] usb 1-2: device descriptor read/64, error -71

Validation & Testing

Run a command that emits both streams and confirm errors land in their own file while stdout is filtered. Pass criteria: stderr captured separately, the pipeline filters correctly, and tee writes the full stream to a file while passing it onward.

Advanced: Troubleshooting

Errors not captured: redirect 2> explicitly; pipes only carry stdout by default.
Both streams merged: use 2>&1 after the stdout redirect, order matters.
tee not writing: add -a to append rather than overwrite.

Key Results

Captured stdout and stderr to separate files in one command.
Built filter pipelines that reduce noisy logs to relevant lines.
Branched output to file and screen simultaneously with tee.
Internalized the three standard streams for reliable scripting.

Linux Redirection and Pipelines: stdin, stdout, stderr, and tee