The index and working tree

Problem

What becomes confusing, fragile, or impossible without understanding the index and working tree? 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

The index and working tree belongs to “Git as a content-addressed database”. the index and working tree is learned by comparing readable, constrained, and deliberately clever implementations against same evidence.

For the index and working tree, 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 the index and working tree three ways using identical fixture: Commit one file, then inspect blob, tree, commit, HEAD, and index with plumbing commands.

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: Amend content with same filename and compare object IDs.
4. Failure case: Force every implementation through: Detach HEAD, make a commit, move away, then recover it from reflog.

RESULT
All versions preserve Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.; 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: the index and working tree' > "$lab/input.txt"
printf 'bytes: ' && wc -c < "$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: the index and working tree

1. Create second case from blank file: Run all three implementations against: Amend content with same filename and compare object IDs.
2. Then create controlled failure: Force every implementation through: Detach HEAD, make a commit, move away, then recover it from reflog.
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 Changed bytes create new blob/tree/commit IDs; reflog retains recoverable commit after ref moves.; 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 the index and working tree failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.