Pipes and redirection

Problem

What becomes confusing, fragile, or impossible without understanding pipes and redirection? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Before integrated development environments, the operating system, terminal, files, and text streams were the environment. Their small composable interfaces still underpin modern tooling.

Concept explanation

Pipes and redirection belongs to “Processes, streams, and the shell”. pipes and redirection must be connected to layer below by tracing representation, ownership, control, and failure. A process transforms input streams into output streams and reports status independently.

For pipes and redirection, trace concrete input, state transition, output, and failure through an operating-system boundary made visible through bytes, files, processes, and exit status.

Programs are processes. They read bytes, transform state, write bytes, open files and sockets, then report success or failure through observable boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.sh. Focus: Trace pipes and redirection end-to-end while running: Pipe three newline-delimited records through sort and uniq -c.

1. Run: chmod +x lab.sh && ./lab.sh
2. Save baseline evidence. Use wc, od, stat, ps, lsof, curl, or Git plumbing as appropriate; save raw output before interpretation.
3. Boundary case: At every pipes and redirection boundary, label owner and representation during: Include spaces, an empty line, and a literal * without accidental shell expansion.
4. Failure case: Locate first layer where evidence diverges during: Make middle pipeline command exit non-zero and preserve its diagnostic on stderr.

RESULT
Trace explains stdout contains counted sorted records; stderr stays separate; final exit behavior is explicitly observed. without skipping any conversion, queue, process, protocol, or storage boundary. Starter-level baseline: Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.

Starter material

Create lab.sh, paste this exact content, then run the command below.

#!/usr/bin/env sh
set -eu
lab="${TMPDIR:-/tmp}/reforging-lab"
mkdir -p "$lab"
printf '%s\n' 'baseline: pipes and redirection' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"

STOP / CLEANUPNo background process. Keep generated evidence files for your notebook.

Independent exercise

Remove one abstraction: pipes and redirection

1. Create second case from blank file: At every pipes and redirection boundary, label owner and representation during: Include spaces, an empty line, and a literal * without accidental shell expansion.
2. Then create controlled failure: Locate first layer where evidence diverges during: Make middle pipeline command exit non-zero and preserve its diagnostic on stderr.
3. Use Ghostty · tmux · hx · shell · curl · Git to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• Trace explains stdout contains counted sorted records; stderr stays separate; final exit behavior is explicitly observed. without skipping any conversion, queue, process, protocol, or storage boundary.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled pipes and redirection failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.