Metadata and permissions

Problem

What becomes confusing, fragile, or impossible without understanding metadata and permissions? 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

Metadata and permissions belongs to “Files are the first database”. metadata and permissions is learned by comparing readable, constrained, and deliberately clever implementations against same evidence.

For metadata and permissions, 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: Solve metadata and permissions three ways using identical fixture: Create notes/hello.txt containing ASCII A, UTF-8 é, and 🙂 on separate lines.

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: Run all three implementations against: Compare composed é with decomposed e + combining acute accent.
4. Failure case: Force every implementation through: Create one lone ff byte and attempt UTF-8 decoding.

RESULT
All versions preserve Valid sample is 10 bytes; composed and decomposed text look similar but have different bytes; ff is invalid UTF-8.; comparison records code size, predictability, and debugging cost. 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: metadata and permissions' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$lab/input.txt"
od -An -tx1 -c "$lab/input.txt"

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

Independent exercise

Compare three versions: metadata and permissions

1. Create second case from blank file: Run all three implementations against: Compare composed é with decomposed e + combining acute accent.
2. Then create controlled failure: Force every implementation through: Create one lone ff byte and attempt UTF-8 decoding.
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

• All versions preserve Valid sample is 10 bytes; composed and decomposed text look similar but have different bytes; ff is invalid UTF-8.; comparison records code size, predictability, and debugging cost.
• Command exits 0, prints a byte count, then prints hexadecimal and character views of the same file.
• Controlled metadata and permissions failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.