Signals and traps

Problem

What becomes confusing, fragile, or impossible without understanding signals and traps? 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

Signals and traps belongs to “Processes, streams, and the shell”. signals and traps is inspected by introducing one realistic failure and proving root cause before repair.

For signals and traps, 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: Capture healthy signals and traps baseline from: 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: Record signals and traps behavior at: Include spaces, an empty line, and a literal * without accidental shell expansion.
4. Failure case: Diagnose this exact signals and traps failure before editing: Make middle pipeline command exit non-zero and preserve its diagnostic on stderr.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: stdout contains counted sorted records; stderr stays separate; final exit behavior is explicitly observed. 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: signals and traps' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"

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

Independent exercise

Prove root cause: signals and traps

1. Create second case from blank file: Record signals and traps behavior at: Include spaces, an empty line, and a literal * without accidental shell expansion.
2. Then create controlled failure: Diagnose this exact signals and traps failure before editing: 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

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: stdout contains counted sorted records; stderr stays separate; final exit behavior is explicitly observed.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled signals and traps failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.